Taxonomy of Proconsul: an Issue of Species

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TAXONOMY OF PROCONSUL: AN ISSUE OF SPECIES

NUMBERS OR SEXUAL DIMORPHISM?

HUGH WEAVER

Submitted in total fulfilment of the requirements of the degree of

Master of Philosophy

October 2015

Department of Anatomy and Neuroscience

University of Melbourne

ABSTRACT

I have investigated the alpha taxonomy of Miocene primate genus Proconsul by performing measurements on photographs of fossil dental material obtained from museum sources. My aim was to assess levels of variation, and thereby evidence for sympatric species, by comparing results with those obtained from sex-matched samples of extant hominoids.

The holotype of the initial species described, Proconsul africanus, came from a mainland site in western Kenya. Additional examples of the genus were obtained subsequently from adjacent sites and from islands within Lake Victoria. By 1951, three species of progressively-increasing size were recognised, seemingly present at both island and mainland localities.

However, subsequent investigations questioned whether, instead of two sympatric species at a particular site, particularly Rusinga Island, size disparities noted reflected the presence of a single sexually dimorphic species.

I addressed this debate by comparing results of measurements of 144 Proconsul specimens with those from sex-matched samples comprising 50 specimens each of four extant primate genera: Pan, Gorilla, Pongo and Hylobates/Symphalangus. The protocol consisted of measuring occlusal views of adult molars to obtain linear measurements and cusp areas.

By selecting samples on a 1:1 sex-matched basis for extant groups, and matching these against those for Proconsul, I minimised the potential that variations within the Proconsul material reflected sexual dimorphism within one species.

I have investigated the situation relevant to each of two clusters of primate fossil sites, respectively on Rusinga and Mfangano Islands (Area 1) and at mainland sites around Koru (Area 2).

The study has addressed a null hypothesis: that at each Area, only one Proconsul species was present from the relevant time-horizon.

Analyses confirmed that levels of variation within Proconsul, at each of Areas 1 and 2, outweighed those within extant hominoids. More than one species of Proconsul existed contemporaneously at each cluster of Miocene fossil sites.

DECLARATION

This is to certify that

(i) the thesis comprises only my original work towards the MPhil except where indicated in the preface. (ii) due acknowledgement has been made in the text to all other material used. (iii) the thesis is less than 50,000 words in length, exclusive of tables, maps and bibliographies.

……………………………………………….. 26 October 2015

PREFACE

The primary material utilised in this study has consisted exclusively of two-dimensional photographic images of primate teeth; this material has been subjected to appropriate measurement using a dedicated software package and the results obtained in turn submitted to statistical analysis.

The material was obtained by my supervisor, Dr. Varsha Pilbrow, from museum sources in 1999. Suitable acknowledgement of this primary source has been provided at appropriate points throughout the study which follows.

However, I can confirm that I performed all measurements undertaken on this material, employing an appropriate software package in order to do so. I checked the data set and then performed a sequence of statistical analyses, from which I was able to draw the conclusions identified later in this paper. To this extent, the work undertaken has been entirely my own.

No part of the work has been submitted for purposes of consideration for the award of any qualification other than the current application for the degree of Master of Philosophy.

No work of any kind, either utilising the primary source material, or of any other kind, had been undertaken prior to my enrolment for candidature.

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ACKNOWLEDGEMENTS

The genesis of this project was provided for me by Dr. Varsha Pilbrow, Lecturer in the Department of Anatomy and Neuroscience, University of Melbourne. I approached Varsha in 2012, asking if she would consider providing me with some suggestions whereby I might undertake a project within the field of physical anthropology. Although I had for many years possessed an amateur interest in this area of academic study, I had never previously had the opportunity to pursue such an interest at any professional level.

Varsha kindly agreed to take me on as a postgraduate student and was entirely responsible for the concept of the project which I have undertaken, namely an attempt to address the continuing debate regarding taxonomy within the primate genus Proconsul. It was her suggestion that I might attempt to address levels of variation within Proconsul dental material by comparing them with levels obtained from similar examples from extant taxa.

As has been indicated in the Preface, Varsha was able to furnish the source material which provided the basis for my project. She has been responsible for suggesting the programs of statistical analysis which were best suited to assess the results obtained from the data set; and she has supplied a critical eye throughout, particularly with regard to the conclusions which might be drawn from the entire process.

It goes without saying that I am grateful to her for her support throughout. Any deficiencies which afflict the final product are my responsibility entirely.

My gratitude goes also to Associate Professor Christopher Briggs, Department of Anatomy & Neuroscience, University of Melbourne. Chris has provided continuing moral support to me throughout the process.

The first draft of the thesis was assessed by two external examiners and I am grateful for the criticisms which they have provided, the result of which I trust is an improved study overall.

I am grateful to my brother, Phillip Weaver, for the assistance which he provided in helping to prepare my bivariate plots. His help was invaluable in making up for some of my deficiencies in word processing.

My thanks go to my wife, Pam, who has, as in so many other areas throughout the course of our partnership, provided her ongoing love and support. Pam encouraged me constantly

iv to persevere on many occasions when I felt as if the project was becoming too much for me. I can only trust that the finished project will go some way towards justifying the faith she showed in me.

TABLE OF CONTENTS

Abstract ...... i

Declaration...... ii

Preface ...... iii

Acknowledgements ...... iv

Table of Contents ...... vi

List of Tables ...... x

List of Figures ...... xii

INTRODUCTION 1

Proconsul: Issues of Taxonomy and Phylogeny ...... 1

This Study...... 2

CHAPTER ONE: HISTORICAL SURVEY AND LITERATURE REVIEW 4

Background ...... 4

Geochronology ...... 8

The island and mainland fossil sites...... 10

Songhor/Meswa Bridge ...... 12

Rusinga Island; The British-Kenya Miocene Expeditions 1948-50; further work at Songhor/Koru ...... 13

Collections on Rusinga Island: Site R106; Whitworth’s ‘pothole’ at R114; R5, the Kaswanga Primate Site ...... 14

Review of Earlier Findings ...... 16

Subsequent Issues: species numbers or sexual dimorphism? ...... 17

Updated Positions ...... 21

CHAPTER ONE: HISTORICAL SURVEY AND LITERATURE REVIEW (cont.)

The Alternative View: two species, rather than extreme sexual dimorphism ...... 21

Distinctiveness of Proconsul heseloni at Rusinga from Proconsul africanus at Koru ..... 22

Support from postcranial material ...... 24

Proconsul meswae ...... 25

Rangwapithecus ...... 25

Ugandapithecus ...... 26

The Initial Concept….………………………………………………………………………………………….. 26

Dissenting Views ...... 27

Further Insights ……………………………………………………….………………………………………. 27

Current Position: Recent Changes..………………………………………………………………………… …30

Species Recognition ...... 30

This Study...... 33

CHAPTER TWO: MATERIALS AND METHODS 36

Introduction ...... 36

The Material Used ...... 36

Extant Primates ...... 36

Selection of Material: Extant Groups ...... 38

Selection of Material: Proconsul ...... 41

Antimeres...... 43

Gap in Data Collection: Loss of M2 Proconsul africanus...... 43

Measurement: Protocols and SigmaScan Pro 5 Program ...... 44

Informal Checking ...... 47

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CHAPTER TWO: MATERIALS AND METHODS (cont.)

Data Screening ...... 47

Comparison between Intraobserver and Interobserver Error Rates ...... 48

Subsequent Analysis Program ...... 50

Coefficient of Variation ...... 51

Bivariate Analyses ...... 51

Independent T-tests ...... 52

CHAPTER THREE: RESULTS 53

Data Analysis ...... 53

Coefficients of Variation (CVs) ...... 53

Bivariate Plots ...... 56

Problematic Specimens ...... 61

Specimen KNM-RU 2088 ...... 64

Summary of Bivariate Plots for all Proconsul Specimens ...... 67

T-tests ...... 68

Specimen KNM-RU 2088 ...... 69

CHAPTER FOUR: DISCUSSION 70

Background ...... 70

Coefficients of Variation (CVs) ...... 71

Bivariate Plots ...... 72

T-tests ...... 73

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CHAPTER FOUR: DISCUSSION (cont.)

Area 1: Rusinga/Mfangano...... 73

(a) Two species present at Rusinga/Mfangano ...... 75

1. General Considerations ...... 75

2. Problematic Specimens (other than KNM-RU 2088) ...... 76

3. KNM-RU 2088 ...... 78

(b) Distinctiveness of P. heseloni from P .africanus ...... 79

Area 2: Separation of P. africanus from P. major ...... 81

Area 2: The current study ...... 84

Current Positions ...... 86

CHAPTER FIVE: CONCLUSIONS 87

Further Work ...... 91

REFERENCES 92

LIST OF TABLES

Table Description Page

Table 1.1 Dating of sites within eastern Africa including those from which hominoid specimens relevant to this study have been found (after various authors) ...... 10

Table 2.1 Extant hominoid study sample: museum sources. For all groups, specimens obtained on a 1:1 ratio, males:females...... 38

Table 2.2 Proconsul data set: Numbers of specimens assessed from each site. Area 1: Rusinga/Mfangano Area 2: Koru/Songhor/Chamtwara/Legetet ...... 41

Table 2.3 Summary statistics for measurements of actual and relative cusp base areas of Proconsul M2 for two observers (digitised images) ...... 49

Table 2.4 Summary statistics for measurements of mesiodistal and mesial buccolingual distances of Proconsul M2 for two observers (digitised images) ...... 49

Table 2.5 Summary statistics for measurements of mesiodistal and (single) buccolingual diameters of Proconsul M2 for two observers (caliper measurements vs. digitised images) ...... 50

Table 3.1 Coefficients of Variation of Total Cusp Base Areas (TCBAs) for all groups. Upper Molars ...... 54

Table 3.2 Coefficients of Variation of Total Cusp Base Areas (TCBAs) for all groups. Lower Molars ...... 54

Table 3.3 Coefficients of Variation of mesiodistal lengths (MD) for all groups. Upper Molars ...... 55

Table 3.4 Coefficients of Variation of mesiodistal lengths (MD) for all groups. Lower Molars ...... 55

Table 3.5 Coefficients of Variation of mesial buccolingual diameters (MesBL) for all groups. Upper Molars...... 55

Table 3.6 Coefficients of Variation of mesial buccolingual diameters (MesBL) for all groups. Lower Molars ...... 55

Table 3.7 Total cusp base areas for M2 and M3 of two specimens compared to ranges of TCBAs of all other specimens of P. heseloni and P. nyanzae at Area 1 ...... 61

Table. 3.8 Comparison of the total cusp base area, trigon and talon of KNM-RU 2088 with means and minimum-maximum ranges of all other P. heseloni and P. nyanzae M2 and M3 from data set ...... 64

LIST OF TABLES (cont.)

Table Description Page

Table 3.9 Summary of bivariate plots for KNM-RU 2088E & F, for all cusp areas relative to TCBAs. + denotes good grouping with P. heseloni, +/- slightly less satisfactory ..... 67

Table 3.10 Proconsul Upper Molars (all specimens): T-tests. (+ indicates p < 0.05) ...... 68

Table 3.11 Proconsul Lower Molars (all specimens): T-tests. (+ indicates p < 0.05) * no analysis possible, because no P. africanus specimens available ...... 69

Table 3.12 Results of T-tests for M2 and M3 of KNM-RU 2088, demonstrating attributions to P. heseloni and P. nyanzae respectively. + = p< 0.05 for each measurement...... 69

Table 5.1(a) Area 1: Hypodigms for Proconsul heseloni and Proconsul nyanzae as identified in this study………………………………………………………………………………………….89

Table 5.1(b) Area 2: Hypodigms for Proconsul africanus and Proconsul major as identified in this study………………………………………………………………………………………….90

LIST OF FIGURES

Figure Description Page

Fig. 1.1 General Map showing Kenya and adjacent regions within eastern Africa ...... 4

Fig. 1.2 Basic Geologic Time Chart...... 5

Fig. 1.3 Regional Map of western Kenya to show fossil sites relating to Area 1 and Area 2 ...... 6

Fig. 1.4 Miocene geology and fossil sites of western central Kenya ...... 7

Fig. 1.5 Rusinga Island: the Kaswanga region/R106/Whitworth’s ‘pothole’ ...... 15

Fig. 2.1 Occlusal view of single molar tooth ...... 37

Fig. 2.2 M 16647, the holotype of P. nyanzae ...... 37

Fig. 2.3a,b Proconsul specimens: examples of ‘acceptable’ and ‘unacceptable’ upper molars .... 43

Fig. 2.4 Gorilla upper molar showing linear dimensions ...... 45

Fig. 2.5 SigmaScan Pro 5 measurement of Proconsul upper molar trigon Specimen No M 14084 ...... 46

Fig. 2.6 SigmaScan Pro 5 measurement of Proconsul lower molar trigonid Specimen No KNM-RU 5871 ...... 46

Fig. 3.1 Bivariate plots for Proconsul M2 trigon and talon, Areas 1 and 2. Reference plots for Gorilla and Hylobates trigon to compare degree of species differentiation within extant genera ...... 58

Fig. 3.2 Bivariate plots for Proconsul M1 trigonid and talonid, Areas 1 and 2. Reference plots for Gorilla and Hylobates talonid to compare degree of species differentiation within extant genera ...... 59

Fig. 3.3 Bivariate plots for Proconsul: least successful plots attempting to demonstrate separation of species within an area. M 16647 is P. nyanzae holotype……………………60

Fig. 3.4a Bivariate plot to demonstrate ‘problematic’ specimens (Walker et al, 1993) M2 trigonids and talonids assessed against total cusp base area ...... 63

Fig. 3.4b Bivariate plot to demonstrate ‘problematic’ specimens (Walker et al, 1993)

M3 trigonids and talonids assessed against total cusp base area ...... 63

Fig. 3.5a Bivariate plots for M2 of specimen KNM-RU 2088E (marker 34): area of trigon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right) ...... 65

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LIST OF FIGURES (cont.)

Figure Description Page

Fig. 3.5b Bivariate plots for M2 of specimen KNM-RU 2088E (marker 34): area of talon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right) ...... 65

Fig. 3.5c Bivariate plots for M3 of specimen KNM-RU 2088F (marker 2): area of trigon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right) ...... 66

Fig. 3.5d Bivariate plots for M3 of specimen KNM-RU 2088F (marker 2): area of talon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right) ...... 66

Fig. 4.1 Comparison of occlusal surfaces of P. africanus (left column) and P. heseloni (right column), upper and lower molars. Total cusp base areas (TCBAs) given ...... 81

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INTRODUCTION

My study is concerned with an investigation into the taxonomy of the early Miocene primate genus Proconsul, specifically by addressing the proposition that levels of variation within the genus, as determined from an analysis of dentognathic material, can be used to identify the presence of sympatric species. A critical component of this study has been the concept of using the results of similar measurements obtained from dental specimens of four groups of extant primates to compare and contrast levels of variation between palaeontological and neontological taxa.

Proconsul: issues of taxonomy and phylogeny

Genus Proconsul has been recognised as comprising a number of taxa which were contained within the Miocene primate family Proconsulidae. (Leakey, 1963; Harrison, 1993, 2002; Cameron, 2004). Hopwood (1933), suggested that the hypodigm was consistent with that of a basal hominoid, ancestral to the common chimpanzee, Pan troglodytes.

Over succeeding decades, the accumulation of many further specimens from a number of fossiliferous sites within Kenya and adjacent regions of East Africa has resulted in the recognition that the genus is represented by several species, the precise number of which has been the focus of debate. Of further significance is the fact that the phylogenetic relationship of Proconsul to other primate groups remains a matter of contention, with some (Andrews, 1985, 1992; Martin, 1986; Walker & Teaford, 1989; Begun, 2007) supporting the argument for a basal hominoid, whilst others such as Harrison, (1987, 1993, 2002) believe that Proconsul is more like an ancestral catarrhine.

Fleagle (1986), noted that two types of catarrhines had been found in the numerous early Miocene localities of Kenya and Uganda, namely the more abundant and diverse apelike Proconsulidae and the less abundant and less diverse early cercopithecoid monkeys. He described how the early Miocene proconsulids were distinctly different in skeletal morphology from extant catarrhines in that they lacked the skeletal specialisations characteristic of either extant cercopithecoids or extant hominoids. Pilbeam (2002) noted that a concordance cladogram constructed by Andrews & Bernor (1999) suggests that most Miocene apes, including Proconsul, occupy part of a monophyletic group with extant large hominoids as sisters and with Hylobates more distant.

Whatever the particular viewpoint, there seems general agreement that the genus does not occupy a position directly ancestral to the later crown hominoids but instead represents an end-group which itself seems to have become extinct some time in the middle Miocene epoch.

Uncertainties about phylogenetic relationships do not form the focus of the current investigation, however; instead my study is concerned with an attempt to address an equally significant issue relating to Proconsul: namely, one regarding alpha taxonomy within the genus. From an early stage in the history of discovery of Proconsul remains in eastern Africa, it became apparent that diverse assemblages, partially with regard to overall morphological appearances, and particularly in terms of significant size disparities between specimens, were present at each cluster of fossiliferous sites being investigated. A debate progressively developed (Greenfield, 1972; Bosler, 1981; Kelley, 1986; Pickford, 1986) as to whether these disparate remnants, which were comprised mainly of jaw fragments and isolated teeth, represented the presence at a particular site, such as Rusinga Island, of a single species, which was itself highly dimorphic; or whether the contemporaneous presence of two or more species, exhibiting substantial morphological differences between them, was the more logical explanation.

These issues, concerning speciation within both extant and fossil groups, have been canvassed extensively in the multi-author work edited by Kimbel & Martin (1993). Reference is made particularly to Groves (1993: 109-121); Cope (1993: 211-237); Plavcan (1993: 239- 263); Harrison (1993: 345-371); Teaford et al (1993: 373-392); Martin & Andrews (1993: 393-427); Kelley (1993: 429-458).

This study

My focus from the outset has therefore been upon this issue of alpha taxonomy and specifically whether levels of variation within the fossil samples might be deduced from undertaking a similar assessment of variation levels within extant taxa, before then looking for correlations or differences between the palaeontological and extant forms. The application of appropriate forms of statistical analysis, in the form of univariate and bivariate analyses, has then been performed to test a null hypothesis, namely: that there has only been one Proconsul species present at each of two separate clusters of fossil sites within central western Kenya.

In particular, given the debate already foreshadowed, as to whether extreme degrees of sexual dimorphism are being demonstrated by the samples from the fossil record, the current investigation has been undertaken with a particular emphasis upon ensuring that the neontological material has been selected on a sex-matched basis. This allows for the concept that levels of variation exhibited within the extant material can be discounted as being due to bias caused by any imbalance between males and females ; and from this that useful conclusions might be drawn regarding the interpretation of variation levels observed within the fossil material.

More substantial discussion will be provided regarding the specifics as to how the material for study was selected, but the point should be made at the outset that all of the material examined, and from which the data set was derived, consisted of two-dimensional photographs of dental material which had been accessed from various museum collections. Even more specifically, the photographs comprised occlusal views of the adult molar teeth of both fossil and extant forms. From these occlusal views, precise measurements of linear dimensions and cusp areas have been recorded.

Although by the present date substantial amounts of other cranial and postcranial material referable to Proconsul have been collected and are represented within museum collections, dentognathic specimens predominate. I have had no access to anything other than the photographed dental material described in the preceding paragraph. My thesis and its conclusions are based totally upon its evidence.

However, purely to round out the historical record as it relates to Proconsul, brief mention will be provided in the ensuing section regarding major events which have occurred leading to the recovery of material other than the dental remains.

CHAPTER ONE: HISTORICAL SURVEY AND LITERATURE REVIEW

Background

Fossilised material relating to a number of species identified as belonging to genus Proconsul has been obtained from a series of Miocene-epoch sites contained within the east African region, mainly Kenya and Uganda, with some additional material from nearby southern Ethiopia (Figs. 1.1 and 1.3 ).

Fig.1.1 General Map showing Kenya and adjacent regions within eastern Africa. Reproduced with permission from Lonely Planet © Lonely Planet 2011.

Millions of Eon Era Period Epoch Years B.P.

Holocene 0.0117

Pleistocene

Quaternary 2.588

Pliocene 5.332

Miocene Neogene

23.03 Cenozoic Phanerozoic Oligocene 33.9 ± 0.1

Eocene

55.8 ± 0.2

Paleogene Paleocene

65.5 ± 0.3

Fig. 1.2 Basic Geologic Time Chart (not to scale). Miocene epoch lasted c. 23 Ma to 5 Ma. Data obtained from internet site of Department of Geosciences, Idaho State University. (www.geology.isu.edu).

The Miocene epoch (Fig. 1.2) lasted for a time interval from approximately 23 million years ago (23Ma) to as recently as 5 million years ago (5 Ma) and, as it relates to the east African region, is significant for the presence of a large number of primate taxa including those ascribed to genus Proconsul.

Fig. 1.3 Regional Map of western Kenya to show fossil sites relating to Area 1 (Rusinga/ Mfangano) and Area 2 (Koru, Songhor, Meswa, Legetet) as discussed in this study.

After Andrews (1981)

The fossiliferous sites (Fig. 1.3) from which the material forming the basis of the current data set has been drawn have been investigated extensively, in terms both of their geochronology and of the extinct primate material which has been retrieved from them (Bishop et al, 1969; Van Couvering & Miller, 1969; Andrews, 1978; Bosler, 1981; Harrison, 1986). Drake et al (1988) have discussed the proposition that the material retrieved has comprised a highly diverse group which includes the earliest true hominoids and also probably the ancestral cercopithecoids. They note that, apart from Proconsul, these sites have yielded evidence of numerous other primate species, including Limnopithecus legetet and Xenopithecus koruensis, each of which was reported by Hopwood (1933) at the same time that he was describing Proconsul.

Fig. 1.3 (Andrews, 1981) and Fig. 1.4 (Drake et al, 1988) illustrate the presence of what have been described as a group of paravolcanic complexes dating from the early to middle

Fig. 1.4 Miocene geology and fossil sites of western central Kenya. Fossil-bearing Lower and Middle Miocene deposits, between c. 23 and 15 Ma in age are indicated in solid black.

After Drake et al (1988)

Miocene epoch, relating respectively to the islands situated in the eastern arm of Lake Victoria and to a cluster of sites situated on the Kenyan mainland, approximately 150 km to the northeast. One complex, noted collectively as Kisingiri (or Rangwa) incorporates Rusinga and Mfangano (previously Mfwangano) Islands (from each of which Proconsul material has been retrieved) and a site at Karungu on the adjacent mainland. Maboko Island, situated approximately 70 km to the northeast of Kisingiri has a separate volcanic history. A group of mainland sites situated in the Nyanza Rift region, collectively referred to the volcanic region of Tinderet (or Timboroa), include the localities of Koru, Songhor, Chamtwara, Legetet and Meswa. The last of these, Meswa, has been described (Harrison & Andrews, 2009) as exhibiting some of the oldest material referable to Proconsul.

Napak, a site in eastern Uganda, has yielded additional material referable to Proconsul, as well as to other primate taxa, but I should note here that none of this material has been available to me for this present study.

The Proconsul photographic material which constitutes the basis for this current study relates respectively to the ‘island’ sites at Rusinga/Mfangano and to the ‘mainland’ complex which includes Koru and Songhor; these are denoted respectively ‘Area 1’ and ‘Area 2’ in this study.

Geochronology

The geochronology of the east African Miocene fossil sites has been investigated extensively over a considerable period (Bishop, Miller & Fitch, 1969; Van Couvering & Miller, 1969; Pickford, 1986c; Drake et al, 1988; Harrison, 1992). Of particular significance for this study is the assessment (Drake et al, 1988) that the sites at Tinderet date from as far back as c. 19.6 Ma, whilst those at Kisingiri are somewhat younger, dating from perhaps a mean 17.8 Ma with a sequence of depositions which might have occurred over a period of no more than 0.5 million years.

Pickford (1981, 1986c) discussed the biostratigraphy of the west Kenyan regions in terms of a sequence of seven distinct Faunal Sets, based essentially upon succession over time. He made the point that the assemblages at Meswa Bridge did not fit convincingly with those from Koru and Songhor and described Meswa Bridge in terms of Pre-Set 1. The bulk of the Miocene fossil sites at Koru and Songhor were placed by him in Faunal Set 1 and, although he recognised some differences between Koru and Songhor, nevertheless regarded the geological mapping to indicate contemporaneity of deposition. He suggested that differences between them, especially amongst gastropod and primate fossils, were probably referable to differences in the palaeo-climate and hence palaeo-vegetation.

He described Faunal Set 11, found at Rusinga, Mfangano, Karungu and Uyoma, as differing from Set 1 importantly in terms of the ruminant, suid, rodent and primate assemblages. For the latter, he noted specifically that Micropithecus clarki, M. songhorensis, Limnopithecus evansi and Proconsul major were all prevalent at Koru and Songhor but had not been found at Rusinga.

Pickford (1986c) provided a biochronological representation of all the west Kenyan mammalian assemblages across a range of nine distinct regions; as already noted, Sets 1 and 11 were represented respectively at Koru/Songhor and Rusinga. This interpretation was essentially accepted by Werdelin (2010).

Drake et al (1988) noted that the Kisingiri sequence included fossil beds at Rusinga, Mfangano, Karungu and Uyoma. The sequences at Rusinga included the older Wayando

Formation, itself contemporaneous with Beds 14-31 at mainland Karungu; whilst later sequences at Rusinga included respectively the Kiahera Formation, Rusinga Agglomerates, Hiwegi Formation and Kulu Formation. They tabulated (1988: 484) the fact that specimens of P. nyanzae had been recovered from across the entire sequence at Rusinga, whilst ‘P. africanus’ (P. heseloni, sp. nov., Walker et al, 1993) had to that point been detected only within the middle sequences, from the Kiahera Formation, Rusinga Agglomerates and Hiwegi Formation.

In their discussion concerning the paleontology of these regions, Drake et al (1988) also noted that Pickford (1982) had described in the Early Miocene faunas from West Kenya sites these ‘faunal sets’ which had been identified by the degree of overlap of taxa in common. Set 1 had been best represented at Songhor in the upper fossiliferous levels of the Timboroa sequence, whilst Set 11 was best represented in the fauna from the Hiwegi Formation of Rusinga. Van Couvering & Van Couvering (1976) had described differences between the faunal groups of Set 1 and Set 11, as had Andrews (1978a). Drake et al (1988: 490) went on to note that ‘(m)ainly because of the confusion over Rusinga dating, such differences were previously attributed to geographical and environmental factors. It now seems likely that the faunal differences are time-sequential and represent changes over a significant period of time.’

Cameron (1991) discussed this approximately 2 Ma temporal separation between the island and mainland sites in terms of its significance for the debate between proponents of the single species hypothesis on the one hand (Kelley, 1986; Kelley & Pilbeam, 1986; Pickford, 1986) and advocates for the dual species hypothesis on the other (Hopwood, 1933; MacInnes, 1943; Le Gros Clark & Leakey, 1951; Bosler, 1981; Teaford, 1988; Ruff et al, 1989).

Table 1.1 provides a summary of the dating schema, based on radiometric dates, which have been provided by various authors for the Miocene sites within eastern Africa from which primate fossils have been found:

Author/Date Locality and Age of Site (Ma) Koru/ Rusinga/ Napak Karungu Fort Ternan Meswa Moroto Songhor Mfangano Bishop et al 19.5± 0.3- 23.2± 0.6- 17.8± 0.4- 15.9± 1.5 – 22.5± 0.4 c.14.0-14.7 12.5± 0.3 1969 19.9± 0.6 19.0± 2.0 19.3± 0.3 25.6± 0.6

Van Couvering c.18.0 & Miller 1969 Pickford 1981 17.5~18.0

Pickford & 19.5-19.7 14.0 22-23 Andrews 1981 Pickford 1986c 19.0-22.5 17.5-18.0 14.0 Drake et al 17.9± 0.5 17.5± 0.06 1988 Gebo et al 1997 20.61±0.05 Pickford et al 17.5 2003

Gradstein et al 19.5±0.4 19.6±0.2 17.2±0.5 17.2±0.5 13.6±0.2 22.2±0.3 20.43±0.3 2004 Pickford et al 13.7±0.3 2006 Harrison 2010 ~19-20 ~17.0-18.5 ~22.5 ~17.0-18.0 Werdelin 2010 19-20 19-20 15.4-18.2 c16.8-18.0 13.6-14.2 21.8-22.8 19.7-21.0

Table 1.1 Dating of sites within eastern Africa including those from which hominoid specimens relevant to this study have been found (after various authors).

The island and mainland fossil sites

Miocene mammalian fossils were first identified in the Karungu region, near the shore of Lake Victoria, Kenya, by G.R. Chesnaye in 1909 (Andrews, 1981). The attempts which were made to determine the palaeontology and stratigraphy of the region have subsequently been described (Andrews, 1981; Walker & Teaford, 1989; Walker, 1991). Although no primate material was found initially at the Karungu site itself, Pickford (1986b:29) has confirmed that it subsequently yielded what he describes as ‘a limited primate fauna’ comprising two taxa based upon two specimens, Dendropithecus macinnesi and Proconsul nyanzae.

In 1927, the first primate fossil was recovered from a site at Maize Crib Quarry in the Koru region, approximately 150 km northeast of Karungu; this specimen comprised a partial left maxillary fragment, C-M3. Hopwood (1933) identified it as material belonging to a member of the family Simiidae, according to then-current nomenclature, and suggested its

10 possible ancestry to the common chimpanzee, Pan troglodytes; indeed it was on the basis of this resemblance that he whimsically named this holotype specimen Proconsul africanus, because up to that time a number of chimpanzees held in zoo collections, notably at Belle Vue Zoo in Manchester, and also in Paris and London, had individually been named ‘Consul’. He believed it to have been a male specimen, based upon size of the canine.

Hopwood (1933) provided the first detailed description of the material, noting that, with regard to the posterior teeth, the first and second upper molars were approximately rhomboidal in outline, the second being larger than the first, whilst the third was considerably reduced. There were substantial cingula, almost encircling the molar circumferences, again more marked in the second molar. The trigons of the first and second molars were distinctly marked; on the first molar there were strong ridges connecting the protocone, paracone and metacone, whilst on the second molar these ridges were not as distinct. There was less marked wrinkling of enamel on the first and third molars compared with the second.

This original specimen, the holotype of P. africanus, was subsequently lodged with the Natural History Museum, London, catalogue number M 14084.

In addition to this holotype specimen of Proconsul, Hopwood described a number of other primate species, notably Limnopithecus legetet and Xeropithecus koruensis, but believed that these species, more gibbon-like in body mass, were not directly related to Proconsul itself. The diagnosis of X. koruensis was based upon a poorly-preserved partial maxilla, M1- M2, catalogue no. M 14081, Natural History Museum, London.

Interest in the Koru region was revived at intervals over subsequent decades. MacInnes (1943) described work which had been undertaken in a series of East African Archaeological Expeditions over the period 1932-42; his paper described further specimens, again ascribed to P. africanus, which had been recovered not only from Koru but also from sites on Rusinga Island. In his description of these additional specimens of P. africanus, MacInnes noted also that there was a size differential of the order M1< M2 > M3 with substantial ridging between the trigon cusps, particularly on M1. MacInnes’ paper questioned the earlier suggestion of Hopwood that Proconsul represented a direct ancestor to the chimpanzee.

MacInnes redefined the specimen of X. koruensis as possibly being a remnant of the earlier taxon Propliopithecus. Andrews (1978a) has however included M 14081 within P. africanus.

MacInnes also raised the concept that the disparate sizes of the various Proconsul specimens described possibly represented the presence of both male and female individuals; because some of this material was larger than the holotype M 14084, he thought that the latter might in fact have been a female.

MacInnes’ doubts regarding the likelihood that Proconsul represented a direct ancestor of Pan was endorsed by Leakey (1943); in reporting upon what he described as an anthropoid mandible retrieved from Rusinga Island the previous year, and which he assigned to Proconsul on the basis of its similarity to Hopwood’s material, Leakey (1943: 320) agreed that Proconsul ‘stands near to the common ape-man stem’.

Songhor/Meswa Bridge

Miocene deposits had also been identified at Songhor, a site approximately 20 km NNW of Koru; these had been first discovered by L.S.B. Leakey and D.G. MacInnes and in a succession of expeditions over subsequent years proved to yield the richest collection of all the Tinderet sites. Songhor was investigated particularly in 1948 during the first of the British-Kenya Miocene Expeditions (see below) and then again in 1966 (Andrews, 1981).

Initially, the Songhor and Koru sites were thought to be of lacustrine origin, following Kent (1944); he had considered that these mainland sites, together with the assemblages up to 160 km to the west, at Rusinga, Mfangano and Karungu, represented the remnants of a single large lake. This was perhaps not surprising, given their proximity to the modern-day Lake Victoria, but had the effect of causing for some time the assumption that the entire local formations were essentially due to a single depositional event during the Miocene. Andrews (1981: 6) reports that it was Bishop (1968) who first showed that the “limestone” deposits so prevalent throughout the region actually represented volcanic tuffs and that the high concentrations of lime derived from carbonatite volcanoes. Bishop (1968) went on to identify four fossiliferous members in the Songhor deposits and by 1972 Andrews had begun to subdivide the main fossil member at Songhor into ten beds (Andrews, 1981). Andrews identified the richest fossiliferous site as Bed 5 and noted further that Leakey’s expeditions in 1948 and 1966 had taken place mainly in beds 5 and 6.

Pickford & Andrews (1981) provide a comprehensive account of the stratigraphy of the entire region of Koru-Songhor and associated localities, including a discussion of the site at Meswa Bridge, investigated somewhat later during expeditions in 1979/80 and dated amongst the oldest in the western Kenyan deposits, at a minimum of 20 Ma. This site yielded substantial collections of mammalian fossils including the catarrhine material subsequently reported (Harrison & Andrews, 2009) as belonging to P. meswae.

Rusinga Island; The British-Kenya Miocene Expeditions 1948-50; further work at Songhor/Koru

The Rusinga Miocene fossil beds were studied during Leakey’s third East African Archaeological Expedition in 1931-32 and again in his fourth expedition in 1934-35 (Leakey, 1943; Pilbeam, 1969). It was during September 1942 that Leakey made a further visit and retrieved the mandible previously mentioned.

Over the period from 1948-50, a series of excavations took place in western Kenya under the auspices of the British-Kenya Miocene Expeditions; these were subsequently reported upon in monograph form (Le Gros Clark & Leakey, 1951). The work involved recovery of fossil primate material both from the mainland sites and also from Rusinga Island, the latter of which had been visited by Louis Leakey as early as 1931. Le Gros Clark & Leakey also reassessed earlier work, including that of MacInnes (1943).

Le Gros Clark & Leakey (1951) reviewed the holotype of P. africanus, M 14084, and suggested that its morphology was indicative of an animal possibly intermediate in size between a large gibbon and a small chimpanzee. They again noted the molar size disparities, with M3 substantially reduced. The main cusps of the trigon were approximately equal in size and the hypocone approximated the area of the protocone. The lingual cingulum was noted to be well-developed.

Importantly, they also described two other specimens, which came to be accepted at the time as holotypes for two potential new species. The first of these was another maxillary specimen, designated M 16647, (Natural History Museum, London), which had initially been retrieved from a deposit at Rusinga Island and described by MacInnes (1943). The teeth had been preserved bilaterally, from C-M3. This was attributed to P. nyanzae, a primate considered to approximate the body mass of a chimpanzee. Le Gros Clark and Leakey noted the presence of a beaded lingual cingulum on the upper molars, with a reduced mesial cingulum and well-developed distal cingulum. The hypocone was smaller than the

13 protocone. The M3 was not as diminished as had been the case with P. africanus. They also described mandibular remnants, mostly fragmentary, including an immature specimen which they also attributed to P. nyanzae (subsequently catalogued as KNM-RU 710).

The third species was represented by a right mandibular fragment, catalogue number M 16648, retrieved from Songhor. The mandible was described as massively constructed, with lower teeth which were similar in proportions and cusp patterns to those which had been identified for P. nyanzae, but which were considerably larger. M1 was reported to be approximately rectangular in shape, but broader in its distal half. The entoconid was suggested to be larger than for the corresponding tooth in P. nyanzae. The presence of a small buccal cingulum was noted, between the protoconid and hypoconid. M2 was partly broken, but again a similarity with the overall shape of P. nyanzae M2 was suggested. M3 was described as notable for its length and for the relative narrowness of the talonid.

This third species was described as approximating in size to a gorilla and was given the name P. major.

Le Gros Clark and Leakey (1951), in reporting their findings, were seemingly accepting the concept that the smallest of the three species, P. africanus, had been present at both the island and mainland sites. They had also suggested that, whilst the holotype of P. nyanzae had been retrieved at Rusinga, a mandible which had been been found at Koru, and previously described as P. africanus by Hopwood (1933) should also be referred to P. nyanzae. They therefore seem to have been advocating the presence of the two smaller species at both the island and mainland sites.

Collections on Rusinga Island: Site R106; Whitworth’s ‘pothole’ at R114; R5, the Kaswanga Primate Site

During the period covered by the British-Kenya Miocene Expeditions, numerous finds attributed to Proconsul, again in the form mainly of dental material, began to be recovered from sites on Rusinga Island. (Fig 1.5, overleaf). An area on the northwest of the island, at Kaswanga (or Kinua) Point provided much of the early material and this general locality has been worked over on many subsequent occasions.

Fig. 1.5

Rusinga Island, showing the Kaswanga region; the site marked R106 from which specimen KNM-RU 7290 had been recovered; and R114, Whitworth’s ‘pothole’ site.

After Walker & Teaford, 1989

Slightly south of this locality, another site, R106, was the area from which Mary Leakey in 1948 retrieved the first elements of the partial cranium which initially acquired the Natural History Museum, London field number R. 1948,50 prior to its subsequent repatriation to Kenya in 1981; henceforth it has been catalogued as KNM-RU 7290. Walker & Teaford (1989) and Walker & Shipman (2005) have described how additional elements belonging to this specimen were subsequently discovered from material which had been misidentified as turtle scutes and which had been misplaced within the collections of the National Museums of Kenya, Nairobi.

Whitworth (1953) reported his findings at site R114, at the western arm of Rusinga Island. This was initially identified as a ‘pothole’ into which it was assumed that material relating to numerous species, including Proconsul, had fallen after having been subjected to predation; and it was from this site that the first forelimb material attributable to Proconsul was identified. This material was extensively discussed by Napier and Davis (1959). Some ankle and foot bones, subsequently ascribed to the same individual, had been found at the same time as the upper limb material, but were initially misidentified as suid bones (Walker & Pickford, 1983; Walker & Teaford, 1989).

Over the period commencing 1984 and during the following field season in 1985, joint expeditions conducted by the National Museums of Kenya-Johns Hopkins University began to survey a new site in the Kaswanga region. This site was situated 3 km east of R114 and was subsequently given the formal designation R5; it is not shown specifically on Figure 1.5, but lies near Kaswanga Point. Less formally, it has become known subsequently as the Kaswanga Primate Site and has been responsible for yielding numerous Proconsul fossils, mainly postcranial material, which appear to be the remnants of at least nine individuals, both adults and juveniles (Walker & Teaford, 1988, 1989).

Review of Earlier Findings

Leakey (1963) proposed the erection of a new family, Proconsulidae, which would include genus Proconsul as well as some specimens of Dryopithecus and Sivapithecus (Tuttle, 2006).

Simons & Pilbeam (1965) concurred with the concept of three taxa as postulated originally by Le Gros Clark & Leakey (1951) but suggested that the entire genus Proconsul represented a subgroup within the European genus Dryopithecus; they did so on the basis of arguing that the range of character differences noted between Proconsul on the one hand and the previously-recognised dryopithecines on the other were no greater than character distinctions noted between individual specimens from within extant genera such as Pan and Gorilla. Accordingly, taxa within Proconsul were reported for some years in subgeneric terms as Dryopithecus (Proconsul) until Andrews (1978a,b) revised the Miocene hominoids and concluded by restoring Proconsul to full generic status.

Numerous additional specimens attributable to all three putative species were recovered over subsequent years and Pilbeam (1969) addressed the concept that all three were necessarily ubiquitous throughout both the Kisingiri and Tinderet sites. He reassigned D.(P.) nyanzae mandibles from Songhor and Koru as females of D.(P.) major and indeed questioned the presence of P. nyanzae at these sites altogether. On the other hand, he questioned the presence of P. major at Rusinga. Pilbeam took into account some large specimens, from Napak and Moroto in Uganda, which had been assigned to P. major; he accepted this attribution and, by comparing them with the specimens from Koru/Songhor, (including the reassigned P. major females), concluded that P. major was a markedly sexually dimorphic species.

Greenfield (1972) considered issues of potential speciation within Proconsul, based upon variations in size between particular specimens, and attempted to consider whether these size differences might more readily be attributed to evidence for sexual dimorphism within a single species. He was concerned particularly by the absence from amongst the material recovered from various sites, apart from the holotype M 14084, of any P. africanus specimens which could be confidently identified as males (see discussion below).

Andrews (1978a) noted that Proconsul (Proconsul) africanus was the most distinctive of the species assigned to the subgenus (as it was then described). He noted again the larger size of M2 as compared with M1, with M3 strongly reduced. These features tended to

16 distinguish it from other potential Proconsul species, although not so much from Limnopithecus legetet; the latter, he suggested, was differentiated in turn by its overall smaller size, elongated P4s and rounded cusps.

He noted that the distribution pattern of P. africanus was very similar to that of P. nyanzae and that the similarity might make it difficult to separate the two species. In this context he also addressed the question raised by Greenfield (1972) regarding the potential ‘absence’ of male P. africanus (as it was then described) from the Rusinga material but concluded that the specimens being disputed by Greenfield should more probably be placed within P. nyanzae.

Andrews further noted (1978a) that P. major differed from P. nyanzae mainly in size, but that differences in the molar dentition were apparent: although the first molars in both jaws were reduced by comparison with the second, there was a difference with the third molars: the M3 was relatively more elongated in P. major and the distal cusps were relatively atrophied, whilst in P. nyanzae it was a broad rectangular tooth with no reduction of cusps. Similarly, M3 was relatively large in P. nyanzae and reduced in P. major.

He referred to the proposition that the larger specimens were all found at Songhor and Napak, while the smaller ones came from this site and from Rusinga Island; and also to the seeming ambiguity that no obvious males had been identified from the Rusinga material.

Andrews (1978b) concluded that there were three species of Pronconsul and that Rangwapithecus should probably be afforded subgeneric status within Proconsul.

Subsequent Issues: species numbers or sexual dimorphism?

Greenfield (1972) had drawn attention to the fact that, although attempts had been made to assign particular specimens either to Dryopithecus africanus or Dryopithecus nyanzae (in the terminology then in use), there had been no attempt made to try to define sexual dimorphism within the species. He was concerned to note that only one specimen, the holotype D.(P.) africanus, M 14084, had been identified as a male, largely on account of its canine size. He looked at samples from both Rusinga and Songhor and considered several criteria in attempting to consider sexual dimorphism. These included issues of summed posterior areas (approximating to the molar occlusal areas) of both maxillae and mandibles, canine size and mandibular robusticity of a number of specimens which had previously been attributed to D.(P.) nyanzae; he suggested that these required to be reattributed as males of D.(P.) africanus. He confirmed a point which had been observed by Le Gros Clark & Leakey

(1951) that, whilst the first and second molars of D.(P.) africanus were approximately equal in size, in D.(P.) nyanzae the first molars were markedly smaller than the second.

He suggested that the issue of sexual variation within current primate species required to be considered and that models derived from this assessment might then be applied to fossil primates.

Bosler (1981) considered the issue of potential species within Proconsul by re-examining and grouping teeth on morphological grounds, taking no account of the sites from which individual specimens had been recovered. She assessed both isolated teeth and also more complete specimens and was able to provide information concerning all tooth-types. Relying for her nomenclature upon that first suggested by Andrews (1978a) she noted that

the morphotypes of molars M1 and M2 from mandibular specimen KNM- SO 1112, which had previously been defined as Rangwapithecus gordoni, another species altogether, were broad and less elongated, whilst M3 was triangular; on this basis, she reassigned the specimen to P. africanus.

Bosler (1981) went on to identify six groups of teeth, Groups 1 to 4 of which corresponded respectively with taxa which she described as P. africanus, P. nyanzae, a small-sized P. major and P. major as previously-recognised. Her other groupings related to the less well-defined Rangwapithecus.

Martin (1981), in discussing new material obtained at Koru, and reporting in the same volume as Bosler, described in particular detail specimen LG 452 from locality 14 at Koru; he referred to the fact that the overall morphotype was consistent with that of either a large P. nyanzae or of a small P. major; however, he noted that the combination of a very robust mandible and posterior dentition, in combination with a fairly small canine, was inconsistent with P. nyanzae and assigned the specimen instead to P. major. His consideration of the possible presence of P. nyanzae at Koru seems to reflect that of Le Gros Clark and Leakey (1951).

Martin (1981) went on to revise the diagnosis of P. major and suggested that the species exhibited considerable size dimorphism; however, he also suggested that there was considerable metrical overlap with P. nyanzae, particularly in the anterior dentition.

Pickford (1986a) and Kelley (1986) then independently raised concerns regarding the level of sexual dimorphism within Pronconsul. Pickford noted the attributions which had been made previously by Greenfield (1972) and by Bosler (1981); he addressed issues of

18 monomorphism and dimorphism within primate species in general and considered matters of variability within both extinct and extant species.

Pickford observed that Andrews (1978a) had regarded P. nyanzae as being strongly sexually dimorphic, especially with regard to the canines whilst Bosler (1981) had suggested that all P. nyanzae canines were male in morphology, but that the P. africanus individuals appeared to be females with the exception of the holotype M 14084.

With regard to the molars, particularly as applied to individual tooth samples, Pickford noted that assignation between P. africanus and P. nyanzae had depended upon the particular author’s viewpoint and that there had frequently been difficulty in defining the position of a particular molar within the tooth row. He remarked upon the fact that the occlusal crests of the Tinderet molars were better defined than those from Rusinga and that the M3 mesiodistal lengths at Tinderet were shorter. Pickford went on to remark not only that the Rusinga specimens of P. africanus were different from those at Tinderet, but that the former had all been identified as females. Given that the P. nyanzae specimens at Rusinga had been classified as males, his conclusion was that, at Rusinga, there were not two separate species present, but rather one highly sexually dimorphic one. He argued that this was P. nyanzae.

At Tinderet, he noted that the P. africanus samples, although small in number, exhibited canine size differences which, when compared with canines from extant species, suggested the presence of larger males and smaller females. The upper and lower first molars clustered closer together. Again, Pickford felt able to argue for the presence of a highly sexually dimorphic species at Tinderet; given that the holotype of P. africanus had been discovered at Koru, this species retained that name.

With regard to P. major at the mainland sites, he noted that bivariate plots of the upper and lower canines and of the upper and lower first molars suggested once again considerable variation within the canines, consistent with the dimorphism present within extant species. Pickford argued for a highly sexually dimorphic P. major.

Kelley’s study (1986) was of similar character and rested upon issues of incisors, canine and postcanine morphology. He commenced his assessment by noting that, whilst previous studies regarding Proconsul taxonomies had concentrated more on the postcanine teeth, and whilst he himself would be giving some consideration to those issues, he was concerned primarily with the anterior dentition and even more specifically the canines. He described

19 how he had initially addressed canine morphs at each of the fossil sites and had attempted to group these in terms of sex-based morphologies. Only subsequent to this did he then attempt to allot particular specimens to a specific taxon.

By so doing, he felt able to argue that, for the Proconsul canines observed at Rusinga, the pattern of variation was such that the specimens exhibiting smaller canines, and which had previously (Le Gros Clark, 1951; Andrews, 1978a; Bosler, 1981) been attributed to a small species, P. africanus, in fact represented females of the seemingly- sympatric larger species, P. nyanzae. With the exception of a small set of canines which he believed were those of a completely distinct and unrelated small-bodied hominoid present also at Rusinga, he suggested that all the Rusinga canines otherwise represented larger male and smaller female members of P. nyanzae. In other words, he reassigned P. africanus at Rusinga as females of P. nyanzae.

Kelley considered ranges of variation within each of extant cercopithecoid and hominoid species as against those for the fossils, noting that the range of canine dimorphism was variable even amongst extant species. Coefficients of Variation (CVs) for crown maximum length and breadth were calculated for the fossil samples now reassigned to P. nyanzae (Groups 1: males and 2: females in his study) with results such that he felt able to argue that variability within each group separately was what one would expect for single-sex samples based on extant hominoid values. If each represented a pooled sample, males and females, within a single species, then the variability was similar to that shown in non-sexually dimorphic gibbons and less than that of great apes. If Groups 1 and 2 were combined, the results matched those of a combined- sex sample of Gorilla. In other words, he felt able to claim support for the proposition that Proconsul at Rusinga was represented by a single sexually-dimorphic species.

He then proceeded to look at the issue of metric variability in the postcanine dentition, this time looking at CVs within both P. nyanzae and P. major for the parameter of buccolingual breadth; again these were matched against a number of extant hominoid and cercopithecoid species. He did have to concede that the results obtained in this instance did seem to exceed variability levels within extant taxa, but had two arguments to counter this finding. He again made the observation that he had not used postcanine metric variability as the criterion for sorting specimens by sex in the first place. Second, he went on to make the observation, pertinent to the other authorities about to be discussed, as to whether the

20 maximum variability observed within extant catarrhines necessarily constitutes an absolute upper limit upon variability within a fossil species, anyway.

As a corollary to his argument for the presence of just one Proconsul species at Rusinga, Kelley did discuss the issue of temporal relationships, noting that, whilst the sites at Koru and Songhor dated from ~19.0-19.5 Ma, the Hiwegi formation at Rusinga is dated at ~17.5Ma. He suggested that P. major and P. africanus (Koru) might not overlap in time with P. nyanzae and that, given the overall similar morphologies for P. africanus and P. nyanzae, there might have been an ancestor-descendent relationship between them.

Updated Positions

I think that it is pertinent at this point to indicate that both Pickford and Kelley have substantially modified their positions concerning speciation within Proconsul over the period since the middle 1980s.

Given the substantial period which has elapsed since the debate concerning numbers of Proconsul species, as opposed to issues of sexual dimorphism, developed in the 1980s, some of the protagonists have modified their views with the passage of time. This point requires to be noted, merely in order to bring the issues raised in this thesis up to the present and to summarise attitudes which are currently held.

I have already noted at length the earlier views of Pickford (1986a) and Kelley (1986). However, by the turn of the millennium (Senut et al, 2000), Pickford had clearly come to accept the concept that two species of Proconsul (P. heseloni, P. nyanzae) had existed contemporaneously at Rusinga; these views were expanded upon substantially (Pickford et al, 2009) as part of the discussion in which an attempt was being made to differentiate Proconsul from Ugandapithecus.

Harrison (2010), although not a protagonist in the initial debate pitting species numbers versus sexual dimorphism, clearly lists P. heseloni and P. nyanzae as separate taxa, albeit making the point that P. nyanzae is morphologically very similar to P. heseloni, differing primarily in its larger size.

The Alternative View: two species, rather than extreme sexual dimorphism

The alternative view from that of Pickford and Kelley was expressed by Teaford et al (1988) in their analysis of a new find, a partial facial skeleton recovered at Rusinga in 1985 and catalogued as KNM-RU 16000. In their analysis of the dentition, they observed that the

21 canines were not only larger than those of any Rusinga specimen of P. africanus recovered up to that time, but also that they were larger than those of any specimen of P. nyanzae other than the holotype. This was despite the fact that the specimen exhibited a fairly small palate and that its posterior teeth were intermediate in size between P. africanus and P. nyanzae.

As regards the molars, M1 was smaller than M2, which was in turn almost matched in size by M3 ; M1 was in the upper range for area usually observed in P. africanus, whilst conversely M2 was within the lower range for P. nyanzae. Of particular note was that M3 did not exhibit the greatly-reduced, almost triangular outline which had frequently been ascribed to P. africanus; by the same token, however, it was small by P. nyanzae standards. The M1 cusps were almost equal-sized, whilst the trigon was less developed than in P. africanus, the talon better marked than in either species. M2 exhibited a marked lingual cingulum which merged with the hypocone to extend the distolingual margin of the tooth. M3 showed a large protocone and lingual cingulum, accounting for the relatively large occlusal surface (certainly compared with P. africanus), but a very reduced hypocone.

Teaford et al (1988) felt constrained to make the choice between accepting KNM-RU 16000 as either: a male of P. nyanzae, but with an unusual combination of large canines and relatively small postcanine dentition; an example of the ‘missing’ males of the smaller species at Rusinga, which so concerned Bosler (1981), i.e. a presumptive male P. africanus; or an example of a P. nyanzae male as defined by either Pickford (1986a) or Kelley (1986). In the latter instance, concerns were again going to be raised regarding the degree of sexual dimorphism being exhibited by P. nyanzae at Rusinga, particularly when consideration was made of elements other than dental samples in isolation (for instance, projections of body mass based upon postcranial material, as per McHenry, (1986)).

Whilst acknowledging the unusual appearance of the specimen overall, with its combination of large canines and relatively small posterior teeth, their conclusion was that KNM-RU 16000 represented a male of the smaller of two species of Proconsul at Rusinga.

Distinctiveness of Proconsul heseloni at Rusinga from Proconsul africanus at Koru

Walker et al (1993) reinforced the argument for the presence of two Proconsul species at Rusinga and indeed further extended it by suggesting that the holotype of P. africanus, M 14084, previously identified at Koru (Hopwood, 1933) was different from the smaller of the two species which they believed to be present at Rusinga; they noted in passing that

22 previous workers (Pickford, 1986a; Kelley, 1986) had commented upon the fact that, notwithstanding the discovery of holotype P. africanus at Koru, most of the succeeding samples had been collected at Rusinga/Mfangano.

First, they considered values for the C1/M1 indices for those specimens of Proconsul which had previously been identified as male and noted that these lay outside the upper limit of values obtained from a similar calculation for extant great apes; on this basis they agreed with Hopwood (1933) that the holotype M 14084 was a male. M 14081, the former X. koruensis from Koru, was regarded by them as a female P. africanus.

A critical consideration for them was that specimen KNM-RU 7290, formerly field reference R. 1948,50 (LeGros Clark & Leakey, 1951), which had been discovered at Rusinga, was a female with relatively small canines. They included it in the hypodigm for P. heseloni and suggested that it was a female.

Walker et al (1993) looked at a number of isolated M1 samples from Rusinga and from Koru, examining coefficients of variation, range-based statistics and maximum/minimum ratios (see also Cope, 1993; Martin, 1993; Martin & Andrews, 1993) for mesiodistal and buccolingual diameters of molars from the two regions. They found that the patterns of variation for the Rusinga samples when considered as a single group exceeded those noted for any extant hominoid species. They performed similar assessments upon all tooth series from canines to molars in the maxillary sequence and on premolars and molars from the mandibles and achieved similar results. On this basis they felt able to falsify the argument (Pickford, 1986a; Kelley, 1986) that only one Proconsul species had been present at Rusinga. With the Rusinga samples divided into two groups, consistent with P. africanus and P. nyanzae respectively, they found that intraspecies levels of variation were no greater than those detected within the (single-species) P. africanus sample from Koru.

They did proceed to examine further the issue regarding morphological differences between the holotype P. africanus from Koru and the smaller species which they had identified from Rusinga. They noted that Koru P. africanus upper molars exhibited well- developed cingula on each of the mesial, lingual and buccal margins; occlusal ridges and a protoconule were well-developed; and that the trigon breadth was approximately 50% of the total breadth of the crown. The lower molars exhibited poorly developed buccal cingula and the M3 tapered distally.

The Rusinga samples were discussed in terms of a number of specimens, of which KNM- RU 2036 was denoted as the holotype; this consisted of a partial skull, with most of the upper and lower teeth, and parts of the postcranial skeleton. (In the current study, KNM-RU 2036DB, a left M1, has been measured as part of the data set.)

Within these Rusinga samples of the smaller species, they found that, by contrast with the Koru specimens, the upper molars showed absence or weak development of a buccal cingulum, that the occlusal ridges were less well developed and that the trigon breadth was greater than that of the total breadth. Within the lower molars the buccal cingulum was more prominent and the M3 was elongated with a massive hypoconulid and moderate distal taper.

They did draw attention to a number of problematic specimens, including KNM-RU 2088. They noted that, whilst Andrews (1978a) had identified this specimen to represent a male of P. africanus, Teaford et al (1988) had regarded it as an example of P. nyanzae. Drawing upon the work of Martin (1983), who had confirmed a C1/M1 ratio for extant hominoids lying within a range of 1.13 to 1.65 as being male, and whilst themselves allowing that fossil specimens with a range between 1.13 and 1.36 were indeterminate, Walker et al (1993) noted that specimen KNM-RU 2088 demonstrated a ratio of 1.47 and that this was consistent with a male individual. Further, given that its inclusion within the P. heseloni sample from Rusinga did not extend variation in that sample beyond comparative samples, on this basis they assigned the specimen as a male of P. heseloni.

On the basis of their findings, Walker et al (1993) felt able to suggest that not only could the presence of two species at Rusinga be confirmed, one smaller and one larger; but also that the smaller of the two species, referred to previously as P. africanus, in fact differed sufficiently from the holotype P. africanus from Koru to be considered a separate taxon, which they named Proconsul heseloni. They specifically recommended that specimen KNM- RU 7290 belonged to the hypodigm of this taxon.

This attribution appears subsequently to have gained considerable acceptance (Rafferty et al, 1995; Harrison, 2002; Nakatsukasa et al, 2004; Begun, 2007; Pickford et al, 2009; Harrison & Andrews, 2009; Hill et al, 2013; Cote et al, 2014).

Support from postcranial material

In addition to the consideration of variation within fossil dental material which has been used to support the contention that Proconsul at Rusinga was represented by more than a

24 single highly dimorphic species, postcranial material has been analysed by a number of workers (Ruff et al, 1989; Walker et al, 1993; Rafferty et al, 1995).

Indeed, much of the argument for separation of P. africanus (as it was then still described at Rusinga) from P. nyanzae at the same site was based upon consideration of such specific measures as comparing cross-sectional diaphyseal femoral diameters, and femoral head articular areas, amongst seven different Proconsul specimens.

Proconsul meswae

Harrison & Andrews (2009) are responsible for a more recent development relating to Proconsul taxonomy in their analysis of a collection of what they consider to be fossil catarrhines, recovered from the Meswa Bridge region of Kenya. They record the diagnosis of Proconsul meswae, a large-bodied primate which they note to have exhibited similar dental size to P. nyanzae. They speculate that, at a date of at least 20 Ma as judged by adjacent fauna, this might represent the oldest species of Proconsul yet recognised, and that it might prove to be the ancestral sister taxon to all other Proconsul.

Further consideration of this matter relating to P. meswae falls outside the scope of this study, as no specimens of the taxon have been examined.

Rangwapithecus

Andrews (1970, 1974 ) described two new specimens, including what came to be known as the Songhor palate, KNM-SO 700, and a partial mandible, KNM-SO 900. He ascribed the former as an ancestral pongid, whilst he initially thought the latter to resemble in overall morphology the Oligocene catarrhine Aegyptopithecus zeuxis. The later recovery of additional specimens, including a partial mandible, KNM-SO 1112, caused him (1974) to propose that these specimens represented a new species within genus Proconsul, which at that time was still described as subgenus Dryopithecus (Proconsul) according to Simons & Pilbeam (1965).

Andrews (1978a) designated this species with a new subgeneric rank and named it Dryopithecus (Rangwapithecus) gordoni. At the same time, he noted that a partial maxilla and upper dentition from Rusinga , KNM-RU 2058, whilst almost identical morphologically to KNM-RU 700, was much smaller; on the basis of tests of ranges of variation and significance, he proposed a second new species, namely Dryopithecus (Proconsul) vancouveringi. These subgeneric and specific rankings are retained in Andrews (1978a).

Bosler (1981) whilst assigning Proconsul teeth amongst six different groups (three Proconsul species, two Rangwapithecus species and one unassigned group) on the basis of their morphologies, whilst ignoring the sites from which they had been retrieved, removed SO-1112 from R. gordoni and reassigned it to P. africanus. This reallocation would have further ramifications later (Senut et al, 2000) when the issue of Ugandapithecus came to be considered.

Harrison (1986) removed R. vancouveringi from Rangwapithecus, reassigned it to the new genus Nyanzapithecus and changed its attribution to N. vancouveringorum (2002). However, R. gordoni has persisted as a species recognised in its own right (Kay, 1977; Cameron, 2004; Harrison, 2010; Hill et al, 2013; Cote et al, 2014) with some agreement that the pattern of its molar shearing crests is indicative of an increased level of folivory in its diet, compared with Proconsul, which seems to have been more frugivorous in its diet (Swindler, 1998; Teaford, 2000; Cameron, 2004; Walker & Shipman, 2005).

Cameron (2004) identified four species (P. africanus, P. major, P. heseloni and P. nyanzae) within the family Proconsulidae; with two genera, Rangwapithecus and Turkanapithecus respectively, contained within the family Rangwapithecidae. He suggested that Rangwapithecus is monospecific, comprised by R. gordoni.

Harrison (2010) discussed Rangwapithecus as a monospecific genus from the early Miocene (~19-20Ma) from Songhor and Lower Kapurtay, Kenya, with R. gordoni the sole representative to date. He differentiated it from Proconsul spp. by noting amonst other features that the upper molars exhibit a strong lingual cingulum, enlarged hypocone, and a rhomboidal arrangement of cusps and occlusal outline; M1

Ugandapithecus

The initial concept

Senut et al (2000) argued for removal altogether of Proconsul major from the genus Proconsul itself and its assignation to a new genus, Ugandapithecus; they did so on the basis that, not only was its dentition markedly different from that of P. africanus, P. nyanzae and P. heseloni; but that postcranial remains retrieved from the sites of Napak and Moroto in

Uganda, and attributed to it, reinforced its distinction from other, smaller, Proconsul species. They suggested that Ugandapithecus major and another large-bodied hominoid, Morotopithecus bishopi, shared these north-eastern Ugandan sites. They also described from Moroto evidence of postcranial remnants which were possibly attributable to P. nyanzae.

Dissenting views

Others have dissented from this view; MacLatchy & Rossie (2005) argue that there is little evidence to support the exclusion of P. major from a clade comprising P. africanus and the Kisingiri species and its allocation to a new genus. They suggested that variation noted within the Napak material might denote the existence of a degree of sexual dimorphism within P. major itself; or, alternatively, the presence of a smaller, P. nyanzae-sized taxon alongside P. major. They discussed various phylogenetic scenarios and argued little evidence for the existence of a clade involving P. africanus and the Kisingiri species to the exclusion of P. major. Suwa et al (2007: 923) referred in inverted commas to “Ugandapithecus gitongai” when discussing one of the purported species within the revised genus. However, their discussion reporting the discovery of a new late Miocene genus, Chororapithecus, did not address phylogenetic affinities within Proconsul/Ugandapithecus any further.

Further insights

Pickford et al (2009) have continued to press for the reattribution of P. major to U. major and have in fact performed a major systemic revision, with four potential species (U. major, U. legetetensis, U. meswae and U. gitongai) under consideration. It is noted in passing that the Songhor mandible KNM-SO-1112, previously discussed, and reattributed by Bosler (1981) from R. gordoni to P. africanus, has been recommended by Pickford et al for inclusion within U. meswae. Pickford et al (2009) discussed Ugandapithecus in terms of an evolutionary trend in which the oldest species is U. meswae, from Meswa Bridge (21.5-19.0 Ma); this species they redefined from the Meswa P. major identified by Andrews et al (1981). Ugandapithecus meswae is also the smallest species within the new genus as defined and they noted that later species exhibited a progressive increase in size. They discussed U. legetetensis, sp. nov. from Koru (20.0-19.0 Ma) as a species intermediate in size with representation also at

Legetet and Chamtwara; with U. major (formerly P. major) represented at Songhor and Napak (19-18 Ma). The largest species, U. gitongai, is defined to have been present at Kipsamaran, Kenya (ca. 14.5 Ma) but possibly also at the older site at Moroto, Uganda (ca. 17.5 Ma).

They believed that U. gitongai is possibly the ancestral group from which one or the other of the Late Miocene East African genera such as Samburupithecus, Chororapithecus or others might have evolved, although they conceded that currently this is difficult to demonstrate given that the latter genera are poorly known.

Their suggestion is that the evolutionary lineages of Ugandapithecus became larger with the passage of geological time whilst at the same time undergoing only modest changes in dental morphology.

Pickford et al (2009) also considered the morphological and metric differences observed in the teeth, maxillae and mandibles of P. africanus and P. major respectively and argued that the degree of heterogeneity exhibited between these specimens precludes their retention within a single genus. They believe that P. africanus is closer in morphology to the two Rusinga species, P. heseloni and P. nyanzae, and that to retain Ugandapithecus major as a species within Proconsul would result in the genus being paraphyletic. They also suggested that even the distinction between Rusinga P. heseloni and the mainland P. africanus needs to be revisited, particularly in view of their attribution of KNM-SO 1112 to U. meswae as noted above.

They revisited the proposition that all the specimens assessed as U. major by Senut et al (2000) represent only one taxon, in which a greater than usual degree of sexual dimorphism was being exhibited. They noted the accumulation of further material attributed to Ugandapithecus species in toto as a consequence of discoveries at Napak, resulting from the Uganda Palaeontology Expedition between 1985 and 2009; and those at Koru, resulting from the Kenya Palaeontology Expedition in 2004-2005. They highlighted the fact that this additional material contributes to great metric heterogeneity within the assemblage overall. They suggested that there is no reason to accept that an unusually high degree of sexual dimorphism exists within the assemblage and that it is therefore preferable to argue instead for the sympatric presence of more than one species. It is on this basis that they proceeded to identify the separate taxa of U. meswae, U. legetetensis, U. major and U. gitongai as already described.

One significant effect of the discussion by Pickford et al (2009) was to emphasise the separate character of the Ugandapithecus material overall, from both Tinderet and Napak, from the holotype of P. africanus, M 14084, and from those small numbers of isolated teeth also attributed to the latter species. They noted that the Tinderet samples of P. africanus group more closely with the two putative species (P. heseloni, P. nyanzae) at Rusinga, notwithstanding that they still noted significant morphological differences between the two smaller species (P.africanus, the holotype, at Tinderet; and P. heseloni at Rusinga).

Harrison (2010) in a major study in which he has reiterated the major morphological features of six described species of Proconsul (P. africanus, P. heseloni, P. nyanzae, P. major, P. gitongai and P. meswae), taking into consideration dental, cranial and (where available) postcranial elements of each of these taxa, has noted amongst other matters that if the concept of Proconsul as a single genus is accepted, there is no less than a fivefold difference in estimated body mass from the smallest to largest of these species. Whilst this is unusual amongst modern genera of mammals, he does note that species of Theropithecus (both extinct and extant) and of Tragelaphus do demonstrate this phenomenon. In other words, the existence of such a range of average body mass does not absolutely preclude the potential for differently-sized species to be considered within one genus.

Harrison (2010) considered alternative cladistic analyses across the range of Proconsul species, but ended up suggesting that, because of their close morphological similarity overall, it is still useful to group them together in a single genus, Proconsul. He did agree with MacLatchy & Rossie(2005) that Ugandapithecus represents a junior synonym of Proconsul. He also further cautioned that the relative paucity of P. gitongai material means that, even though this species does seem to be slightly larger than P. major, the distinctions between them are not marked. Proconsul gitongai is described as possessing upper molars with cusps of higher relief and a more blocky appearance amongst other features. Nevertheless, the distinction from P. major is not marked; he suggested that P. major and P. gitongai might be regarded as conspecific or at least closely related members of a single evolving lineage.

Harrison concluded this analysis by affording P. gitongai no more than provisional acceptance as a separate species. He referred to the requirement for further material to be recovered before this issue can be more definitively resolved. He does clearly advocate a different resolution to the issue of Proconsul gitongai/Ugandapithecus gitongai than that suggested by Pickford et al (2009).

Current Position: Recent Changes

This discussion regarding the status of P. major and its potential revision to U. major is of some relevance for the current study, if only for the fact that a number of the specimens discussed (Senut et al, 2000; MacLatchy & Rossie, 2005; Pickford & Kunimatsu, 2005; Pickford et al, 2009; Harrison, 2010) have formed part of the data set for the current study. These have included M 16648, the holotype of P. major (as it was originally described), as well as a number of other specimens from the sites at Songhor, Legetet and Chamtwara. Nevertheless, these issues are again concerned largely with matters of phylogeny and thus lie outside the scope of the present study.

Species Recognition

A study such as the current one, concentrating upon potential taxonomic distinctions to be recognised within a single genus such as Proconsul, requires at least some consideration of the complex and ongoing debate concerning speciation issues in general. However, from even a basic review of the literature, it becomes apparent that speciation issues within the palaeontological record are even more problematic than those which are encountered when dealing with extant taxa. (Mayr, 1942; Dobzhansky, 1951; Simpson, 1943; Paterson, 1985; Eldredge, 1993; Szalay, 1993; Harrison, 1993.)

In terms of this study, in which an attempt is being made to try to determine the alleged presence of more than one species of Proconsul at either or both of the fossil sites under consideration (i.e. Areas 1 and 2 respectively), consideration has to be made of issues such as the presence of what are arguably sympatric species within either or both of the main areas (such as the contemporaneous presence of P. heseloni and P. nyanzae at Area 1, or P. africanus and P. major at Area 2); or in terms of what are possibly allopatric species, when it is appreciated that Areas 1 and 2 are themselves separated by a substantial distance geographically. Further, the issue of allopatric speciation takes on a potential temporal dimension, when consideration is given to the difference in time-horizons between, for instance, Rusinga/Mfangano on the one hand, as against Koru/Songhor on the other. (Refer to Table 1.1). This is one of the issues previously raised by Kelley (1986) in his argument against the alleged presence of two sympatric species, particularly at Rusinga/Mfangano.

The matter is complex: Mayr (1942:153) himself points out that the delimitation of palaeontological species is arguably made even more difficult by the almost inevitable gaps within the fossil record and that the very term ‘species’ itself possibly has to be considered

30 differently for extinct groups of organisms than it does for extant groups. As an ornithologist by training, he presents tremendous amounts of evidence for issues of speciation within neontological taxa, stressing such issues as geographic variation, the concept of polytypic species and the mechanisms whereby populations, races or subspecies are enabled to diversify particularly in terms of allopatric speciation. However he seems to have less comment with regard to the matter of sympatric speciation and even less as to how such speciation might potentially have occurred in a discrete palaeontological assemblage.

Similarly, Dobzhansky’s (1951) views, relying substantially upon an analysis of the genetic mechanisms underpinning isolating mechanisms, are again less helpful in considering issues of speciation within a palaeontological context. As with Mayr, Dobzhansky seems to focus in particular upon biological mechanisms which lead to a result whereby allopatric species can subsequently be identified.

As a vertebrate palaeontologist, Simpson (1943) is of necessity concerned with issues of speciation as they are considered in terms of temporal sequences, and thus raises the concept of variation, and hence ultimate speciation, from the viewpoint of appraising ‘vertical’ sequences through time, as opposed to the ‘horizontal’ sequences which might be considered by the systematist dealing solely with neontological data. He makes the point that the species definition as applied to neontological taxa is inapplicable to a palaeontologist who has to deal with what he describes as ‘vertical species, dynamic temporal sequences’ (Simpson, 1943: 146).

Much of the modern discussion of these issues relates still to the Biological Species Concept as defined by Mayr (1942: 120). Even with the attempt by Paterson (1985) to define the mechanism of a Recognition Species Concept, operating via a Specific Mate Recognition System, the focus still relates substantially to a consideration in terms of modern species and thus arguably has less relevance when paleontological material is being considered.

The Hennigian Species Concept (1966) and the Phylogenetic Species Concept (Nelson & Platnik, 1981; Wiley, 1981) should also be taken into consideration. Hennig (1966) discussed amongst other matters the concept of species extinction as a stem species splits into two daughter species, whilst the Phylogenetic Species Concept allowed instead for the alternatives of anagenesis (gradualistic change) or of cladogenesis, a branching event in

31 which the stem species gives rise to two daughter species, as alternative models allowing for the subsequent recognition of two or more species at a particular site. This concept of potential sympatric speciation is of relevance when considering issues such as those raised by Harrison (1993) in his consideration of the Early Miocene catarrhines including the Proconsuloidea.

Harrison (1993), perhaps somewhat updates Simpson (1943) when he considers the issues in light of the kinds of cladistic analyses which have gathered strength since Hennig’s original work (1966). Harrison notes the discordance between the manner in which cladists view palaeontological samples which possess a realised evolutionary history, as against modern species which are themselves the terminal products of such history and which themselves might only have a potential to produce descendant species. Whilst himself seeming to accept that a case might be made for a cladistic approach in general, he does qualify it by suggesting that it might be appropriate to consider both cladistic and gradistic (taxic) elements. He does stress the difficulties involved in attempting to deal with stem groups from fossil taxa.

Despite his reservations in this regard, it is noteworthy that Harrison (1993) still suggests that it is valid to accept the concept of the Proconsuloidea as a taxonomic grouping and that, despite current uncertainties about phyletic relationships within the group, it seems appropriate at present to utilise the considerable fossil material to try to construct a primary taxonomy.

Of significance for the current study is the work of Gingerich (1974, 1977; Gingerich et al, 1982) who has analysed coefficients of variation of length and breadth of mammalian molars; he has suggested that what he describes as the peakedness of M1 is of particular importance in diagnosing closely related fossil species from the same locality and stratigraphic horizon because the measurements with peaked distributions (low variability) in one species have the best chance of appearing bimodal when the mixed measurements of two species are considered.

Plavcan (1993) makes the suggestion that the fossil record cannot be interpreted only with reference to itself and goes on to make the point that to justify the claim (as per Pickford, 1986a; Kelley, 1986) that the biological characteristics of fossil species differ from those of living species on the basis of fossil evidence is a circular argument, since the proof rests on the data that are being questioned. He then makes the point that, if tooth-size

32 variation in a fossil sample exceeds that in all living species, we have little choice but to infer that the sample is composed of more than one species, rather than two sexes, unless there is other evidence to support the dimorphism hypothesis.

Over the past 25 years, substantial efforts have been made to consider inclusion of postcranial material with regard to determining speciation within Proconsul. Ruff et al (1989) estimated body mass within Proconsul species by a consideration of cross-sectional diaphyseal and articular dimensions of femora from the fossil material and from a selection of extant catarrhines. Teaford et al (1993) were similarly concerned about attempting to address the issue of uniformitarianism arising just from comparisons of dental material; so they went on to consider not just coefficients of variation (CVs) from within this material, but also the CVs relating to talar and calcaneal samples. Rafferty et al (1995) utilised a particular distal tibial specimen from Napak, Uganda, together with a talus from Songhor and various humeral and femoral fragments from Rusinga/Mfangano.

It would be suggested that, in summary, these different workers have in recent years used a combination of dental and postcranial material in an effort to determine specific taxonomic criteria, and have provided substantial evidence to refute the concept that a single highly-dimorphic species is present at a site such as Rusinga.

Most of their work lies outside the scope of the current study. However, Rafferty et al

(1995) did also consider the issue of M1 areas and body weights as had previously been considered (Gingerich, 1977; Gingerich et al, 1982) and utilised these in their attempt to confirm the presence of sympatric species within Proconsul. I can only note that Rafferty et al felt obliged to suggest that the postcranial material allowed for better separation amongst the three Proconsul species, whereas the estimates from dental material provided a much more continuous distribution. They were clearly opting for a combined approach which would utilise assessment of both dental and postcranial samples.

This study

I therefore revert to the kinds of considerations made by others (Cope, 1993; Plavcan, 1993) in attempting to deal with the issue of speciation within Proconsul. My study comprises the accumulation of a data set in which I obtained measurements from sex- matched samples of molars from amongst four groups of extant primates. Consideration was made of results obtained from measurements of occlusal views, concentrating upon

33 both linear distances and cusp areas, so that matters of size and of shape could be considered.

I then obtained similar information from amongst a similar collection of photographs of fossilised material which had been attributed previously to one or other species of Proconsul.

The rationale for concentrating upon these occlusal views of molar teeth has been discussed by Pilbrow (2003: 41-42, 48-49) in which she refers to the distinction between attempting to obtain measurements from the earlier and more traditional use of sliding calipers as opposed to the more comprehensive data which can be gleaned from the analysis of photographic images as discussed in the current study.

The data obtained from caliper measurements is much more constrained, given that these assessments are confined to linear measurements of mesiodistal lengths and buccolingual diameters in isolation. If any attempt was to be made to assess occlusal surface areas, this had to be performed on the basis of calculating the product of length and breadth, thus assuming that the molar occlusal surface was essentially quadrilateral in shape. This was potentially inaccurate, and would create a tendency to overestimate the occlusal area, given the curved outlines which normally exist at each of the meeting points of adjacent borders in even relatively square or oblong molars. Neither could calipers provide useful information regarding other features such as crest sizes.

Prior to the introduction of morphometric image analysis, of the kind upon which this current study relies, morphological features such as cusps thus had to be described essentially in qualitative terms only. The introduction of photographic image measurement, combined with a suitable software analysis program, has afforded the potential for these features to be measured accurately and thus recorded in quantitative format. As has been demonstrated in this study, accurate measurements of individual cusp areas of upper and lower molars can be obtained, as well as measurements of more complex features such as trigons/trigonids, talons/talonids and total cusp base areas.

Provided that measurement protocols are specified with the kind of rigor exemplified by earlier workers (Wood & Abbott, 1983; Uchida, 1992), and reinforced by Pilbrow (2003, 2006, 2007, 2010) and Bailey et al (2004), it is feasible to utilise these occlusal views of molar teeth to construct data sets which are then available to subsequent independent

34 investigators who themselves might not have had access to the original specimens. This forms the rationale for the current study.

Pilbrow (2003) drew attention to the fact that an analysis of the kind which forms the basis for the current study has distinct advantages. Whereas hominoid ante molar teeth, whether incisors, canines or premolars, possess cusps which exhibit a high degree of relief in the occlusal plane, it is not feasible to measure them using an image analysis technique. By contrast, the relatively bunodont character of hominoid molars renders them very suitable for measurements to be obtained from occlusal views; and it is on this basis that a study of the present kind can be justified.

I refer in particular to the fact that the size of the data set obtained for this study, comprising no fewer than 144 specimens of Proconsul molars from various meristic positions, has afforded a very adequate sample from which statistical analyses might be undertaken. In other words, the size of such a data set helps to obviate any criticisms which might have been made concerning the fact that only molar teeth had been examined.

Once this material had been accumulated and checked for potential errors, I was able to apply a series of appropriate statistical tests to determine whether the degree of variation from within the Proconsul set outmatched that from within the extant taxa; and also whether it was possible to identify the presence of significant discontinuities which in themselves validated the argument that more than one species was present at a particular palaeontological site.

The intention of this study was to readdress the issue as to whether two or more species of Proconsul had existed sympatrically at each of the clusters of East African Early Miocene sites which have previously been described; or whether variations in size noted within the Proconsul material extracted from within those sites were due instead to enhanced degrees of sexual dimorphism within a single species at a particular site.

CHAPTER TWO: MATERIAL AND METHODS

Introduction

The aim of this project can be summarised as follows: whether is it feasible to address a major controversy concerning genus Proconsul by determining levels of variation, using a statistical analysis of the data obtained from measurements relevant to molar teeth of both Proconsul and of extant hominoids. The controversy to which I refer questions whether levels of variation observed amongst samples of Proconsul dental material, obtained from a particular site, are due to the presence of two or more sympatric species at that site; or whether they are due instead to the presence of a mixed sample of male and female individuals from within one species. In particular, by utilising two-dimensional photographs of the occlusal views of primate molars, my focus has been upon obtaining accurate measurements of linear dimensions and cusp areas, and to subject the data obtained from those measurements to statistical analysis, in order to find whether levels of variation within Proconsul exceed those which are obtained from an identical analysis of sex-matched samples drawn from four recognised groups of extant primates.

The Material Used

At the commencement of this project, Dr Varsha Pilbrow (Dept of Anatomy & Neuroscience, University of Melbourne) made available to me the photographs of dental material which she had collated from various museum sources (Pilbrow, 2003). These form the basis for the entire data set which has been utilised in this study. Tables 2.1 and 2.2 indicate the numbers of specimens assessed.

I took the decision at the outset to concentrate upon occlusal views of the molar teeth of all the primate specimens which were being studied; this was in accordance with the views expressed by previous workers regarding the potential to calculate coefficients of variation (CVs) from linear measurements of molar length and breadth (Gingerich, 1974; Gingerich & Schoeniger, 1979; Cope & Lacy, 1992) and that morphological traits relating to the occlusal surfaces could be obtained from digitised images (Hills et al, 1983; Wood & Abbott, 1983; Wood & Xu, 1991; Uchida, 1996b). My study focused on the adult dentition.

Extant Primates

I measured all images in accordance with the protocols described by Pilbrow (2003) and by Bailey et al (2004); indeed, their protocols formed the basis of this entire study. Their

36 works described how images of the molar occlusal surfaces had been photographed using a camera mounted directly above the specific tooth being measured and with the lens positioned in a plane parallel to the occlusal surface itself. Single teeth were photographed directly in this fashion (Fig. 2.1).

Fig. 2.1 Occlusal view of single molar tooth. Image: courtesy Dr. Varsha Pilbrow.

For those specimens which comprised a partial or complete tooth row, in most such instances each tooth had been photographed in turn, so that a sequence of consecutive images had been obtained. For each meristic position the camera had been centred over that particular tooth in order to limit the potential for parallax to be a problem.

In a small number of instances there was only one image for an entire jaw: an example is the image for M 16647, the maxilla which represents the holotype for P. nyanzae (Fig. 2.2).

Fig. 2.2 M 16647, the holotype of P. nyanzae Image: courtesy Dr. Varsha Pilbrow

However, as each image of this kind had been obtained by application of the same criteria as had been adopted for the individual specimens, as described by Pilbrow (2003, 2006, 2010), this did not pose any problem. The calibrated scale had been applied on the same horizontal scale as the occlusal surface, so that individual molars within the tooth could each be measured in turn; the software program allowed for magnification to be applied so that the appropriate measurements for that tooth could be obtained.

Selection of Material: Extant Groups

I was aware that the entire debate concerning speciation vs. sexual dimorphism within Proconsul required an attempt for one variable to be minimised as far as possible, specifically the influence that variation levels within a particular group were dependent upon the sex distribution within the material being examined.

For this reason, selection from within the extant primate groups, which were going to be used essentially as controls for the Proconsul material, was undertaken strictly on a 1:1 basis between males and females; this accounts for the fact that all elements in Table 2.1 are represented by even numbers. However, this requirement did in itself pose some difficulties and it is for this reason that the cells within Table 2.1 are rather disparate in terms of the actual figures represented.

Number of Specimens

Subspecies 1 2 3 N M M M M1 M2 M3 Pan troglodytes troglodytes 24 24 24 24 24 24

Pan troglodytes schweinfurthii 10 10 10 10 10 10

Pan troglodytes verus 6 6 6 6 6 6

Pan paniscus 10 10 10 10 10 10

Total Pan 50 50 50 50 50 50 300

Gorilla gorilla gorilla 26 28 22 30 30 28

Gorilla gorilla diehli 10 8 12 4 4 2

Gorilla beringei beringei 6 6 8 8 8 10

Gorilla beringei graueri 8 8 8 8 8 10

Total Gorilla 50 50 50 50 50 50 300

Pongo pygmaeus 36 36 36 36 36 36

Pongo abelii 14 14 14 14 14 14

Total Pongo 50 50 50 50 50 50 300

Hylobates lar 28 28 28 28 26 26

Symphalangus syndactylus 22 22 22 22 24 24

Total Hylobates/ 50 50 50 50 50 50 300 Symphalangus Table 2.1 Extant hominoid study sample: museum sources. For all groups, specimens obtained on a 1:1 ratio, males:females.

For instance, within Pan it clearly proved difficult to find substantial numbers of Pan paniscus available for measurement and this proved to be the case across the entire spectrum of museums from which material had been accessed. To some extent the paucity of material for Pan paniscus is itself indicative of the limited numbers of that species in the wild; and that the sex imbalance which is exhibited even within that museum material is possibly due in part to the preponderance of males within the wild-shot samples. These issues have been canvassed by Pilbrow (2003:27-30; 2006:649) who has noted that, although she comprehensively sampled all major U.S. and European museum collections to obtain her study material, sample sizes for a number of localities were small and that the sex ratios were unbalanced.

Similarly, within Gorilla, it was equally difficult to obtain useful numbers for the subspecies, G.g. diehli, a problem which was again accentuated by the male:female imbalance exhibited by the museum material. This proved such that I could obtain only a single M3 specimen which had been identified as female, resulting in the single paired sample which is noted in Table 2.1.

Within Pongo, the limited representation for P. abelii is demonstrated by the fact that no more than 28% of the entire selection for genus Pongo was made up from this species.

The situation as regards the gibbons was complicated for a different reason. The museum material was best represented by H. muelleri and by H. lar within Hylobates, but in insufficient numbers of unworn or near-unworn specimens to enable a full set of 50 individuals, with a 1:1 male:female ratio, to be measured from these two recognised species in isolation. There was however a substantial selection of another species, Symphalangus syndactylus, from within which a sex-matched sample could be achieved. I took the decision to concentrate upon a separation between Hylobates and Symphalangus, treating them on the basis of the same kind of between -species separation as had been utilised between the two species each of Pan, Gorilla and Pongo.

I took the decision to treat these two species of gibbons essentially as though they were two species of Hylobates against the complex background of assessment of the Hylobatidae overall. There had in the past been considerable argument concerning the potential merits of considering Hylobates and Symphalangus within one genus. Simpson (1945) regarded Hylobates and Symphalangus as two distinct genera, whilst both Anderson (1967) and

Corbet & Hill (1980) treated both as individual species within Hylobates. Chivers (1984, 2006) initially regarded Hylobates and Symphalangus as distinct species within a single genus, but later regarded Symphalangus as a genus in its own right. Wilson & Reeder (2005: 178) noted that ‘the family (Hylobatidae) is usually awarded a single genus’ but accepted that Goodman et al (1998) had argued for full generic separation, whilst both Roos & Geissmann (2001) and Groves (2001) had each postulated that a case could probably be made for Symphalangus to be afforded full generic status.

Goodman et al (1998) had suggested that, based upon a combination of DNA and fossil evidence, and given an estimated Last Common Ancestry date of 8 Ma, an age-related classification caused them to argue for two separate genera. It was on the basis of these factors that I elected to combine the available samples of Hylobates lar and Symphalangus syndactylus to make a sex-matched sample of 50 individuals within a single genus, fully acknowledging that some authors place them in distinct genera.

Mention should be made of the fact that, as far as possible, within the samples being assessed for inspection from within the extant groups, care was taken to ensure that the samples being utilised for measurement were not too worn. I adopted the protocol outlined by Pilbrow (2003: 40-41; 2006: 649) and of Bailey et al, 2004 (Table 1: 325) of utilising 0 to represent an unworn specimen; 1 for a specimen within which just the molar cusp tips had been exposed; and 2 for a specimen exhibiting more significant damage in terms of wear or damage. These latter either showed such exposure of the underlying dentine that I would have found it difficult to measure specific elements such as particular cusp areas; or else there were breakages involving cusps or tooth margins which would obviate accurate measurements of areas being performed.

Missing molars from a particular dental arcade also proved a problem, with substitutes having to be made from another specimen.

In most instances there were sufficient examples of a particular subspecies present to allow me to choose appropriate unworn examples; this was the case with common species such as P. t. troglodytes or G.g. gorilla; again, however, this concern to ensure quality of sampling only accentuated the problems already outlined within those subspecies for whom lower numbers had been present at the outset.

In the manner which has just been described, I obtained 50 examples of each of the great apes, Pan, Gorilla and Pongo; and a mixed sample of 50 specimens for

Hylobates/Symphalangus. Not only was each group selected on a sex-matched basis, but in addition there was a 1:1 apportionment made between upper and lower jaws.

Once each particular extant sample had been selected, it was subjected to a measurement protocol which was identical to that used for the Proconsul specimens.

Selection of Material: Proconsul

As demonstrated in Table 2.2, 144 adult Proconsul specimens were examined in total, comprised of material from Areas 1 and 2. Molar attributions followed those provided by Andrews (1978a); Pickford (1986a); Teaford et al (1988); Uchida (1996a) and Walker et al (1993). There were 35 molar specimens which I rejected as being unsuitable for measurement, either because they were broken or were too worn to permit adequate delineation of the cusp boundaries.

1 2 3 M M M M1 M2 M3 Totals Area 1 15 18 9 16 16 11 85 Area 2 9 13 9 13 9 6 59 Totals 24 31 18 29 25 17 144

Table 2.2 Proconsul data set: Numbers of specimens assessed from each site. Area 1: Rusinga/Mfangano. Area 2: Koru/Songhor/Chamtwara/Legetet.

Compared with the extant groups, the Proconsul material was somewhat more circumscribed but was nonetheless clearly suitable for acquisition of a data set. A breakdown of Table 2.2 reveals that there were 42 upper molars and 43 lower molars from Area 1, whilst the corresponding figures for Area 2 were 31 upper and 28 lower; upper and lower jaws were thus almost evenly represented from each area. The data set overall was also comprehensive from the viewpoint of representation of all meristic positions.

There was one unsatisfactory feature of my Proconsul sample overall and I refer to an issue arising from previous attribution of specimens as recorded in the literature. From the specimens available for me to examine, none had been identified previously as representing an M2 of P. africanus from Area 2; of the 9 M2s available for assessment from these sites, all had previously been suggested as belonging to P. major. This absence of P. africanus M2 subsequently prevented some statistical analyses from being conducted for this meristic position.

Uchida (1996a) had similarly reported on the relative paucity of Proconsul mandibular specimens overall in her study; she did have access to three isolated teeth which had been attributed to P. africanus M2, but none of her specimens was available for me to study.

The character of the current study is again emphasised as having followed previous analyses, e.g. that of Wood & Abbott (1983) and more particularly that of Pilbrow (2003,2006,2010) and of Bailey et al (2004).

As has been discussed earlier, the nature of the measurement process adopted as the basis for this study, relying upon the accurate measurement of two-dimensional images, increased the amount of information to be obtained, certainly in comparison with that obtainable from caliper measurements performed on actual teeth. Pilbrow (2003) has described the relatively bunodont character of primate molars whose low height renders them suitable for measurement of the two-dimensional character of the occlusal surfaces and this criterion provides much of the basis for the current study.

As would be anticipated from collections of fossil material, the molars themselves comprised both individual teeth as well as partial or complete tooth rows. Within the tooth rows, on most occasions a single image had been obtained of each meristic position successively, with the camera centred overhead, as had been the case with the extant groups. Single molars had by definition been photographed individually. By means of one or other of these alternatives a single specific image had been obtained for each molar within the collection.

Examples of the kinds of Proconsul molar specimens respectively considered ‘acceptable’ and ‘unacceptable’ for measurement are shown in Figs. 2.3(a) and 2.3(b) overleaf.

Fig. 2.3(a) Fig. 2.3(b)

Proconsul specimens:

Figs. 2.3(a) and 2.3(b): examples of ‘acceptable’ and ‘unacceptable’ upper molars respectively.

Images: courtesy Dr. Varsha Pilbrow.

Antimeres

Within the Proconsul material considered suitable for measurement there were 18 examples in which both left and right-sided antimeres had been photographed; examples were M 16647 and KNM-RU 7290 in which entire maxillae and/or mandibles had been photographed. In order to confirm that the data set would not be skewed I chose to select only left-sided antimeres for inclusion in my analysis.

Gap in Data Collection: Lack of M2 Proconsul africanus

Attributions from the literature concerning species assignations for the Proconsul material were obtained from a number of sources (Le Gros Clark & Leakey, 1951; Greenfield, 1972; Martin, 1981; Andrews, 1986a; Pickford, 1986a; Walker et al, 1993; Uchida, 1996a; Harrison, 2010 and others).

I subsequently found, from the Proconsul photographic images which were available for study, that there were no M2 from Area 2 which had been assigned to P. africanus. For instance, none of the specimens measured by Andrews (1978a, Table 17: 166-167) were available to my study. In comparison there were 9 specimens which had been assigned to P. major.

Uchida (1996a: 491) had referred to specimen KNM-CA 395 as an M2, but she unfortunately listed this specimen both as P. africanus and also as P. major. My own measurement of this specimen revealed a total cusp base area of 110.95 sq.mm., a figure

43 which falls within the midrange of all P. major specimens from Area 2: these ranged from a figure of 93.95 sq.mm. in the case of KNM-CA 2229 up to 176.77 sq.mm. for KNM-SO 415.

Given that M1 and M3 P. africanus specimens from this region ranged in size respectively from 51.79 sq.mm. (KNM-CA 1773) to 52.02 sq.mm. (KNM-SO 903) for the two M1 specimens and from 83.07 sq.mm. (KNM-LG 1389) to 90.84 sq.mm. (M 14087) for the two

M3, I felt obliged to accept that KNM-CA 395 is indeed P. major. This reinforces my belief that no M2 P. africanus was available for measurement.

This was the only significant missing element within what appeared otherwise to be a very satisfactory data set; the absence of P. africanus specimens still allowed for a determination of Coefficients of Variation for M2 from Area 2 but, if I relied upon the attributions which had previously been provided by multiple previous authors, it rendered impossible the undertaking of an independent T-test to contrast means and standard deviation between P. africanus and P. major for M2 at this area. This point has been noted in the relevant table (Table 3.11) in Results.

This issue relating to the overall paucity of mandibular P. africanus specimens from the mainland sites is one which has been discussed by numerous previous authors (Greenfield, 1972; Kelley, 1986; Pickford, 1986a; Walker et al, 1993; Uchida, 1996a). Once increased sample sizes relating to these meristic positions, particularly with regard to specimens of lower second molars, have become available by the accumulation of further specimens from the mainland sites, it would be hoped that the conclusions reached in the current study might be further strengthened. This would provide further evidence to support the argument for the presence of sympatric Proconsul species at the Tinderet sites

Measurement: Protocols and SigmaScan Pro 5 Program

In the case of both the extant groups and for the fossil material, an identical protocol was observed. All measurements were performed using software program SigmaScan Pro 5, an image analysis package most recently developed by SYSTAT Software Inc. This enables measurements of linear distances, angles and areas. In the current study, however, I confined it to measurements of distances and areas only.

For each photographic image analysed, an initial calibration was performed using a measured scale which had been placed alongside the image at the time it had been taken; the scale had been placed at the level of the molar occlusal surface. By this manoeuvre pixel counts were converted into metric numerals. Once calibration had been undertaken,

44 measurements of linear distances, of individual cusp areas, of trigon/talon for the upper molars, of trigonid/talonid for lower molars, and finally of total cusp base areas were performed.

I performed measurements of dental morphology as previously defined (Le Gros Clark, 1971, following Simpson, 1937; Wood & Abbott, 1983; Uchida, 1996a; Swindler, 2002) with specific reference to measurement of upper molar cusp base areas as defined in Bailey et al (2004) and those of mandibular measurements (Wood & Abbott, 1983). For the purposes of this study, linear measurements for both upper and lower molars were performed as in Fig. 2.4 overleaf: a mesiodistal length was measured, together with mesial and distal buccolingual diameters constructed in a line connecting the appropriate cusp tips (protocone and paracone, and metacone and hypocone, respectively, in the case of upper molars; protoconid and metaconid, hypoconid and entoconid, respectively, for the lower molars) as explained in Pilbrow (2003,2006,2010).

In this study, area measurements for the cusps of both upper and lower molars were conducted according to the protocol adopted by Bailey et al (2004) for Pan M1s in which the profile of a specific cusp was measured by identifying the primary fissures within the occlusal basin and tracing the lines of the fissure to the outer margin of the crown base.

Particular attention was paid to the measurement of trigon (Fig. 2.5) and talon; and to the trigonid (Fig. 2.6) and talonid.

Fig. 2.4 Gorilla upper molar showing linear dimensions (After Pilbrow, 2010)

Fig. 2.5 SigmaScan Pro 5 measurement Fig. 2.6 SigmaScan Pro 5 measurement of Proconsul upper molar trigon of Proconsul lower molar trigonid Specimen No. M 14084 Specimen No. KNM-RU 5871

Images: courtesy Dr. Varsha Pilbrow

Overall, for both extant species and the Proconsul material, the following measurements were performed:

Upper Molars Lower Molars Distances Distances Mesiodistal Length Mesiodistal Length Mesial Buccolingual Diameter Mesial Buccolingual Diameter Distal Buccolingual Diameter Distal Buccolingual Diameter

Cusp Areas Cusp Areas Trigon Trigonid Talon Talonid Protocone Protoconid Paracone Hypoconid Metacone Metaconid Hypocone Entoconid Hypoconulid Total Cusp Base Area (TCBA) Total Cusp Base Area (TCBA)

Informal Checking

An informal ongoing checking process was undertaken throughout the entire measurement program. Randomly but regularly I took the opportunity to check the sum obtained by adding manually the areas obtained by SigmaScan Pro 5 for trigon/trigonid and talon/talonid as opposed to the total cusp base area (TCBA); or similarly the summation of individual cusp areas (four for upper molars, five for lower molars) compared to the TCBA. In every instance checked, I found that the program revealed an error of less than 1% between what were essentially two methods of defining TCBAs.

This informal pattern of intraobserver checking appears to accord with the findings obtained by Bailey et al (2004) in their consideration of intra- and interobserver error determinations.

Data Screening

All statistical analyses for this study were undertaken using an IBM SPSS Statistics software package (Statistical Package for the Social Sciences).

After I had undertaken the primary data collation, I performed informal tests for data screening, with the intention of assessing normality within the distribution of scores for all groups surveyed. In particular, I was concerned to identify the presence of potential outliers within each group and then to ascertain whether these were true outliers or whether they represented errors made during measurement. Boxplots, Histograms, Normal Q-Q Plots and Detrended Normal Q-Q Plots were generated in order to obtain a visual representation as to how closely the results obtained for each group approximated to a normal distribution.

From this information from the primary data set I was able to identify 2.67% of measurements which presented as potential outliers. In each of these instances I undertook a second set of measurements to check the results and with the exception of a tiny subset within this group, I found that the second measurement was never more than 1% different from the first; in instances of this kind, I elected simply to choose the first of the two measurements which had been performed.

In 0.13% of the entire data set, I found that there was a considerable discrepancy between the first and second sets of measurements, so that it was apparent that some error must have occurred. With this small subset I now performed a third set of measurements and in each case I found that two out of the three were now within 1% of each other, no

47 matter which measurement of distance or area had been the subject of examination. Again, I opted to choose the first of the two results which had proven to be closest to each other.

Comparison between Intraobserver and Interobserver Error Rates

Once I had obtained an indication of the possible degree of error associated with my own measurements I had to consider the question as to how the measurements obtained by image analysis might compare with the results obtained by an independent observer utilising the same protocols. An additional consideration was whether the measurements obtained from digitised images matched those obtained using sliding calipers.

To address these issues I performed analyses on some of the Proconsul material, in the first instance considering the results obtained from 16 specimens which had been analysed independently by myself and by my project supervisor. Analyses relating to cusp areas and linear dimensions were performed in line with the methods outlined by Bailey et al (2004).

Tables of means and standard deviations for each element of analysis were constructed, together with absolute differences in measurements and percentage errors of differences between the two observers (Tables 2.3 and 2.4). The most obvious feature of the error study was the discrepancy between the area measurements for the metacone and hypocone: this was as high as 3.7% for the hypocone and 11.2% for the metacone. This was a possibility anticipated by Bailey et al (2004), where they suggested that the presence of accessory crown features, and particularly the presence of a well-developed crista obliqua, might contribute to a measurement error of this kind. I suspect that this was the very reason why such a discrepancy between metacone and hypocone areas was noted in this current study.

With regard to the measurement of linear distances undertaken by the two observers using digitised images, the concordance was again somewhat variable: although the discrepancies for mesiodistal and distal buccolingual diameters, at 2.3% and 2.1% respectively were acceptable, the figure of 5.0% discrepancy for distal buccolingual diameters is both more difficult to accept and difficult to explain. The criteria for measurement of buccolingual diameters, as set out in Figure 2.4 above, really give no suggestion as to why the results for mesial and distal buccolingual diameters should be so discordant.

Difference Percentage Mean Mean Cusp SD CV SD CV VCP/HLW error (VCP) (HLW) (mm) Actual cusp base areas Protocone 24.7 8.1 32.8 24.3 8.0 32.9 0.4 <1 Paracone 19.3 6.0 31.1 18.9 6.6 34.9 0.4 2.1 Metacone 18.8 6.9 36.7 16.8 5.9 35.1 2.0 11.2 Hypocone 18.5 6.1 33.0 19.2 5.7 29.7 0.7 3.7

Relative cusp base areas Protocone 30.5 4.4 14.4 30.8 5.2 16.9 0.3 1.0 Paracone 23.8 3.7 15.5 23.7 3.8 16.0 0.1 0.4 Metacone 22.9 2.4 10.5 21.2 3.5 16.5 1.7 7.7 Hypocone 22.8 3.0 13.2 24.3 1.7 7.0 1.5 6.4

Total cusp base area 81.3 24.9 30.6 79.2 23.5 29.7 2.1 2.6 n = 16, d.f. = 30

Table 2.3 Summary statistics for measurements of actual and relative cusp base areas of Proconsul M2 for two observers (digitised images).

Difference Percentage Mean Mean Length Measure SD CV SD CV VCP/HLW error (VCP) (HLW) (mm)

Mesiodistal 9.0 1.5 16.7 8.8 1.3 14.8 0.2 2.3 Mesial Buccolingual 10.3 1.4 13.6 9.8 1.4 14.3 0.5 5.0 Distal Buccolingual 9.8 1.4 14.3 9.6 1.4 14.6 0.2 2.1 n = 16, d.f. = 30

Table 2.4 Summary statistics for measurements of mesiodistal and mesial buccolingual distances of Proconsul M2 for two observers (digitised images).

I next performed an analysis upon 16 Proconsul molars comparing mesiodistal and buccolingual lengths measured using standard calipers by Andrews (1978a) with those which I measured by photographic analysis (Table 2.5). (I should advise, however, that these 16 specimens were not identical to the ones which had been compared as digitised images by my supervisor and myself; so that a direct comparison between all three observers, for identical specimens, is not being claimed.)

With regard to this attempted comparison, a significant point must be made. Andrews (1978a: 87) advises that his buccolingual measurements were taken at the widest point of the tooth and were therefore performed essentially at right angles to the longitudinal axis; by comparison, as I have already indicated by reference to Figure 2.4, my measurements were performed in line with the protocols described by Pilbrow (2003) and therefore

49 consisted of two separate measurements, each essentially oblique to the longitudinal diameter and performed along lines intersecting the tips of the major cusps. It is thus immediately apparent that identical protocols had not been adopted.

With regard to the current interobserver comparison, I elected to consider my measurements obtained from the mesial buccolingual diameters in isolation (and hence the term ‘buccolingual’ as identified in Table 2.5 really refers more appropriately to Andrews’ consideration of a single measurement than it does to my own, which clearly comprised two separate measurements).

Under these circumstances, it is perhaps less than surprising that there turned out to be significant discrepancies between the figures gleaned by Andrews (1978a) using caliper measurements, as opposed to those which I obtained from analysis of the photographic images; thus, I noted a percentage error of 19.5% for mesiodistal dimensions and 6.4% for buccolingual dimensions.

Mean Mean Difference Percentage (PA) SD CV (HLW) SD CV (mm) error caliper image Mesiodistal 9.8 2.1 21.4 8.2 1.0 12.2 1.6 19.5 Buccolingual 11.6 2.3 19.8 10.9 2.1 19.3 0.7 6.4 n = 16, d.f. = 30

Table 2.5 Summary statistics for measurements of mesiodistal and (single) buccolingual diameters of Proconsul M2 for two observers (caliper measurements vs. digitised images).

Subsequent Analysis Program

Once the primary data set had been obtained and then checked both for potential errors and outliers, and once consideration had been given to the requirement at least to discuss seeming discrepancies on either an intraobserver or interobserver basis, I proceeded with the analysis.

This analysis was predicated upon the basis that the use of digitised images of molars was an appropriate way to obtain accurate measurements of lengths and of cusp areas displayed by these molars. Another consideration was that there were adequate numbers within the collection of fossil specimens to afford the possibility that a statistical analysis could be

50 worthwhile. The character of the investigation, which was essentially based upon the concept of distinguishing species one from another, could be defined in terms of the conducting of both univariate analysis (in the form of calculation of Coefficients of Variation) followed by bivariate analyses in terms of construction of bivariate plots and the undertaking of T-tests.

Coefficient of Variation (CV)

A Coefficient of Variation was calculated for each measurement (linear dimensions and cusp areas) for each of the extant groups and for Proconsul according to the standard formula prescribed (Thomas, 1986; Cope & Lacy, 1992; Plavcan, 1993; Martin and Andrews, 1993):

CV = 100 x S x̄ where S is the standard deviation and x-bar is the sample mean.

Bivariate Analyses

I then proceeded to perform two sets of additional calculations of a bivariate character, namely the construction of bivariate scatter plots and the undertaking of T-tests.

For the scatter plots, graphs were constructed on the basis of separation by Area 1 and Area 2, with specimens previously attributed to Proconsul heseloni and P. nyanzae highlighted at Area 1, and specimens attriuted to P. africanus and P. major highlighted at Area 2. Genus Hylobates was regarded as constituted by two species, respectively Hylobates lar and Symphalangus syndactylus. Genus Gorilla was split into species as G. gorilla and G. beringei, Pan as P. troglodytes and P. paniscus and Pongo as P. pygmaeus and P. abelii. In each instance, calculations were performed within groups taking into consideration the separate characters of mesiodistal and buccolingual diameters for distances, whilst area measurements considered the following: each of the individual cusps for upper and lower molars respectively, for trigon and talon in upper molars, trigonid and talonid in lower molars, and finally for total cusp base areas.

Given the large number of plots generated throughout this process, only a representative sample has been supplied in the Results section. These were chosen entirely at random but are believed to demonstrate outcomes which were essentially consistent throughout. In

51 order to demonstrate the degree of separation by species which seems to have been delineated successfully for Proconsul, reference plots have been provided to show comparisons with the other genera studied.

Independent T-tests

Independent T-tests were conducted for each of the same elements, distances and areas, across all meristic positions except M2s for P. africanus at Area 2, as explained above.

CHAPTER THREE: RESULTS

Data Analysis

The statistical analyses of the results obtained from the data set comprised:

 Univariate analysis utilising a Coefficient of Variation (CV)  Bivariate plots for each of the extant species and Proconsul  Independent T-tests

Coefficients of Variation (CVs)

Coefficients of Variation were calculated for all groups, of both extant species and Proconsul, for all meristic positions; the results are provided in Tables 3.1 to 3.6 inclusive. Tables 3.1 and 3.2 summarise the results obtained in terms of cusp area analyses for upper and lower molars respectively; Tables 3.3 and 3.4 reveal the position as regards mesiodistal lengths, whilst Tables 3.5 and 3.6 show the results for the mesial buccolingual diameters.

As demonstated from the majority of figures provided in these tables, whether considered from the viewpoint of assessment of individual cusp areas against total cusp base areas, or in terms of linear measures (mesiodistal lengths and mesial or distal buccolingual diameters), there seems to be a fairly consistent pattern.

The position with regard to CVs conducted for cusp area analyses seems straightforward: throughout, and without exception (Tables 3.5 and 3.6), there is a clear indication that variation levels are highest within Proconsul, at each of Areas 1 and 2, next highest amongst the gibbons, and then substantially lower in terms of results obtained for each of the great ape genera.

There are only relatively minor variations to this situation within the tables registering levels for mesiodistal lengths and the buccolingual diameters. Examples of this are the findings from Table 3.3, where both Areas 1 and 2 exhibit levels of mesiodistal length for M2 lower than for the gibbons and Gorilla (10.8 and 8.2 for Proconsul Areas 1 and 2, compared with 17.3 and 10.0 for Hylobates and Gorilla respectively); and from Table 3.4 which demonstrates a distortion with lower levels for M2 of 9.8 each for Proconsul Area 1 and Hylobates, compared with 15.8 for Proconsul Area 2 and 10.2 for Gorilla.

For the mesial buccolingual diameters, there were similar minor discrepancies: Table 3.5 shows that for M1 there are values for Proconsul Area 1, Proconsul Area 2 and Hylobates of

10.4, 15.3 and 9.5 respectively; whilst M3 for Proconsul Area 1, Proconsul Area 2 and

Hylobates are 9.7, 16.3 and 14.3 respectively. Finally, from Table 3.6, M1 shows values of 9.6, 11.2 and 11.8 respectively for Proconsul Area 1, Proconsul Area 2 and Hylobates; whilst

M2 shows values, again for these groups respectively, of 11.5, 17.5 and 12.1

With these relatively few exceptions throughout, the tables seem to confirm the proposition that there was a much greater degree of variation within Proconsul, whether considered from the viewpoint of upper or lower molars, meristic positions or the fossil sites from which the samples had been drawn, compared with the results relating to any of the extant groups. The next highest levels of variation overall were those noted from within the gibbons and this feature in itself possibly reflects the issue, previously discussed, as to whether the variation levels within this group reflect intergeneric as opposed to interspecific factors.

It seems to be the case that, for all of the extant great apes, levels of variation within these genera, as measured in terms of Coefficients of Variation, are strikingly lower than the levels noted within Proconsul.

TCBA M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 74.1 18.5 25.0 81.0 23.7 29.3 100.5 29.5 29.3 Area 2 75.1 22.5 30.0 104.8 40.0 38.2 128.1 43.6 34.0 Hylobates 36.1 7.8 21.6 41.2 10.7 26.0 34.6 11.2 32.4 Gorilla 178.4 26.5 14.8 202.1 35.0 17.3 171.9 33.9 19.7 Pan 94.0 15.2 16.2 91.2 11.7 12.8 78.3 10.3 13.2 Pongo 133.2 14.1 10.6 136.8 21.0 15.4 118.6 20.7 17.4 Table 3.1 Coefficients of Variation of Total Cusp Base Areas (TCBAs) for all groups. Upper Molars.

TCBA M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 62.2 11.8 19.0 81.7 17.6 21.5 88.3 31.7 35.9 Area 2 86.4 19.6 22.7 112.3 34.7 30.9 122.5 34.5 28.2 Hylobates 33.2 8.0 24.1 39.3 10.6 27.0 39.0 11.9 30.5 Gorilla 178.1 23.5 13.2 221.5 42.7 19.3 212.2 41.9 19.8 Pan 86.8 13.8 15.9 96.9 13.5 13.9 84.7 13.3 15.7 Pongo 129.9 18.0 13.9 148.3 17.8 12.0 138.3 23.2 16.8 Table 3.2 Coefficients of Variation of Total Cusp Base Areas (TCBAs) for all groups. Lower Molars.

MD M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 8.5 1.2 14.1 8.3 0.9 10.8 9.9 2.0 20.2 Area 2 8.5 1.4 16.5 7.3 0.6 8.2 11.5 2.1 18.3 Hylobates 6.4 0.9 14.0 6.9 1.2 17.3 6.2 1.3 21.0 Gorilla 14.6 1.3 8.9 15.9 1.6 10.0 14.7 1.6 10.9 Pan 10.0 0.9 9.0 9.9 0.8 8.1 9.2 0.7 7.6 Pongo 12.1 0.7 5.8 12.2 0.9 7.4 11.3 1.1 9.7 Table 3.3 Coefficients of Variation of mesiodistal lengths (MD) for all groups. Upper Molars.

MD M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 8.9 0.9 10.1 10.2 1.0 9.8 11.4 2.2 19.3 Area 2 10.5 1.3 12.4 12.0 1.9 15.8 13.7 2.0 14.6 Hylobates 7.0 1.0 14.3 10.2 1.0 9.8 7.4 1.4 18.9 Gorilla 15.8 1.0 6.3 17.6 1.8 10.2 17.9 2.0 11.2 Pan 10.7 0.8 7.5 11.2 0.8 7.1 10.4 0.8 7.7 Pongo 13.1 0.8 6.1 13.8 0.8 5.8 13.7 1.2 8.8 Table 3.4 Coefficients of Variation of mesiodistal lengths (MD) for all groups. Lower Molars.

MesBL M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 9.6 1.0 10.4 10.0 1.6 16.0 11.4 1.1 9.7 Area 2 9.8 1.5 15.3 11.6 2.2 19.0 12.3 2.0 16.3 Hylobates 6.3 0.6 9.5 6.7 0.8 11.9 6.3 0.9 14.3 Gorilla 14.0 1.1 7.9 14.9 1.4 9.4 13.8 1.2 8.7 Pan 10.5 1.0 9.5 10.7 0.8 7.5 10.2 0.8 7.8 Pongo 12.6 0.8 6.4 13.3 1.0 7.5 12.7 1.1 8.7 Table 3.5 Coefficients of Variation of mesial buccolingual diameters (MesBL) for all groups. Upper Molars.

MesBL M1 M2 M3 Mean SD CV Mean SD CV Mean SD CV Area 1 7.3 0.7 9.6 8.7 1.0 11.5 9.3 1.8 19.4 Area 2 8.9 1.0 11.2 10.3 1.8 17.5 10.7 1.4 13.1 Hylobates 5.1 0.6 11.8 5.8 0.7 12.1 5.7 0.8 14.0 Gorilla 12.6 1.0 7.9 14.3 1.6 11.2 14.0 1.4 10.0 Pan 9.0 0.7 7.8 9.8 0.8 8.2 9.3 0.9 9.7 Pongo 11.2 0.7 6.3 12.2 0.8 6.6 11.7 1.1 9.4 Table 3.6 Coefficients of Variation of mesial buccolingual diameters (MesBL) for all groups. Lower Molars.

Bivariate Plots

The bivariate plots depicted below represent only a sample drawn at random from those generated throughout the analysis, but are nonetheless consistent with the appearances noted from the majority of these outputs. In the case of both upper and lower molars it was feasible to calculate linear elements such as buccolingual diameters against mesiodistal lengths, whilst each of the individual cusp areas, as well as trigon/trigonid and talon/talonid ratios, were assessable against total cusp base areas (TCBAs). In the majority of cases, analyses of cusp areas vs. cusp base areas were the most meaningful, in the context that they seemed to provide good evidence of separation of clusters into two distinct groups , most particularly in the case of the Proconsul sites.

Two examples of bivariate outputs generated have been provided in Figs. 3.1 and 3.2; these concentrate largely upon consideration of trigon/trigonid and talon/talonid ratios to cusp areas for various molars, including again the meristic position with the largest Proconsul representation of 32 specimens at M2 (19 at Area 1, 13 at Area 2). They seem to exhibit good evidence for the presence of two distinct clusters at each of Areas 1 and 2. For reference, I have also included examples from extant groups Gorilla and Hylobates and these also seem to demonstrate good separation of the species within each of the genera.

The two examples shown for Proconsul in Figs. 3.1 and 3.2 are consistent with the majority of bivariate plots for this genus from all meristic positions, in the context that they demonstrated good separation of specimens into two distinct clusters at each site.

Given the discussion which I provided earlier concerning the selection of gibbon species, Figs. 3.1 and 3.2 seem to support the concept for good separation within genus Hylobates, particularly if one accepts the distinction between H. lar and S. syndactylus as species. Again, I believe that these findings are consistent with the results obtained from coefficients of variation.

There were relatively few examples from with the Proconsul assessments, from either of Areas 1 and 2, in which good evidence was not obtained for a separation of specimens into two distinct clusters. Fig. 3.3 shows one of the least convincing results in this regard from amongst the entire data set; this relates to the M3s at Area 1. Interestingly, the specimen labelled M16647 is the holotype specimen for P. nyanzae; the plots depict the results for each cusp element when plotted against TCBAs and it can be seen that the protocone and

56 hypocone of this specimen, in particular, show poor evidence for a clear separation between species. The paracone and metacone demonstrate perhaps slightly better evidence, whilst the trigon and talon provide results which are more consistent with findings for all other Proconsul specimens from all meristic positions.

Shown for reference is KNM-RU 2088F, whose potential attribution is the subject of later discussion. The varying patterns seen with regard to these two specimens might be an explanation for the difficulties sometimes associated in attempting to differentiate large specimens of P. heseloni from smaller ones of P. nyanzae. This is an issue which has been reported on numerous previous occasions (Andrews, 1978a; Bosler, 1981; Kelley, 1986; Pickford, 1986a; Walker et al, 1993).

Fig. 3.1 Bivariate plots for Proconsul M2 trigon and talon, Areas 1 and 2. Reference plots for Gorilla and Hylobates trigon to compare degree of species differentiation within extant genera.

Fig. 3.2 Bivariate plots for Proconsul M1 trigonid and talonid, Areas 1 and 2. Reference plots for Gorilla and Hylobates talonid to compare degree of species differentiation within extant genera.

Fig. 3.3 Bivariate plots for Proconsul: least successful plots attempting to show separation of species within an area. M 16647 is P. nyanzae holotype.

Figures 3.4a and 3.4b (page 63) are included in order to demonstrate the difficulties which can be encountered in attempting to attribute a particular specimen to one or other Proconsul species when the specimen in question exhibits cusp areas which are potentially either at the upper range of the smaller species or conversely at the lower range of the larger one. This is a situation which has been encountered particularly at Area 1, where debate has arisen concerning attribution of a particular specimen either to P. heseloni or to P. nyanzae.

Problematic Specimens

Walker et al (1993: 52) considered a number of specimens which they regarded as ‘problematic’, including KNM-RU 1674 and KNM-RU 2087. Walker et al noted that KNM- RU 1674 had been regarded by Andrews (1978a) as an example of P. nyanzae; however, they themselves noted that its C1/M1 ratio was clearly that of a male, whilst its molars were only slightly larger than others within the P. heseloni sample. On this basis, they attributed it to P. heseloni as earlier proposed also by Greenfield (1972).

They similarly (Walker et al, 1993: 54) regarded KNM-RU 2087 as a ‘comfortable’ P. heseloni, notwithstanding that it had been attributed by Andrews (1978a) to P. nyanzae. They considered that ‘the addition of its tooth dimensions into those from the P. heseloni sample does not lead to excessive variation in tooth size’.

If the total cusp base areas for specimens KNM-RU 1674 and KNM-RU 2087 are considered against the mimimum, mean and maximum values of TCBAs for M2 and M3 at Area 1, excluding these two specimens from the rest of the data set, these were the results which I obtained (Table 3.7):

P. heseloni KNM-RU KNM-RU P. nyanzae min. mean max. 1674 2087 min. mean max.

M2 total cusp base area 59.6- 69.4- 76.8 84.5 86.2 92.5- 103.1- 120.8

M3 total cusp base area 50.9- 64.2- 77.7 91.8 97.0 91.8- 132.4- 139.9

Table 3.7 Total cusp base areas for M2 and M3 of two specimens compared to ranges of TCBAs of all other specimens of P. heseloni and P. nyanzae at Area 1.

In other words, both M2 and M3 of KNM-RU 1674 appear to remain above the upper end of the range for P. heseloni, whilst those for KNM-RU 2087 are similarly below the range for

61 all the other specimens of P. nyanzae for M2 and towards the lower end of the minimum range for M3.

The issue clearly remains complex. In this study, in order to create bivariate plots which include these specimens, I was obliged to make attributions of some kind, even if initially on a speculative basis. I chose to attribute KNM-RU 1674 to P. heseloni and KNM-RU 2087 to

P. nyanzae; Figs. 3.4a and 3.4b show the results when the areas of M2 and M3 trigonids and talonids are plotted against total cusp base areas. The situation with regard to the trigonids seems to suggest that my attributions hold up, but the talonids fail to support the attribution to the same extent: in this case, there was literally no observable separation between the specimens with regard either to M2 or to M3

I undertook these bivariate plots for all cusp areas and for both molars and I can confirm that, with the exception of the trigonids as just discussed, it was literally impossible to find any meaningful separation of the two specimens in question.

This issue will be addressed further in Discussion. At this stage I can only remark that these outcomes confirm why there has been such dissension regarding attribution of these specimens amongst previous assessors.

Fig. 3.4a Bivariate plot to demonstrate ‘problematic’ specimens (Walker et al, 1993) M2 trigonids and talonids assessed against total cusp base area. 59 is KNM-RU 1674 (attr. P. nyanzae, Andrews 1978a, P. heseloni, Walker et al, 1993) 60 is KNM-RU 2087 (attr. P. nyanzae, Andrews 1978a, P. heseloni, Walker et al, 1993) This study: KNM-RU 1674 attributed to P. heseloni, KNM-RU 2087 to P. nyanzae.

Fig. 3.4b Bivariate plot to demonstrate ‘problematic’ specimens (Walker et al, 1993) M3 trigonids and talonids assessed against total cusp base area. 633 is KNM-RU 1674 (attr. P. nyanzae, Andrews 1978a, P. heseloni, Walker et al, 1993) 634 is KNM-RU 2087 (attr. P. nyanzae, Andrews 1978a, P. heseloni, Walker et al, 1993) This study: KNM-RU 1674 attributed to P. heseloni, KNM-RU 2087 to P. nyanzae.

Specimen KNM-RU 2088

I refer to this specimen separately, even though it was one which had also been discussed by Walker et al (1993: 52) in their set of ‘problematic’ specimens. In fact, I found that, of the entire data set of 144 Proconsul molar specimens, this for me too was the most problematic in terms of its attribution to a particular species.

KNM-RU 2088 consists of a number of specimens which are believed to have come from the one individual. As well as a canine and a premolar, which clearly have nothing to do with the current study, the remnants are comprised of a right sided M2/M3 composite and the separated left antimeres of these same meristic positions. Given that I had concentrated on left-sided specimens, only these, respectively KNM-RU 2088E and 2088F, were used in my analysis.

Walker et al (1993) noted that, similar to the above cases of KNM-RU 1674 and KNM-RU 2087, Andrews (1978a) had attributed KNM-RU 2088 to P. africanus, whilst Teaford et al (1988) had treated it as P. nyanzae. Walker et al (1993) ended up favouring P. heseloni (sp. nov.).

I first attempted to deal with this question by undertaking a comparison of the total cusp base areas and of the areas of trigon and talon, of KNM-RU 2088, for each of M2 and M3, and comparing them with the means and minimum-maximum ranges for all the other specimens of P. heseloni and P. nyanzae excluding KNM-RU 2088 (see Table 3.8):

2 2 P. heseloni M KNM-RU 2088 P. nyanzae M min. mean max. min. mean max. TCBA 47.3- 70.4- 95.0 90.4 118.5- 124.7- 136.1 Trigon 36.8- 55.1- 73.8 70.1 89.7- 96.9- 102.7 Talon 10.9- 15.4- 22.1 20.9 21.7- 28.3- 33.5

3 3 P. heseloni M KNM-RU 2088 P. nyanzae M min. mean max. min. mean max. TCBA 66.4- 76.9- 88.1 82.3 103.1- 128.6- 145.2 Trigon 54.6- 62.5- 68.0 68.5 81.9- 97.7- 105.5 Talon 11.7- 14.9- 20.9 14.7 22.2- 32.4- 42.7 Table. 3.8 Comparison of the total cusp base area, trigon and talon of KNM-RU 2088 with means and minimum-maximum ranges of all other P. heseloni and P. nyanzae M2 and M3 from data set.

I next proceeded to address this question of attribution of KNM-RU 2088 to a particular species by assuming that it would be reasonable to test the specimens by the use of bivariate plots and T-tests, using attributions respectively to P. heseloni and then to P. nyanzae. The results for a sample of the bivariate plots generated by testing for particular cusp areas against total cusp base areas are set out below (Figs. 3.5a-3.5d):

Fig. 3.5a Bivariate plots for M2 of specimen KNM-RU 2088E (marker 34): area of trigon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right).

Fig. 3.5b Bivariate plots for M2 of specimen KNM-RU 2088E (marker 34): area of talon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right).

Fig. 3.5c Bivariate plots for M3 of specimen KNM-RU 2088F (marker 2): area of trigon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right).

Fig. 3.5d Bivariate plots for M3 of specimen KNM-RU 2088F (marker 2): area of talon vs.TCBAs. Attribution to P. heseloni (left), P. nyanzae (right).

I can confirm that bivariate plots of cusp areas for trigon, talon, paracone, metacone, protocone and hypocone were generated against total cusp base areas for both KNM-RU 2088E and KNM-RU 2088F (Table 3.9). Referring to the kinds of plots pictured above, these were the overall findings for M2 and M3 whereby a very good result, in terms of grouping of KNM-RU 2088 with P. heseloni, is represented by + and a less convincing result by +/-:

M2 M3 Trigon + + Talon + +/- Paracone + + Metacone +/- + Protocone + + Hypocone + +/-

Table 3.9 Summary of bivariate plots for KNM-RU 2088E & F, for all cusp areas relative to TCBAs. + denotes good grouping with P. heseloni, +/- slightly less satisfactory.

Summary of Bivariate Plots for all Proconsul Specimens

Notwithstanding these uncertainties regarding a minority of specimens, overall the results obtained from these bivariate plots for Proconsul did suggest the presence of two clusters at each of Areas 1 and 2, for all meristic positions save M2 from Area 2 (where, as has been noted previously, no specimens attributed by previous authorities to P. africanus were available for assessment). There appeared to be good separation between two species at each site, an appearance which was at least as marked for these fossil species as had been the case for the extant genera.

T-tests

Independent T-tests were performed to test for the proposition that the differences in means within each genus conformed with a level of significance p < 0.05 that there was a separation between Proconsul species at each of the areas under consideration.

The situation with regard to the entire Proconsul data set is displayed in Tables 3.10 and 3.11. These demonstrate that, particularly with regard to cusp area analyses, and for most meristic positions, there was good evidence for a separation of the two species identified at each site. This was certainly the case for all upper and lower molars at area 1 and for the first molars, upper and lower, at Area 2; but considerably less so for the Area 2 third molars, both upper and lower. The situation for lower molars at Area 2 was dominated by the fact that, as has been explained previously, no P. africanus M2 were available for measurement in the specimens which I examined; consequently it was not possible to generate an analysis for that region. This is the reason why asterisks appear for Area 2 M2 in Table 3.11.

Mesiodistal diameters for M2s at both Areas 1 and 2, and M3 at Area 2, also failed to satisfy the requirement of p < 0.05. However, these less satisfactory results for an element of linear measure are themselves partially offset by the fact that buccolingual diameters, whether mesial or distal again suggest significant differences in mean figures between two Proconsul species tested for at each of Areas 1 and 2.

Overall, the T-tests supported the concept that there was more than one Proconsul species present at each of Areas 1 and 2.

Area 1 Area 2 M1 M2 M3 M1 M2 M3 n 15 18 9 9 13 9 TCBA + + + + + + Trigon + + + + + + Talon + + + + + + Paracone + + + + + - Metacone + + + + + - Protocone + + + + + + Hypocone + + + + + - MD + - + + - - BL Mesial + + + + + + BL Distal + + + + + + Table 3.10 Proconsul Upper Molars (all specimens): T-tests. (+ indicates p < 0.05)

Area 1 Area 2

M1 M2 M3 M1 M2 M3 n 16 16 11 13 9 6 TCBA + + + + * - Trigonid + + + + * - Talonid + + + + * - Protoconid + + + + * - Hypoconid + + + + * - Metaconid + + + + * - Entoconid + + + + * - Hypoconulid + + + - * - MesioDistal + + + + * - BL Mesial + + + + * + BL Distal + + + + * +

Table 3.11 Proconsul Lower Molars (all specimens): T-tests. (+ indicates p < 0.05) * no analysis possible, because no P. africanus specimens available.

Specimen KNM-RU 2088

Compared with the bivariate plots, I obtained less satisfactory results when subjecting the M2 and M3 of this specimen to alternative analyses using T-tests, attributing it first to P. heseloni and then subsequently to P. nyanzae. Table 3.12 shows that there was no obvious distinction for M2 in this context, although the results were more convincing for M3, where all measurements except for the hypocone indicated that the specimen belongs with P. heseloni.

Walker et al (1993:52) had alluded to the difficulty in determining to which species this specimen belonged; they concluded that it was P. heseloni but without detailing why they had reached this conclusion.

M2 M3 P. heseloni P. nyanzae P. heseloni P. nyanzae TCBA + + + + Trigon + + + + Talon + + + + Paracone + + + + Metacone + + + - Protocone + + + - Hypocone + + - + Mesiodistal Length - - + + Mesial Buccolingual + + + - Distal Buccolingual + + + - Table 3.12 Results of T-tests for M2 and M3 of KNM-RU 2088, demonstrating attributions to P. heseloni and P. nyanzae respectively. + = p< 0.05 for each measurement.

CHAPTER FOUR: DISCUSSION

I believe that the results obtained from this study challenge the null hypothesis that only one Proconsul species existed at each of the clusters of fossil sites in western Kenya, namely at Rusinga/Mfangano on the one hand and at Koru, Songhor and the associated mainland assemblages on the other. The results obtained from coefficients of variation in themselves arguably support only the contention that higher levels of variation were expressed at each fossil site than is the situation observed from within extant primate groups. However, a consideration of the results obtained both from bivariate plots and from T-tests supports the contention that there were at least two Proconsul species present sympatrically at each of Areas 1 and 2.

Background

As has been discussed extensively in Chapter One, the impetus behind the current study has been an attempt to address an issue relating to genus Proconsul which has been the subject of controversy for a period now exceeding forty years. Although Le Gros Clark & Leakey (1951) seemingly conceptualised Proconsul as comprising three separate species, differing essentially in terms of their size rather than on any other morphological basis, that concept first began to be challenged as a consequence of the observations of workers such as Greenfield (1972) and Bosler (1981).

Pickford (1986a) and Kelley (1986) independently began to argue the proposition that distinctions within Proconsul, particularly as noted at the island sites, but also to some extent on the mainland, were a reflection of extremes of sexual dimorphism rather than of speciation. This view was then in turn challenged by such workers as Ruff et al (1989) and Rafferty et al (1995), who considered respectively the materials from Rusinga/Mfangano and Napak, and who each challenged the ‘highly-dimorphic single-species’ concept as a violation of the concept of uniformitarianism. It is noteworthy that in each of these latter instances, part of the counter argument, i.e. for the presence of more than one species at each set of assemblages, was based upon consideration of postcranial material as well as the more numerous fossilised dental remains.

The current study has been performed against a background relating to a consideration of speciation within primate groups in general, as these issues apply both to extant genera and to the particular case of an extinct genus such as Proconsul. In this context, specific reference is again made to the various studies collated in Kimbel & Martin (1993).

This study has consisted of a specific approach, in which I have attempted, by the use of dental material in isolation, to address the potential for speciation within Proconsul by undertaking a statistical analysis in which the results obtained from a set of measurements from the palaeontological material have been compared with those obtained from a similar set of measurements derived from sex-matched samples of four extant groups.

I performed this current analysis in terms of an acceptance of the views put forward by others (Ruff et al, 1989; Rafferty et al, 1995) regarding the issue of uniformitarianism: namely, that there is every reason to believe that the same biological rules which operate for modern taxa, with regard to selection factors for variation, and ultimately speciation, have held true consistently throughout the biological record.

I would argue that the overall results of the current study support my statement that the null hypothesis outlined above can be challenged. It seems that each element of the statistical analysis performed in this study lends support to the argument that more than one species of Proconsul existed previously at each of the fossil sites which I have designated Area 1 and Area 2.

Coefficients of Variation (CVs)

The results obtained from the Coefficients of Variation constructed for the current study suggest that the levels of variation within Proconsul, both at Area 1 and Area 2, are greater than those observed within other groups; this is certainly the case for the great apes, Pan, Gorilla and Pongo. The samples for all extant groups were deliberately chosen on a sex- matched basis between males and females, precisely in order to minimise, if indeed not to eliminate, the possibility that variation within those groups was in any manner due to extremes of sexual dimorphism.

The results obtained from this univariate analysis have been set out in Tables 3.1 to 3.6. As has been noted in the Results chapter, the analysis suggests that variation levels within Proconsul, at each of Areas 1 and 2, are greater than those observed within any of the sex- matched sampled for the extant species; this was certainly the case with regard to cusp area analyses, whilst the relatively few deviations from this pattern, in terms of aberrations related to linear measures, have been discussed in some detail.

Reference is made at this point to the results obtained from assessment of the gibbons: overall, it seemed that levels of variation within this group were overall lower than for Pronconsul from either Areas 1 or 2, but were also fairly consistently greater than those for

71 the great apes. This issue has been discussed above, concerning the manner in which the gibbon samples were selected, and I would argue that the relatively high levels of variation noted within the gibbons are in themselves probably reflective of the continuing debate, also discussed previously, as to whether there should be an interspecific or intergeneric separation between Hylobates and Symphalangus. I believe that the most appropriate means of taking this matter further would perhaps be to try to obtain additional samples, perhaps from two closely-related species, such as Hylobates lar and H. muelleri, in order to see whether, by thus narrowing the focus to this level, the levels of variation exhibited would be substantially lower than those obtained from the current study.

Overall, I conclude that the CVs support the proposition that levels of variation within Proconsul, at each of areas I and 2, exceed levels noted from within sex-matched samples taken from the great apes. The evidence obtained from the gibbons is slightly less supportive and the inference that I would draw from this is that, were the current project to be taken any further, then I would make a further attempt to elicit from the museum specimens a more restricted, indeed species-specific, data set. I think that it would be appropriate to try to obtain equal numbers of two of the more common Hylobates species, such as H. lar and H. muelleri, for such an analysis.

Bivariate Plots

Bivariate plots were constructed for all elements, both linear measurements and cusp areas for all molars, both upper and lower; a selection of these has been presented in Chapter Three (see Figs. 3.1 to 3.4 inclusive). Overall there was a consistency of results obtained throughout, particularly insofar as the Proconsul assessments were concerned. In both Areas 1 and 2 there was good separation of two clusters, with very little overlap no matter which particular cusp area was being considered.

The general character of these findings, which suggest that there is a significant distinction between two clusters of specimens at each of Areas 1 and 2, are arguably supported by the fact that previous observers have commented upon the differences in the four Proconsul species under consideration in the current analysis: namely, P. heseloni (formerly described as P. africanus) and P. nyanzae at Area 1; P. africanus and P. major at Area 2.

Without wishing to address the sometimes vexed issues regarding taxonomies within the greater and lesser apes in general, I would merely suggest that the bivariate plots

72 obtained for Proconsul in this study reveal at least as good a degree of differentiation between potential species as was the case for each of Pan, Gorilla, Pongo and Hylobates/Symphalangus.

T-tests

The results of these are again generally consistent throughout and suggest that more than one species is present at each of Areas 1 and 2.

In terms of cusp area analyses, as seen in Table 3.10, all the upper molars in Area 1 showed good evidence for separation into two species, as did the M1s and M2s at Area 2. M3 at Area 2 was slightly less convincing, as only the trigon, talon and protocone showed levels of p < 0.05 to support separation of P. africanus from P. major. There is no obvious explanation for this, given that 9 specimens for M3 were available for measurement from each area.

Within the lower molars, (Table 3.11) M1 again suggested good evidence for two species within each area. For the M3s, whilst Area 1 showed good separation for all cusp areas, the result at Area 2 was unsatisfactory; this outcome is possibly explained by the fact that, with just 6 specimens available for analysis (2 P. africanus, 4 P. major) the figures were too low to permit meaningful analysis.

Table 3.12 shows that, with regard to cusp area analyses, the results for M3 can be varied according to whether specimen KNM-RU 2088 is attributed to P. heseloni or P. nyanzae. However, the balance taken across all cusps seems to favour attribution to P. heseloni.

As far as analysis of linear measures was concerned, the T-tests were slightly less supportive for separation into two species at each area than were the cusp area analyses. The results for mesiodistal lengths showed a mixed result, although by comparison both the mesial and distal buccolingual results were consistent throughout.

Area 1: Rusinga/Mfangano

As has been noted in the historical survey, Walker et al (1993) reviewed the situation with regard to the smaller of each of the two species alleged to be present at each of the two assemblages, Area 1 and Area 2. They point to a number of issues which have contributed to the debate regarding the number of Proconsul species, including the fact that, although the holotype of P. africanus, and indeed the first Proconsul specimen of any

73 kind, had been retrieved from Koru, the bulk of the subsequent material attributed to P. africanus had been obtained from the island sites of Rusinga and Mfangano. The island sites had subsequently assumed the referent role for the species P. africanus.

Greenfield (1972) might perhaps be regarded as having initiated much of the debate between the proponents of the concept that two Proconsul species were present at a particular site, as opposed to those (Kelley, 1986; Pickford, 1986a) who argued instead for high levels of sexual dimorphism within each Proconsul taxon. Greenfield made the point that, up to the time of him reporting, the only specimen of P. africanus which had been identified as male was the holotype, M 14084.

He reviewed a number of specimens which had been attributed previously to P. nyanzae and argued, mainly on the basis of constructing indices for both canines and posterior teeth, that a number of smaller specimens, previously attributed as females of P. nyanzae, were actually males of P. africanus. He considered both maxillary and mandibular specimens and as part of his analysis drew for comparison from similar indices constructed for Pan and Gorilla.

Greenfield did not identify the source of most of his specimens, but it is clear that he was discussing M 14084, derived from Koru, and contrasting its indices with a number of specimens of P. nyanzae; most if not all of the latter had presumably been secured from the island sites (he does not indicate specifically whether he believed P. nyanzae to have been present on the mainland at all). Given the period from which he was reporting, there had been no suggestion at that stage that P. africanus from the mainland differed in any regard from the smaller specimens obtained at Rusinga/Mfangano.

Greenfield was obviously not attempting to differentiate between assemblages obtained from different areas, but rather seems to have been attempting to differentiate P. africanus in toto from P. nyanzae, across all sites without distinction, based upon specific dental features. He was able to argue for the concept of a bimodal sexual distribution within this smaller species (Dryopithecus africanus as it was then known).

His hypothesis had been subjected to further consideration, particularly by Bosler, (1981), who had somewhat similarly attempted to separate Proconsul samples into groups, based upon morphological considerations, and, in her case, specifically ignoring sites of origin.

The debate which underpins the current study had subsequently arisen (Kelley, 1986; Pickford, 1986a): whether the smaller examples from Rusinga were just representative of female specimens of the larger P. nyanzae at the same site; or whether, as had been argued initially by Le Gros Clark and Leakey (1951) and subsequently by others (Andrews, 1978; Teaford et al, 1988, 1993; Cameron, 1991), that two species, of substantially different sizes, were present at the island sites. Pickford (1986a) had argued additionally that P. africanus itself, first identified at Koru, was also highly sexually dimorphic, although he did accept that two very distinct species, P. africanus and P. major, had been present contemporaneously at the mainland sites.

Notwithstanding which side of the multiple species/sexual dimorphism debate had been adopted by any particular authority, Walker et al (1993) made the point that all protagonists were essentially agreed upon the proposition that the smaller of the two groups of specimens retrieved at Rusinga were not conspecific with the examples of P. africanus from Koru. Again, the question arose as to whether the approximately 2 Ma time separation between island and mainland sites had been a factor in influencing this seeming differentiation.

(a) Two species present at Rusinga/Mfangano

1. General Considerations

Walker et al addressed two separate issues in their paper (1993); first, they used a combination of dental and postcranial material in statistical analyses to substantiate the presence of two contemporaneous Proconsul species, markedly dissimilar in body weight, at Rusinga/Mfangano; secondly, they addressed the issue that P. africanus as demonstrated at the Tinderet sites was different in character from the smaller of their two putative species at Rusinga.

Depending upon the particular criteria employed (such as femoral cross sections, C1/M1 ratios or combinations), they felt able to conclude that an adult of the smaller species at Rusinga, the one originally considered conspecific with P. africanus at Koru, had weighed somewhere between 8.9 Kg if based upon femoral cross section, 16.7 Kg if based upon dental evidence. The larger species had by contrast exhibited weights of up to 36.8 Kg based on postcranial bones and 37.7 Kg based on dental evidence.

These results helped to validate the original assessments by Le Gros Clark and Leakey (1951) of a small, gibbon-sized animal and a larger creature, about the body mass of a female chimpanzee, at Rusinga.

2. Problematic Specimens (other than KNM-RU 2088)

In their discussion, Walker et al (1993) reported a number of specimens which they considered problematic: KNM-RU 2088, KNM-RU 1674 and KNM-RU 16000; they also made some remarks concerning KNM-RU 2087, which they ended up attributing as a large male P. heseloni.

The situation regarding KNM-RU 2088 will be considered separately below.

KNM-RU 1674 had been assigned by Andrews (1978a) to P. nyanzae. Walker et al (1993) noted that its C1/M1 ratio defined it as a male; given that its molars were little larger than others in the P. heseloni sample, they also assigned it to the smaller species, in line with Greenfield(1972).

KNM-RU 16000 was discussed in detail by Walker et al in their analysis (1993). Recognising it as a male, they discussed potential attributions either to P. heseloni or P. nyanzae, but ended up suggesting that it was a small male of P. nyanzae with large canines but relatively small molars. I had no access to specimen KNM-RU 16000 in my study.

As far as specimens KNM-RU 1674 and KNM-RU 2087 in my analysis are concerned, on the basis of bivariate plots considered in isolation, KNM-RU 1674 was probably P. heseloni, whilst the position with regard to KNM-RU 2087 was more complex.

For M2 the plots overall seemed to suggest that both KNM-RU 1674 and KNM-RU 2087 cluster towards the upper limits for P. heseloni generally, although in some plots there was evidence of overlap with the lower range for P. nyanzae. For instance, whilst the ratios of trigonid areas to TCBAs seemed more to be suggestive that KNM-RU 1674 grouped with P. heseloni and KNM-RU 2087 with P. nyanzae, the ratios for talonids to TCBAs failed to separate the two of them. The metaconid to TCBA ratios weakly suggested attribution of KNM-RU 1674 to P. heseloni and KNM-RU 2087 to P. nyanzae, whilst the remaining cusps seemed not to favour one taxon over the other.

For M3 the plots for talonid and hypoconid seemed slightly to favour attribution of KNM-RU 2087 to P. nyanzae, but even this appearance was not dramatic; in most of the

76 remaining plots the two specimens under consideration fell midway between the clusters for P. heseloni and P. nyanzae.

Nevertheless, I note that Kelley (1986) based his attributions very substantially upon canine morphology and in this context argued that both KNM-RU 1674 and KNM-RU 2087 were males; he also attributed both to P. nyanzae. A potential complicating feature relates to the possibility that some degree of metrical overlap might have occurred between sympatric species at the same site, anyway. Particularly if considered in terms of the usual size disparities between males and females within a single extant primate species, it is not inconceivable that, at Rusinga, a specimen which has been identified as a large male of P. heseloni might just as easily turn out to be a small female of P. nyanzae. In such an instance, Pickford’s (1986a) initial allocation of sexes based upon canine morphology might be of very considerable significance indeed.

These findings are perhaps not surprising, given the context of the entire debate regarding the situation which had existed at Rusinga. Whilst the bivariate plots tend to confirm that two separate clusters were present, the presence of such ‘problematic’ specimens as KNM-RU 1674 and KNM-RU 2087 does raise the issue as to whether the apparent separation between P. heseloni and P. nyanzae is being blurred by the presence of individuals such as these whose measurements indicate that they are either extremely large members of the smaller species, or, conversely, smaller members of the larger one.

An additional complication is the fact that both KNM-RU 1674 and KNM-RU 2087 seem to be regarded (Teaford et al, 1988; Walker et al, 1993) as males. Consequently, if KNM-RU 2087 is to be regarded as belonging to P. nyanzae, then it must have been a smaller male specimen.

Whilst I felt that the results obtained from the current study, relating to KNM-RU 1674 and KNM-RU 2087 in isolation failed to endorse dramatically the superiority of one attribution over another, I believe that the findings from these bivariate plots cannot be considered in isolation, but have to be considered against the overall results obtained from the analyses performed as part of the current study. Given that the study was constructed with a view to attempting to eliminate sex-based differences, by selecting from the extant groups on a 1:1 basis as between males and females, the fact that two clusters can be demonstrated from the bivariate plots for Area 1 lends support to the concept that more

77 than one species was present. These bivariate plot results are supported by the findings obtained separately from the coefficients of variation and the T-tests.

Consequently, the presence of ‘problematic’ specimens such as KNM-RU 1674 and KNM- RU 2087 is not believed to undermine the overall argument for the presence of two sympatric species.

On the basis of my own findings, I would be inclined to agree with Walker et al (1993) in their recommendation that KNM-RU 1674 is probably a larger male of P. heseloni and I would therefore assign it to that species. However, KNM-RU 2087 exhibits a TCBA which, whilst it falls below the minimum figure for P. nyanzae, demonstrated both in its trigonids and talonids, for M2 and M3, a tendency to group with P. nyanzae. I would therefore suggest that it possibly belongs to that species.

This conclusion is unavoidably at odds with that discussed by Walker et al (1993: 54); they seem to make no mention of the canines specifically and the impression received is that, when they state that specimen KNM-RU 2087 ‘makes a comfortable male P. heseloni’, they are assessing it on the basis of tooth dimensions overall and not from the size of the canines in isolation.

Assessment of canines has not formed any part of this study and all that I can suggest that, if KNM-RU 2087 is indeed a male, then it is one whose molar characteristics are indeterminate between P. heseloni and P. nyanzae. On the basis of the trigonids, it groups with P. nyanzae; but, as far as the talonids are concerned, there is no obvious separation between the cusp-area-to-TCBA ratio for this specimen and for that of KNM-RU 1674.

3. KNM-RU 2088

Andrews (1978a) attributed the Rusinga specimen KNM-RU 2088 to P. africanus, according to the nomenclature then current; whilst Pickford (1986a) attributed it to P. nyanzae. Walker et al (1993) described this specimen as being one of four individuals from Rusinga which they regarded as problematic. They noted that Andrews (1978a) had described it as a male of P. africanus whilst Teaford et al (1988) had attributed it to P. nyanzae. Walker et al stated that the specimen was definitely a male and that its molars lay at the upper end of the range for P. heseloni. They concluded by attributing it to this taxon.

Quite independently, I also found this specimen to be the one to have afforded me greatest difficulty with attribution in my own analysis. This took the form of a metric

78 analysis in which I considered the situation of KNM-RU 2088, with regard to its total cusp base areas and areas of trigon and talon, set against the minimum-mean-maximum ranges of similar area measures for all other specimens of P. heseloni and P. nyanzae. Table 3.12 demonstrates that the area measures for the TCBAs, trigons and talons for M2 and M3 of KNM-RU 2088 each provided figures which lay towards or (for the talon) slightly above the maximum figures for P. heseloni in general, but were clearly substantially lower than the same figures which applied for P. nyanzae. This metric analysis provided a first indication that P. heseloni was the appropriate attribution for this specimen.

In order to try to test this supposition I then performed both the bivariate plots and the T-tests for KNM-RU 2088 in two forms, allowing for attributions respectively to P. heseloni and then to P. nyanzae (Figs. 3.5a-d, Table 3.8 summarise the bivariate plots; Table 3.12 summarises the T-tests for this specimen).

For M2, as far as area measures were concerned, these plots indicate a strong association for KNM-RU 2088E with P. heseloni for all cusps with the possible exception of the metacone. The linear measures also seemed to favour P. heseloni with the exception of a slight overlap for the mesiodistal length.

As regards M3, the results were again consistent with this specimen being an example of P. heseloni: indeed, the only slight anomaly was with regard to the hypocone which identified more with P. nyanzae. The plots for linear measures also seemed supportive of attribution to P. heseloni.

Under the circumstances, I believe that my results validate an attribution of KNM-RU 2088 to P. heseloni.

(b) Distinctiveness of P. heseloni from P. africanus

On the basis of their findings, and noting the possible distinction between the measurements obtained from dental specimens of the smaller of two species believed present at Rusinga, as against those of the holotype P. africanus at Koru, Walker et al (1993) suggested that the smaller Rusinga specimens represented a new species, Proconsul heseloni, which has gained wide acceptance (Rafferty et al, 1995; Harrison, 2002; Cameron, 2004; Nakatsukasa et al, 2004; Begun, 2007; Pickford et al, 2009; Harrison & Andrews, 2009; Hill et al, 2013; Cote et al, 2014). The holotype specimen was KNM-RU 2036.

Fig. 4.1 provides a comparison of the appearances of some molars of P. africanus and P. heseloni. Walker et al (1993) drew attention to the fact that the maxillary molars of P. africanus, as exemplified by specimen M 14084, show evidence of strongly-developed cingula involving each of the mesial, lingual and buccal margins, with occlusal ridges well- developed.

They suggested that the P. africanus lower molars exhibit poorly developed buccal cingula (see KNM-CA 1773 below). I am inclined, however, to query their observation that

M3 ‘tapers strongly distally’ (Walker et al, 1993: 50). If specimen M 14087, as pictured below, is any guide, then an element of distal tapering, whilst present, does not seem excessive.

They reported that in contrast P. heseloni showed less development of buccal cingula on the upper molars, an observation which again seems slightly suspect, given the appearance of KNM-RU 2016 as noted below; nor was I convinced regarding their suggestion that the lower molar buccal cingula were necessarily more developed (compare M3 KNM-RU 1927 with M2 M 14084, for instance). Comparing the hypoconulid on M 14087, P. africanus, with that present on KNM-RU 1927 , P. heseloni, seemed to me not to demonstrate any dramatic difference between the two specimens.

In terms of overall size of the total cusp base areas, there does seem to be a good match for all of the upper molars and for the M1s; M2s cannot be compared because of the lack of a P. africanus specimen, whilst the P. africanus M3 area is clearly much larger than that of P. heseloni.

In summary, for the specimens from both regions which I had the opportunity to study, the distinctions between P. africanus and P. heseloni did not appear dramatic. There are grounds for arguing that the somewhat subtle differences which appear to exist between the two species perhaps reflect the effects of the different time horizons at the Tinderet and Kisingiri sites.

M1: Left: M 14084 Proconsul africanus Right: KNM-RU 2036-DB Proconsul heseloni

M2: Left: M 14084 Proconsul africanus Right: KNM-RU 2016 Proconsul heseloni

M1: CA 1773 Proconsul africanus

M3: Left: M 14087 Proconsul africanus Right: KNM-RU 1927 Proconsul heseloni

Fig. 4.1 Comparison of occlusal surfaces of P. africanus (left column) and P. heseloni (right column), upper and lower molars.

All images: courtesy Dr. Varsha Pilbrow

Area 2: Separation of P. africanus from P. major

Le Gros Clark & Leakey (1951) were the first to define Proconsul in terms of three species, based on variations in size. They had considered material from both the island and mainland sites and seem to have concluded that the three species were each present across the two regions (i.e. Area 1 and Area 2 as defined in the current study). With the benefit of hindsight it might be assumed that they were hindered by not having at that time access to modern dating techniques, so that the significant time-gap of approximately 2 Ma between the older sites at Koru/Songhor on the one hand and Rusinga/Mfangano on the other was not apparent to them.

Pilbeam (1969) noted that the evidence for presence of P. nyanzae at Koru and Songhor was meagre and he had no difficulty in reassigning P. nyanzae from these sites as female individuals of P. major; this included an edentulous mandibular specimen, KNM-SO 404.

Martin (1981: 143) considered newly discovered material from Koru, including the mandibular specimen LG 452 and, whilst noting that there were some resemblances to the P. nyanzae material from Rusinga, described how the canine, in his words, ‘fell well down the P. nyanzae range in size’, whilst all the other teeth, and the mandibular proportions in themselves, were at the top of the range for P. nyanzae. Considering these disparities, he was inclined to contest that the specimen was attributable to P. nyanzae; additionally, it seemed to resemble the edentulous mandible, KNM-SO 404, which had been assigned by Pilbeam (1969) to P. major.

He considered additionally a number of newly-recovered right crowns, P3-M3, from Chamtwara (KNM-CA 387/391/390,388/389) and, whilst noting that there were resemblances to P. nyanzae material previously described, including the fact that M1 was smaller than M2, the sizes of these teeth were much larger than those of the P. nyanzae holotype. Similar considerations applied to isolated M1s and other specimens.

Martin noted that much of the new material from Koru, whilst falling towards the upper range for P. nyanzae, was morphologically more similar to larger P. major specimens from Songhor; he noted that Bosler (1981) had made a similar comment with regard to her Group 3 specimens. Martin concluded by suggesting the presence of rather smaller specimens of P. major from Koru as opposed to those from Songhor, but that there was considerable overlap in the size of specimens from the two sites. He propounded the presence of a sexually-dimorphic single species, P. major, between Koru and Songhor overall; a corollary of his argument seems to be the concept of removing P. nyanzae from consideration as a representative at the mainland sites altogether.

He did make the observation that when the largest and smallest Kenyan P. major specimens were considered in terms of males and females respectively, the degree of sexual dimorphism was equal to or slightly greater than the range noted within Pan but was well within the maximum observed within Gorilla; so that there seemed support for his conception of sexual dimorphism within P. major.

Pickford (1986a) argued strongly for the concept that sexual dimorphism levels within Proconsul overall were very high, particularly at Rusinga. However, with regard to the

82 alleged presence of P. nyanzae at the Tinderet sites, he suggested that the specimens at Songhor and Koru which had been considered (Le Gros Clark & Leakey, 1951; Bosler, 1981) to belong to this taxon were actually female examples of P. major. In so doing, he excluded P. nyanzae from Tinderet altogether. Similarly, he suggested that the alleged presence of P. major at Rusinga, resting solely upon the evidence of a single deciduous molar, KNM-RU 1767, was due to incorrect labelling as a result of confusion with a suid molar possessing the same catalogue number. On this basis, he doubted any presence for P. major at Rusinga.

Pickford (1986a) further noted that the paucity of P. africanus material from Tinderet, particularly based upon the presence of no more than four canines from that region, did somewhat undermine the potential for determining levels of sexual dimorphism within this taxon. As far as P. major at Tinderet was concerned, he suggested a likelihood of high levels of sexual dimorphism, but again conceded that the limited specimen numbers made this possibility difficult to confirm.

To sum up his position, Pickford argued for the likely presence of two different species at the Tinderet sites (i.e. P. africanus and P. major), each of which was probably sexually dimorphic. As for Rusinga, and as has been noted previously, he argued for the presence of a single, highly sexually-dimorphic species which he felt was P. nyanzae. Perhaps prefiguring Walker et al (1993), Pickford noted that the holotype P. africanus, the smaller species at Tinderet, was distinct morphologically from the smallest specimens from Rusinga, although these latter had also been ascribed to P. africanus (Le Gros Clark & Leakey; Bosler, 1981). In Pickford’s case, however, this was due to his belief that the small Rusinga specimens were females of P. nyanzae; as distinct from the case made later (Walker et al, 1993) that there were two species present at Rusinga but that the smaller of these was sufficiently different from P. africanus at Tinderet to be afforded the status of P. heseloni, a species in its own right.

Kelley (1986) whilst independently agreeing with Pickford’s position regarding the presence of just one Proconsul species at Rusinga, had no difficulty in endorsing the presence of two species, respectively P. africanus and P. major, at the mainland sites, Koru and Songhor. He noted that the earlier P. major specimens recovered appeared fairly monomorphic and possibly comprised mainly male examples; however, he also referred to the fact that both Pilbeam (1969) and Martin (1981) had identified a number of probable females and that the later addition of these to the collections had reinforced the proposition that considerable sexual dimorphism had existed within this taxon.

Walker et al (1993) referred again to the different time horizons at Tinderet and Rusinga as perhaps contributing to the distinction between P. africanus and P. heseloni. They also noted, with regard to the status of P. africanus if assessed in isolation at Area 2 (i.e. without contrasting P. africanus and P. major), that all analysis confirmed this material to support the null hypothesis of a single species. They had performed CV analysis as well as range- based estimates and, with the exception of a slightly high CV value for MD length of M1, the material referable to P. africanus in isolation confirmed that the single-species concept for P. africanus was valid. Variation levels within the taxon were compatible with those of modern species in terms of dimorphism.

This was in contrast to the argument made by Pickford (1986a) that a significant degree of sexual dimorphism might have existed within P. africanus in isolation.

Area 2: the current study

As regards Area 2 in the current study, I am obliged to reiterate that the material available from that region for this study was slightly less numerous than had been the case for Area 1. Within the material available to me for analysis, the smallest M2 from Area 2, in terms of total cusp base area, was KNM-CA 2229, which I measured to have a value of 93.95 sq.mm. The other eight specimens, all attributed to P. major, exhibited TCBAs ranging between 99.0 sq.mm. and 176.8 sq.mm. so that the value for KNM-CA 2229 was just below the lower boundary of this range. In addition to this fact I noted that Uchida (1996a) had clearly attributed the specimen to P. major. These facts reinforced my belief that it seemed unlikely that there were any P. africanus M2s included in my sample set.

Uchida (1996a), in her analysis of Proconsul material from the Tinderet sites, has referred to the relative paucity of mandibular specimens; her comments are certainly reinforced by the situation relating to the current study. Reference to the relevant bivariate plots will confirm that, for P. africanus at Area 2, as previously attributed in the literature, I had no more than two specimens to study from each of M1 and M3.

Notwithstanding these concerns, on the basis of the material which was analysed for this study, the results obtained from the various statistical analyses were supportive again of the contention that, for Area 2, as had been the case for Area 1, the null hypothesis was incorrect. There is more than one Proconsul species present at Area 2, the mainland sites. The coefficients of variation (Tables 3.1 to 3.6) and the bivariate plots (Figs. 3.1 and 3.2) support this proposition. The T-tests for the various meristic positions (Tables 3.10 and 3.11)

84 were less supportive of this proposition, although the results with regard to M1, M2,, M3 and

M1 again suggested the presence of two species, showing significant differences in mean values at each of Areas 1 and 2. The results for M2 at Area 2 were less supportive, as were those for the third molars in both upper and lower jaws.

For the bivariate plots (Figs. 3.1 and 3.2), it can be seen that there is good evidence for the presence of two distinct clusters at the Area 2 sites. This is again believed to represent strong evidence for the presence of two species within this region, in particular because there is no significant overlap at any point between the two taxa which have been designated P. africanus and P. major.

Insofar as the situation concerning Area 2 is concerned, the results obtained from the current study are believed merely to support a contention which has effectively existed ever since Le Gros Clark & Leakey (1951) first categorised Proconsul as consisting of three separate species. Although their supposition was that the three species were essentially sympatric across both the island and mainland sites, that initial proposition has been disputed and arguably disproven as discussed previously in this account (Kelley, 1986; Pickford, 1986a; Walker et al, 1993; and others). These other authorities seem to have had little difference in accepting the presence of two species, highly separable in terms of their sizes, at Area 2; most of the subsequent debate has centred around the much more vexing proposition at Area 1, where the argument has involved the alternate concepts of a single highly sexually-dimorphic species on the one hand, as opposed to the presence of two species.

Even Pickford (1986a) was prepared to countenance the presence of two species at Area 2; his concern seems to have been to argue that even P. africanus at this group of sites was sexually dimorphic. Walker et al (1993) in turn have shown that for the material attributed to P. africanus, when considered in isolation, there is evidence of sexual dimorphism, but not at a level which is inconsistent with that observed within extant primate genera such as Pan and Gorilla. Levels of variation demonstrated within this taxon do not controvert the null hypothesis.

I can only suggest that the current study supports the argument that more than one species was present at Area 2, the same proposition which I have advanced for Area 1. I believe there is evidence to justify the overturning of the null hypothesis at both areas.

Current Positions

It is hardly surprising that, given the substantial period which has elapsed since the debate concerning numbers of Proconsul species, as opposed to issues of sexual dimorphism, developed in the 1980s, some of the protagonists have modified their views with the passage of time. This point requires to be noted, merely in order to bring the issues raised in this thesis up to the present and to summarise attitudes which are currently held.

Throughout this discussion I have noted at length the earlier views of authorities such as Pickford (1986a) and Kelley (1986). However, by the turn of the millennium (Senut et al, 2000), Pickford had clearly come to accept the concept that two species of Proconsul (P. heseloni, P. nyanzae) had existed contemporaneously at Rusinga; these views were expanded upon substantially (Pickford et al, 2009) as part of the discussion in which an attempt was being made to differentiate Proconsul from Ugandapithecus.

I think it worthy of note also that Harrison (2010), although not a protagonist in the initial debate pitting species numbers versus sexual dimorphism, clearly lists P. heseloni and P. nyanzae as separate taxa, albeit making the point that P. nyanzae is morphologically very similar to P. heseloni, differing primarily in its larger size.

CHAPTER FIVE: CONCLUSIONS

I have undertaken the current study in an attempt to determine taxonomic attributions within the extinct primate genus Proconsul by performing a statistical analysis upon measurements obtained from their molar teeth; the intention has been to try to determine how the levels of variation observed with regard to specific dental measurements from the fossil specimens compare with those which have been shown to exist within sex-matched samples of extant hominoids.

The investigation has focused upon taking measurements from dentognathic samples obtained from both extant genera and genus Proconsul, with a specific emphasis upon the results achievable from examining digitised images of molar teeth. For fossil groups, teeth are the most numerous samples to remain following the effects of taphonomic processes and molars are generally well-represented in any assemblage. For a study such as the current one, molar teeth are particularly useful for examination, given that their relatively low profile renders them suited to assessment by two-dimensional analysis of linear measures and areas.

This study has concentrated upon assessing the results of measurements performed upon a collection of 144 examples of Proconsul molars which had themselves formed part of a much wider collection of dental material harvested from island and mainland sites within western Kenya. I have tested levels of variation, within both the Proconsul samples and from a sex-matched set of molar teeth taken from four extant groups of hominoids, by formally calculating coefficients of variation. Subsequently, I have performed analyses of the number of species present at each of these two assemblages of fossil sites by constructing bivariate plots and by performing independent T-tests upon the mean values obtained for the linear measures and cusp area analyses of Proconsul specimens at each of the fossil sites.

The current study has attempted to address the null hypothesis, that only one species was present at each of Areas 1 and 2, by considering levels of variation within Proconsul as against levels which might be identified amongst four groups of extant anthropoids. In particular, by having specifically sampled from amongst these extant groups on a strict 1:1 basis as between males and females, the intention has been to try to mitigate the effects of any sex imbalance in the fossil material.

On the basis of calculating the coefficients of variation as well as T-tests and bivariate analyses, I believe that this study supports the proposition that more than one species of Proconsul occurred sympatrically at each of two sets of assemblages which are known to have existed throughout the lower Miocene epoch in the region of western Kenya.

Both the bivariate plots and T-tests provided support overall for the proposition that two species of Proconsul existed sympatrically at each of Areas 1 and 2 as defined earlier in this study. There was generally good evidence for separation between species at each location, with only a limited number of exceptions.

For instance, my analysis suggested that specimen KNM-RU 1674 was an example of P. heseloni, an attribution which agreed with that of Walker et al (1993) but was in opposition to the original attribution by Andrews (1978a) to P. nyanzae. Conversely, it is my belief that specimen KNM-RU 2087 groups more satisfactorily with P. nyanzae, although the evidence is slightly less firm. My attribution would thus accord with the original designation by Andrews (1978a) and in this instance would contradict that of Walker et al (1993).

Finally, I found that specimen KNM-RU 2088, which comprised a number of separate maxillary remnants, seemed to group more satisfactorily with P. heseloni. The attribution of this specimen has in the past been a source of dispute between a number of researchers (Andrews, 1978a; Teaford et al, 1988; Walker et al, 1993).

From the molar material examined, I would make the following suggestions regarding hypodigms within each area: please refer to Tables 5.1(a) and 5.1(b) overleaf.

Area 1: P. heseloni Holotype: KNM-RU 2036 Area 1: P. nyanzae Holotype: M 16647

M1 M2 M3 M1 M2 M3

KNM-RU 1741 KNM-RU 1671 KNM-RU 2088A M16647L M 16647L M 16647L KNM-RU 1742 KNM-RU 1672 KNM-RU 2088F M16649 KNM-RU 1677A KNM-RU 1677 KNM-RU 1795 KNM-RU 1747 KNM-RU 1920 KNM-RU 1677A KNM-RU 1677H KNM-RU 1697 KNM-RU 1904 KNM-RU 2088E KNM-RU 1922 KNM-RU 1696 KNM-RU 2061 KNM-RU 1936 KNM-RU 1835 KNM-RU 7290 KNM-RU 1929 KNM-RU 1973 KNM-RU 1861 KNM-RU 2036DB KNM-RU 1904 KNM-RU 2036A KNM-RU 1954 KNM-RU 7290L KNM-RU 1973 KNM-MW 161 KNM-RU 2016 KNM-RU 2045 KNM-RU 2036A KNM-RU 7290 KNM-RU 1693 KNM-RU 1803

M1 M2 M3 M1 M2 M3

KNM-RU 1706L KNM-RU 1674L KNM-RU 1674L KNM-RU 1679 KNM-RU 1694 KNM-RU 1735 KNM-RU 1824L KNM-RU 1706 KNM-RU 1728 KNM-RU 1711 KNM-RU 1710 KNM-RU 1764 KNM-RU 2093 KNM-RU 1823 KNM-RU 1820 KNM-RU 1780 KNM-RU 1734 KNM-RU 1982 KNM-RU 2036L KNM-RU 1873 KNM-RU 1927 KNM-RU 1789 KNM-RU 1736 KNM-RU 2087L KNM-RU 7290L KNM-RU 1945 KNM-RU 1945 KNM-RU 1818 KNM-RU 1982 KNM-RU 1678 KNM-RU 1959 KNM-RU 2038 KNM-RU 2000 KNM-RU 2087L KNM-RU 5871 KNM-RU 2036L KNM-MW 50 KNM-RU 2032 KNM-RU 7290L KNM-RU 1767 KNM-RU 1676 KNM-RU 2087L KNM-RU 5871

N = 85

Table 5.1(a) Area 1: Hypodigms for Proconsul heseloni and Proconsul nyanzae as identified in this study.

Area 2: P. africanus Holotype: M 14084 Area 2: P. major Holotype: M 16648

M1 M2 M3 M1 M2 M3 M 14084 M 14084 M 14084 KNM-SO 418 KNM-SO 381 M 14331 M 14081 M 14081 KNM-SO 442 KNM-SO 542 KNM-SO 382 KNM-LG 1815 KNM-SO 528 KNM-SO 946 KNM-SO 934 KNM-SO 485 KNM-CA 389 KNM-LG 902 KNM-CA 568 KNM-SO 939 KNM-LG 924 KNM-CA 397 KNM-CA 1872 KNM-CA 390 KNM-CA 388 KNM-CA 1299 KNM-CA 1893 KNM-CA 392 KNM-CA 1780 KNM-KO 100 KNM-CA 2250

M1 M2 M3 M1 M2 M3

KNM-SO 903 M 14087 M 16648 KNM-SO 415 M 16648 KNM-CA 1773 ------KNM-LG 1389 KNM-SO 470 KNM-SO 472 KNM-SO 920 KNM-SO 541 KNM-SO 917 KNM-LG 452 KNM-SO 914 KNM-LG 47 KNM-CA 393 KNM-SO 915 KNM-LG 1472 KNM-SO 1113 KNM-LG 452 KNM-LG 452L KNM-CA 395 KNM-CA 394 KNM-CA 1298 KNM-CA 1856 KNM-CA 2229 KNM-SO 916 KNM-LG 1390

N = 59

Table 5.1(b) Area 2: Hypodigms for Proconsul africanus and Proconsul major as identified in this study.

Further Work

The current study would hopefully be strengthened by the retrieval of additional fossil samples from the regions in western Kenya and adjacent regions in eastern Africa from which these specimens have been drawn. In particular, it would be hoped that future studies could be strengthened by the retrieval of additional mandibular specimens, particularly those of P. africanus, from the mainland sites.

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Minerva Access is the Institutional Repository of The University of Melbourne

Author/s: Weaver, Hugh

Title: Taxonomy of Proconsul: an issue of species numbers or sexual dimorphism?

Date: 2015

Persistent Link: http://hdl.handle.net/11343/56376

File Description: Taxonomy of Proconsul: An Issue of Species Numbers or Sexual Dimorphism?