Phylogenetic, Ontogenetic, and Functional Implications of Hominoid Mandibular Corpus Shape Variation

Mary Kathleen Pitirri

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Anthropology University of Toronto 2019

by Mary Kathleen Pitirri

Department of Anthropology University of Toronto 2019

Abstract

Mandibular fragments are among the most commonly preserved elements in the primate fossil record. These specimens are often studied through linear measurements of mandibular corpus height and breadth, which are used to calculate mandibular robusticity (MR). Presently, the significance of mandibular corpus variation in both living and fossil hominoids remains unclear. Here, three separate analyses are conducted to develop and evaluate an alternative method to quantify hominoid mandibular corpus shape and to investigate the dietary, phylogenetic and ontogenetic significance of hominoid mandibular corpus shape variation in order to help interpret corpus shape variation in the primate fossil record. These analyses use landmarks and semilandmarks to capture the shape of the outline of the mandibular corpus in cross-section in a sample of extant great apes, corporal cortical bone distribution (CBD) in a sample of extant and fossil hominoids, and to assess ontogenetic changes in corpus shape and the

ii relationship between these changes and molar crypt length, breadth and height in a sample of extant hominoids.

These results show that quantification and comparison of the shape mandibular corpus in cross-section is a preferred alternative to MR in studying the hominoid fossil record.

Additionally, extant hominoids are found to have significant CBD shape differences that are phylogenetically significant and do not match morphological predictions based on diet.

Investigation into ontogenetic changes in corpus shape shows clear differences in growth patterns among all three species prior to the emergence of M1, and finds a significant covariance between molar crypt form and corpus shape during the developmental stage marked by the emergence of M1.

This research is significant because it provides support for the hypothesis that hominoid mandibular corpus shape is influenced by the development of the molars in their crypts during development. These results also indicate that both corpus shape and corporal CBD shape are taxonomically and phylogenetically significant and do not match morphological predications based on diet in hominoids. Additionally, this research shows that quantification and comparison of the outline of the mandibular corpus is a preferred alternative to MR in studying the hominoid fossil record.

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Acknowledgements

I would like to take this opportunity to express my gratitude to my exceptionally patient and supportive doctoral supervisor, David Begun. It has taken us a very long time to get here.

You inspired me as an undergraduate student to change the course of my life to this point. You tolerated every annoying question, every stupid idea and every terrible sentence with unwavering support. I’ve learned so much from you and I cannot thank you enough for everything you have done for me.

I am also indebted to the other members of my core committee, Mary Silcox and Michael

Schillaci. Without your guidance and input I would have been lost. Your dedication to me as your student has given me such an amazing opportunity to learn and grown into a much stronger researcher than I would have without you.

I would also like to thank Jay Kelley for access to invaluable scans of fossil material and for his collaborative input in this research. Additionally, I would like to thank the Smithsonian’s

Division of Mammals and Human Origins Program for the scans of USNM specimens and Dr.

Emmanuel Gilissen for use of the scans of the RMCA specimens in this research.

Finally, I would like to thank my family and friends for their encouragement and support during every step of this process. Steve Dorland and Amy Beresheim thanks for the coffee breaks, football games and nights out. Halszka Glowacka, thank you for expertise, proof reading and decades of friendship. Mom, Connie, Jim, Ang, Tam, Dee, Will, Nick and RJ thank you for everything you have done to encourage me from childhood through to adulthood. To Ben and

Russ, thank you for inspiring me to achieve my goals, hopefully I am giving you the same gift.

Table of Contents Abstract ...... ii Acknowledgements ...... iv List of Tables ...... viii List of Figures ...... xi List of Appendices ...... xiii List of Appendix Figures ...... xiii Chapter 1 Introduction ...... 1 1.1 Background ...... 2 1.1.1 Diet and the Hominoid Mandibular Corpus ...... 2 1.1.2 Growth and the Hominoid Mandibular Corpus ...... 6 1.1.3 Phylogeny and the Hominoid Mandibular Corpus ...... 7 1.1.4 Measuring the Hominoid Mandibular Corpus ...... 8 1.2 Research Goals ...... 9 1.3 Organization of Thesis ...... 9 1.4 Literature Cited ...... 10 Chapter 2 A New Method to Quantify Mandibular Corpus Shape in Extant Great Apes: Implications for Interpreting the Hominid Fossil Record...... 20 2.1 Abstract ...... 20 2.2 Introduction ...... 21 2.3 Materials and Methods ...... 24 2.3.1 Sample ...... 24 2.3.2 MR Data Acquisition and Analyses ...... 24 2.3.3 Corpus Outline Data Acquisition and Analyses ...... 25 2.3.4 Comparison of MR with Analysis of Corpus Outline Shape ...... 27 2.4 Results ...... 27 2.4.1 Analysis of Corpus Shape from MR Approach ...... 27 2.4.2 Analysis of Corpus Outline Shape ...... 30 2.4.3 Comparison of MR Analysis with Analysis of Corpus Outline Shape ...... 33 2.5 Discussion ...... 37 2.6 Conclusion ...... 38 2.7 Literature Cited ...... 39

Chapter 3 Functional and Phylogenetic Implications of Cortical Bone Distribution in the Mandibular Corpus of Extant and Fossil Great Apes...... 48 3.1 Abstract ...... 48 3.2 Introduction ...... 49 3.3 Materials and Methods ...... 58 3.3.1 Sample ...... 58 3.3.2 Data Acquisition ...... 59 3.3.3 Geometric Morphometric Analysis ...... 62 3.3.4 Phylogenetic Analysis ...... 63 3.3.5 Analysis of Fossil Taxa ...... 65 3.4 Results ...... 65 3.4.1 Shape of corporal cortical bone distribution in extant great apes ...... 65 3.4.2 Analysis of phylogenetic signal in extant apes ...... 72 3.4.3 Corporal cortical bone distribution in Miocene hominoids ...... 72 3.5 Discussion ...... 74 3.5.1 Significance of patterns of corporal cortical bone distribution in extant hominoids ....74 3.5.2 Phylogenetic signal of corporal cortical bone in extant hominoids ...... 77 3.5.3 Affinities of Miocene hominoid corporal cortical bone distribution...... 78 3.6 Conclusions ...... 79 3.7 Literature Cited ...... 80 Chapter 4 The Relationship Between Growth, Dental Development and Mandibular Corpus Shape in Hominoids...... 95 4.1 Abstract ...... 95 4.2 Introduction ...... 96 4.2.1 Previous studies of mandibular corpus ontogeny ...... 97 4.2.2 Previous studies on the relationship between mandibular corpus growth and dental development ...... 98 4.2.3 Homology in the mandibular corpus during growth ...... 99 4.3 Materials and Methods ...... 100 4.3.1 Sample ...... 100 4.3.2 Data Acquisition and Analyses ...... 101 4.4 Results ...... 103 4.4.1 Ontogeny of mandibular corpus shape variation – PCA in shape-space ...... 103

4.4.2 Mandibular corpus ontogenetic trajectories – PCA in form-space ...... 105 4.4.3 Relationship between ontogeny of mandibular corpus shape and dental development ...... 107 4.5 Discussion ...... 112 4.5.1 Ontogenetic shape variation in the great ape mandibular corpus ...... 112 4.5.2 Relationship between molar crypt form and corpus shape during growth ...... 113 4.5.3 Implications for interpreting mandibular corpus shape in the fossil record ...... 115 4.6 Conclusions ...... 115 4.7 Literature Cited ...... 116 Chapter 5 Conclusion...... 125 5.1 Measuring the Mandibular Corpus ...... 125 5.2 Factors Influencing Mandibular Corpus Shape ...... 126 5.2.1 Diet ...... 126 5.2.2 Growth ...... 126 5.2.3 Phylogeny ...... 128 5.3 Interpreting Mandibular Corpus Shape in the Primate Fossil Record ...... 129 5.4 Literature Cited ...... 130 Appendix A: List of Abbreviations by Order of Appearance ...... 132 Appendix B: Full Research Sample ...... 134 Appendix Figures ...... 140

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List of Tables

Table 1: Hominoid taxa used in both analyses ...... 25

Table 2: Definitions of landmarks used in geometric morphometric analyses ...... 26

Table 3: ANOVA results from analysis using linear measurements testing for mandibular robusticity differences between genus and species and their interactions with sex ...... 27

Table 4: P-values obtained from Mann-Whitney pairwise comparison testing for species differences in mandibular robusticity based on linear measurements ...... 29

Table 5: P-values obtained from Mann-Whitney pairwise comparisons of all taxonomic groups testing for sexual dimorphism in corpus shape based on linear measurements ...... 29

Table 6: Procrustes ANOVA testing for shape and size differences between species and sex, and interactions between them ...... 31

Table 7: P-values obtained from pairwise comparisons across great ape species, based on 10,000 permutations of Procrustes distances, after standardising shape data for the effects of sex ...... 32

Table 8: P-values obtained from pairwise comparisons of all taxonomic groups, based on 10,000 permutations of Procrustes distances, testing for sexual dimorphism in corpus shape ...... 33

Table 9: Results of correlation and regression analyses comparing determinants of corpus shape from the MR method and the outline method ...... 36

Table 10: Mechanical properties for great ape diets ...... 54

Table 11: Morphological predictions based on diet ...... 56

Table 12: Extant hominoid taxa used in this study ...... 58

Table 13: Fossil hominoids used in this study ...... 59

Table 14: Definitions of landmarks used in geometric morphometric analyses ...... 61

Table 15: Definitions of curves used in geometric morphometric analyses ...... 61

Table 16: Procrustes ANOVA for analysis of extant hominoids testing for shape and size differences between species and sex, and interactions between them ...... 68

Table 17: Analysis of extant taxa p-values obtained from pairwise comparisons across great ape species, based on 1,000 permutations of Procrustes distances, after standardizing for the effects of sex ...... 70

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Table 18: Analysis of extant hominoids, p-values obtained from pairwise comparisons of all taxonomic groups, based on 1,000 permutations of Procrustes distances, testing for sexual dimorphism in corpus shape ...... 70

Table 19: Samples used in this analysis ...... 100

Table 20: Definitions of landmarks used in geometric morphometric analyses ...... 101

Table 21: Descriptive statistics for measurements of M1, M2 and M3 crypts in mm per DS .....108

Table 22: Results for the first two dimensions of the 2B-PLS analyses ...... 110

List of Figures

Figure 1: Cross section of Pongo pygmaeus CT scan in between right M1 and M2. A: Depiction of linear measurements traditionally used to quantify mandibular robusticity (width=solid line, height = dashed line). B: Depiction of the three landmarks (blue circles) and 60 semilandmarks (red circles) used to quantify mandibular shape in this analysis. L = Lingual, B = Buccal ...... 23

Figure 2: A) Plot of mandibular robusticity index (breadth/height) on geometric mean in great apes; B) Thin plate splines depicting the shape changes along the scores of PC1 and PC2 to illustrate the shape changes from the negative to the positive ends of the axes. Thin plate splines are computed by warping the respective PC scores onto the mean shape of all the specimens in the sample. L = Lingual, B = Buccal; C) PCA plot in shape-space showing variation along PC1 and PC2; D) Plot of multivariate regression of Procrustes shape coordinates on centroid size ...28

Figure 3: Comparison of cluster analyses results showing higher taxonomic discrimination from results produced by analysis of corpus outline relative the results of analysis produced using the mandibular robusticity index. A) Dendrogram produced from cluster analysis of linear assessment of corpus shape (UPGMA, cophen. corr.=0.7887); B) Dendrogram produced from cluster analysis of CVA results from assessment of the outline of corpus shape (UPGMA, cophen. corr.=0.8281). C) Dendrogram produced from cluster analysis of Procrustes distance matrix from assessment of the outline of corpus shape (UPGMA, cophen. corr.=0.7178). POP = P. pygmaeus; POA = P. abelii; GB = G. beringei; GG = G. gorilla; PT = P. troglodytes; PPA = P. paniscus ...... 30

Figure 4: Species mean corpus shapes warped to mean shape of the entire sample, enhanced by a factor of 2. L = Lingual, B = Buccal ...... 34

Figure 5: CVA plot of corpus shape variation in great apes based on the first two CVs, representing 71.84% of the total between species variation. Colors and shapes are the same as in Figure 2 ...... 35

Figure 6: Corporal cross-sections of extant and fossil hominoids included in this analysis. Images are not to scale. See methods for explanation of data acquisition for D. fontani specimen ...... 60

Figure 7: Depiction of evolutionary relationships of extant and fossil taxa included in this analysis ...... 61

Figure 8: Depiction of landmarks (red) and semilandmarks (blue) used to quantify the outline of cortical bone in the mandibular corpus in this study ...... 62

Figure 9: Phylogenetic tree of extant hominoids used in this study (10ktrees.fas.harvard.edu; Arnold et al., 2010) ...... 64

x hominoids; C) Thin plate splines depicting the shape changes along the scores of PC1 and PC2 to illustrate the shape changes from the negative to the positive ends of the axes. Thin plate splines are computed by warping the respective PC scores onto the mean shape of all the specimens in the sample. D) PCA plot in shape-space showing variation along PC1 and PC2 for analysis of fossil and extant hominoids; E) PCA plot in shape-space showing variation along PC1 and PC3 for analysis of fossil and extant hominoids ...... 66

Figure 11: Allometric results from analysis of extant great apes. A) Plot of multivariate regression of Procrustes shape coordinates on centroid size; B) Thin plate splines depicting the maximum and minimum allometric shape changes. Thin plate splines are computed by warping the respective shape coordinates onto the mean shape of all the specimens in the sample ...... 69

Figure 12: Thin plate splines of species mean shapes for analysis of extant hominoids. Each species mean shape is warped to the mean shape of the entire sample ...... 71

Figure 13: CVA plot of corporal cortical bone distribution in fossil and extant hominoids based on the first two CVs representing 83.77% of the total variation. D = D. fontani, R = R. hungaricus, S = S. sivalensis ...... 73

Figure 14: Neighbour Joining Tree based on Euclidean distances of CVA derived from analysis of the shape of corpus cortical bone distribution in fossil and extant hominoids. POP = P. pygmaeus; POA = P. abelii; GB = G. beringei; GG = G. gorilla; PT = P. troglodytes; PPA = P. paniscus; YPM = S. sivalensis ...... 74

Figure 15: Depiction of data used in this study. A) Coronal cross section of Pongo pygmaeus specimen between M1 and M2 depicting the three landmarks (blue circles) and 60 semilandmarks (red circles) used to quantify corpus shape in this analysis. B) Coronal cross section of M1 crypt of P. pygmaeus CT scan depicting measurements of crypt height (solid line) and crypt width (dotted line). C) Transverse cross section of M1 crypt of P. pygmaeus CT scan depicting measurement of crypt length (dashed line). L = Lingual, B = Buccal ...... 102

Figure 16: Bivariate plot of shape-space PC1 and PC2. A) Convex hulls indicating taxonomic groups with thin plate splines depicting the shape changes along the scores of PC1 and PC2. Thin plate splines are computed by warping the respective PC scores onto the mean shape of all the specimens in the sample. B) Convex hulls indicating DS. Pongo pygmaeus are represented in blue, Pan troglodytes in purple and Pan paniscus in red...... 104

Figure 17: Thin plate splines showing ontogenetic shape change within each species ...... 106

Figure 18: Bivariate plot of the first two PCs in form–space. Pongo pygmaeus are represented in blue, Pan troglodytes in purple and Pan paniscus in red ...... 107

Figure 19: Results of 2B-PLS analysis of DS1 corpus shape with M1 crypt form. A) Plot of first 2B-PLS dimension; B) 2B-PLS weights for the first dimension of M1-M2 crypt form; C) Thin plate splines (TPS) depicting the shape changes along the scores of the first dimension of DS1 corpus shape to illustrate the shape changes from the negative to the positive ends of the axis.

TPS were computed by warping the respective scores onto the score at zero. Colors and orientation of TPS are the same as in Figure 16 ...... 109

Figure 20: Results of 2B-PLS analysis of DS1 corpus shape with M1 and M2 crypt form. A) Plot of first 2B-PLS dimension; B) Plot of second 2B-PLS dimension; C) 2B-PLS weights for the first dimension of M1 and M2 crypt form; D) 2B-PLS weights for the second dimension of M1 and M2 crypt form; E) Thin plate splines (TPS) depicting the shape changes along the scores of the first dimension of DS1 corpus shape to illustrate the shape changes from the negative to the positive ends of the axis. F) TPS depicting the shape changes along the scores of the second dimension of DS1 corpus shape to illustrate the shape changes from the negative to the positive ends of the axis. TPS were computed by warping the respective scores onto the score at zero. Colors and orientation of TPS are the same as in Figure 16 ...... 111

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List of Appendices

Appendix A: List of Abbreviations by Order of Appearance ...... 132

Appendix B: Full Research Sample ...... 134

List of Appendix Figures

Figure A1: PCA on regression residuals showing very minimal change between and within extant hominoids (see Figure 10A) when size is regressed out of the analysis. Colors and shapes are the same as those in Figure 10...... 140

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Chapter 1

Introduction

Mandibular corporal fragments are among the most commonly preserved elements in the hominoid fossil record. Consequently, the mandibular corpus has been used to describe and interpret fossils representing every hominoid group, including Miocene apes (Brown, 1997;

Ravosa, 2000; Güleç and Begun, 2003; Koufos and deBonis, 2005; McNulty et al., 2015), australopithecines (Wood, 1991; Silverman et al., 2001; Ward et al., 2001; Kimbel et al., 2004;

Haile-Selassie, 2010; Haile-Selassie et al., 2015; Glowacka et al., 2017) and members of the genus Homo (Fabbri, 2006; Skinner et al., 2006; Lague et al., 2008; Brown and Maeda, 2009;

Chang et al., 2015). In order to understand the significance of the mandibular corpus variation in the fossil record, this aspect of anatomy has also been studied in extant apes and humans (Wood et al., 1991; Humphrey et al., 1999; Ravosa, 2000; Taylor, 2002, 2003, 2006a,b,c, 2009; Antón et al., 2011; Laird et al., 2017). However, despite being the focus of numerous analyses, the significance of variation in hominoid mandibular corporal morphology remains poorly understood. Primate mandibular corporal morphology has been hypothesized to be influenced by several factors including: diet, allometric scaling, sexual dimorphism, growth, and phylogeny

(Ravosa, 1991, 2000; Humphrey et al., 1999; Daegling and Jungers, 2000; Daegling and Grine,

2006; Ross et al., 2012). Consequently, further analysis of the significance of hominoid mandibular corpus shape in living great apes is necessary in order to help us interpret the significance of corpus variation present in the hominoid fossil record.

1.1 Background

For decades, researchers have attempted to find a link between diet and mandibular form in primates. To date, these studies have yielded mixed results and have not shown a clear relationship between diet and mandibular morphology, especially in the corpus of great apes

(Taylor, 2002, 2006; Daegling, 2007; Ross et al., 2012). It is often suggested that the problem is a lack of information on the physical properties of foods, ingestion methods, and/or chewing cycles (Bouvier, 1986; Ravosa, 1991; Taylor et al., 2008; Ross et al., 2012). It has also been suggested that mandibular corporal morphology is determined by non-mechanical factors such as growth (Cole, 1992; Daegling, 1996; Taylor, 2002; Boughner and Dean, 2004, 2008; Boughner,

2011; Ross et al. 2012; Coquerelle et al., 2010, 2011) or phylogeny (Bouvier, 1986; Brown,

1997; Ravosa, 1991; Singleton, 2000; Collard and Wood, 2001; Ross et al., 2012). In this section, I provide a brief review of the factors that are hypothesized to influence the morphology of the hominoid mandibular corpus.

1.1.1 Diet and the Hominoid Mandibular Corpus

The focus on the relationship between diet and mandibular corporal morphology in great apes has been largely driven by results from experimental studies on monkeys (Bouvier and

Hylander, 1981; Hylander, 1979a,b,c; Hylander et al., 1987). These experimental studies have suggested that increased magnitudes of bone strain in the primate mandible correlate with tougher and/or harder diets. This has led to the hypothesis that primates with relatively harder or tougher diets (such as folivores and hard-fruit eaters) and/or higher amounts of time spent chewing these types of foods, will exhibit mandibular morphologies that reflect their diet

(Bouvier and Hylander, 1981; Hylander 1979a,b,c). Relatively wider, taller corpora with

2 increased amounts of cortical bone have been suggested to be optimal for countering increased masticatory stress (Bouvier and Hylander, 1981; Hylander, 1979a,b,c; Hylander, 1984, 1985;

Daegling, 2007).

The apparent dietary differences present among extant great apes are hypothesized to result in differences in masticatory stress, resulting in differences in mandibular corporal morphology ((Taylor 2002, 2003, 2006a,b,c, 2009; Taylor et al., 2008). At the genus level,

Gorilla is the most folivorous of the great ape taxa (Tutin and Fernandez, 1993; Rogers et al.,

2004; Masi, 2008; Masi et al., 2009; Head et al., 2011), Pan is the most frugivorous (Tutin and

Fernandez, 1993; Conklin-Brittain et al., 1998; Wrangham et al., 1998; Yamagiwa and Basabose,

2009; Head et al., 2011; Watts et al. 2012; McLennan, 2013) and Pongo prefers ripe fruit, but during periods of fruit scarcity, this taxon incorporates hard fruits, terrestrial herbaceous vegetation (THV) and bark into its diet (Galdikas, 1998; Knott, 1998; Vogel et al. 2008; Vogel et al., 2009; Kanamori et al., 2010; Vogel et al., 2015). However, each of these genera are known to demonstrate intraspecies variation that makes these broad dietary generalizations at the genus level overly simplistic.

Within Gorilla, Gorilla beringei includes more THV than Gorilla gorilla (Watts, 1984;

Williamson et al., 1990; Remis et al., 2001; Goldsmith 2003; Rogers et al., 2004). Within Pongo,

Pongo pygmaeus is known to masticate more hard fruits, THV and bark relative to Pongo abelii

(Knott, 1998; Fox et al., 2004; Wich et al., 2006). At present, dietary differences between Pan species are less clear. Research shows that P. paniscus regularly incorporates THV in their diet, while P. troglodytes increases the amount of THV consumed during times of fruit scarcity

(Malenky and Wrangham, 1994). Additionally, P. panicus is known to switch to different types of fruits and THV when their preferred foods are not available (Serckx et al., 2015).

A growing body of research on the mechanical properties of great ape food items have found that these dietary differences translate into differences in dietary toughness and hardness

(Vogel et al., 2008, 2014; Coiner-Collier et al., 2016). Presently, these data are not available for all of the great apes. The mechanical properties of P. paniscus and G. gorilla diets remain unknown, which makes interpretations of interspecies differences in masticatory stress in African apes difficult to assess. We know that P. troglodytes has the least tough diet of all the great apes,

P. abelii has the second least tough diet, Pongo pygmaeus wurmbii and Gorilla beringei have similarly tough diets and Pongo pygmaeus morio has the highest levels of dietary toughness

(Coiner-Collier et al., 2016). In terms of hardness, both of the Pongo species have diets that are significantly harder than that of P. troglodytes (Vogel et al., 2008; Coiner-Collier et al., 2016), with P. p. wurmbii having a diet that is relatively harder than that of P. abelii (Coiner-Collier et al., 2016).

Due to the lack of available data for P. paniscus and G. gorilla diets, we can only infer relative mechanical properties of their diets based on what we know about their dietary regimes.

Both P. paniscus and P. troglodytes incorporate THV into their diets (Malenky and Wrangham,

1994), therefore it is possible that these taxa do not experience large differences in masticatory stress due to differences in dietary toughness. However, it is also possible that Pan species may experience differences in masticatory stress due to differences in the amount each Pan species spends chewing relatively tough foods, with P. paniscus eating THV more regularly than P. troglodytes (Malenky and Wrangham, 1994). Additionally, because G. gorilla masticates lower amounts of THV than G. beringei (Watts, 1984; Williamson et al., 1990; Remis et al., 2001;

Goldsmith 2003; Rogers et al., 2004), it is possible that their diets exhibit similar degrees of

4 toughness, but that G. beringei experiences higher levels of masticatory stress due to increased chewing time.

If diet is the driving factor influencing the mandibular corpus shape in extant great apes, it is hypothesized that taxa with relatively tougher and/or harder diets will exhibit morphologies that reflect increased masticatory stress (Bouvier and Hylander, 1981; Hylander, 1979a,b,c;

Hylander, 1984, 1985; Taylor, 2002, 2003, 2006a,b,c; Taylor et al., 2008; Vogel et al., 2014).

Under this hypothesis, Pongo and Gorilla species are predicted to have wider, taller corpora with increased amounts of cortical bone than Pan species (Taylor, 2006c). Also, Gorilla species are expected to exhibit wider, taller corpora with increased cortical bone than Pongo species (Taylor,

2006c). At the intrageneric level, Pan species are expected to have similar corpus shapes, G. beringei is predicted to have increased cortical bone with a taller, wider corpus than G. gorilla, and P. pygmaeus is expected to exhibit a relatively taller, wider corpus with more cortical bone than P. abelii (Taylor 2002, 2003, 2006a,b,c, 2009; Taylor et al., 2008).

Support for these morphological predictions in great ape mandibular corpus shape have been limited (Ravosa, 2000; Taylor, 2002; Taylor, 2006a; Taylor, 2009). Additional evidence against the hypothesis that hominoid corporal morphology is primarily determined by diet comes from biomechanical analyses of mandibular cortical bone (Daegling and Grine, 1991; Daegling,

2007). Analysis of corporal cortical bone thickness has found significant differences in extant great apes, with P. pygmaeus specimens characterized by less cortical bone than G. gorilla, P. troglodytes and humans (Daegling and Grine, 1991; Daegling, 2007). These results are counterintuitive to hypotheses of biomechanics and seemingly indicates that there is no apparent connection between diet and patterns of corporal cortical bone distribution in great apes

(Daegling, 2007). However, previous analyses of cortical bone distribution in hominoid

5 mandibular corpora have been limited to P. pygmaeus, G. gorilla, P. troglodytes and humans

(Daegling and Grine, 1991; Daegling, 2007) and the inclusion of additional hominoid species may clarify the relationship between cortical bone distribution and diet in hominoids.

1.1.2 Growth and the Hominoid Mandibular Corpus

Because hominoid mandibular corpus morphology is not consistent with predictions based on diet, several scholars have suggested that variation in mandibular corporal morphology is related to ontogenetic factors such as the growth of permanent molars in the corpus during development (Daegling, 1996; Taylor, 2002; Boughner and Dean, 2004, 2008; Boughner, 2011;

Ross et al. 2012; Coquerelle et al., 2010, 2011). However, a relatively small body of research has focused on hominoid mandibular growth and development (Daegling, 1996; Taylor and Groves,

2003; Taylor, 2002, 2003; Boughner and Dean, 2004, 2008; de Ruiter et al., 2013; Singh, 2014;

Terhune et al., 2014; Martinez-Maza et al., 2016). Of these studies, very few have quantified the shape of the mandibular corpus and data is currently limited to Pan and Gorilla species (Taylor and Groves, 2003; Taylor, 2002, 2003; Boughner and Dean, 2004, 2008; Martinez-Maza et al.,

2016). From these analyses, we know that the mandibular corpus develops separately from other parts of the mandible (Daegling, 1996, Singh, 2014, Martinez-Maza et al., 2016). Ontogenetic analyses of the entire mandible have found that P. paniscus and P. troglodytes have parallel allometric trajectories and that species-specific patterns of adult mandibular shape are established prior to the eruption of deciduous teeth (Boughner and Dean, 2008; Singh, 2014).

Even less research has focused on the relationship between dental development and mandibular corporal morphology in hominoids and is limited to P. paniscus, P. troglodytes

(Boughner and Dean, 2004, 2008; Boughner, 2011) and modern humans (Coquerelle et al., 2010;

Coquerelle et al., 2011). Boughner and Dean’s (2004, 2008) analyses of mandibular shape and

6 the timing of dental development in chimpanzees did not find a relationship between corpus shape and the timing of dental development. However, Coquerelle and colleagues (2010, 2011) have found a strong correlation between mandibular shape and dental development during the first two years of life in humans. Boughner and Dean’s (2004, 2008) approaches did not quantify the mandibular corpus using the same methodology as Coquerelle et al. (2010, 2011), which might explain why Boughner and Dean (2004, 2008) did not find a similar relationship between dental development and corpus shape in chimpanzees. Additionally, Boughner’s (2011) analysis of mandibular growth in P. troglodytes and P. paniscus provides some support for covariation between mandibular growth and dental development. However, it should be noted that the data used in Boughner (2011) do not quantify corpus height or width. Therefore, with the exception of humans, the relationship between dental development and mandibular corpus shape has not been evaluated in any of the extant great ape taxa. Consequently, the relationship between the shape of the corpus and dental development requires further investigation in all of the extant hominoids.

1.1.3 Phylogeny and the Hominoid Mandibular Corpus

Of the factors that are hypothesized to influence great ape corpus morphology, phylogeny has received the least amount of attention, with very few studies focusing on the phylogenetic significance of mandibular corpus morphology (Brown, 1997; Singleton, 2000; Collard and

Wood, 2001). Analyses of corporal cortical bone thickness has found significant differences in extant great apes, with P. pygmaeus specimens characterized by less cortical bone than G. gorilla, P. troglodytes and humans (Daegling and Grine, 1991; Daegling, 2007). Daegling (2007) interprets these differences as purely functional and does not account for phylogeny. It is possible that pattern of cortical bone distribution seen in P. pygmaeus is a primitive character

7 unrelated to diet in living Pongo. In order to examine this issue, it is critical to assess the internal morphology of P. abelli, G. beringei and P. paniscus. Additionally, assessing patterns of cortical bone distribution in fossil hominoids from the Miocene epoch may also provide clarification of the evolutionary significance of this trait.

1.1.4 Measuring the Hominoid Mandibular Corpus

Central to interpreting the significance of variation in the hominoid mandibular corpus is how the mandibular corpus is measured. Currently, the standard method used to study the primate mandibular corpus is mandibular robusticity (MR), which is a ratio of corpus breadth and height. Even though MR is commonly used in interpretations of human and great ape evolution, the measurements used to calculate MR are not standardized. Corpus breadth and height measurements are problematic for several reasons. Since it has been demonstrated that both mandibular breadth and height scale with allometry in primates (Smith, 1983; Ravosa,

1991, 2000), it is possible that these metrics may be so influenced by size that they are not useful in interpreting variation among taxa. Additionally, measurements of breath do not provide any information regarding the superioinferior location of maximum mandibular breadth, meaning that the measurements used to calculate MR only accounts for a portion of the shape of the mandibular corpus. Furthermore, MR does not account for variation in the inferior portion of the mandibular corpus, which may be significant in distinguishing among taxa. Consequently, it is unlikely that mandibular breadth and height measurements accurately capture the shape of the mandibular corpus. Analyses using MR may actually provide inaccurate depictions of variation in mandibular shape in fossil and living hominoids, possibly resulting in misinterpretations of the primate fossil record.

In light of these problems, several researchers have attempted to study the shape of the mandibular corpus by examining the outline of the corpus in cross-section (Brown, 1997; Kimbel et al., 2004; Kondo et al., 2016). However, these studies have been limited by qualitatively assessing features of the corporal outline. Consequently, these assessments are subjective and the comparative power of these studies is relatively low. I suggest that an alternative approach is to use a geometric morphometric approach to study the shape of the cross-sectional outline of the mandibular corpus.

1.2 Research Goals

With the aim of contributing to this body of research, the goals of this dissertation are to:

1) Develop and test an alternative method to develop a better understanding of

mandibular corpus shape variation in extant and fossil hominoids.

2) Re-evaluate the factors that are hypothesized to influence mandibular corpus shape in

extant hominoids.

3) Provide a framework to help interpret the significance of mandibular corpus shape

variation in the hominoid fossil record.

1.3 Organization of the Thesis

In order to achieve these research goals, this dissertation includes three separate analyses of hominoid mandibular corpus shape. The first analysis evaluates the taxonomic utility of using landmarks and sliding semilandmarks to quantify the shape of the cross-sectional outline of the corpus in extant great apes and compares the results to those of the traditional mandibular robusticity index (Chapter 2). This analysis is a rethinking of the approach to studying corporal fragments in the primate fossil record by providing an alternative method to quantify and

9 compare mandibular corpus shape in primates. The second analysis investigates cortical bone distribution in extant and fossil hominoids (Chapter 3). The goal of this research is to evaluate the possibility that corporal cortical bone displays a phylogenetic pattern, which may be important to interpretations of cortical bone distribution in the primate fossil record. The final analysis in this dissertation focuses on ontogenetic changes in mandibular corpus shape in extant great apes and the relationship between permanent molar crypts and corpus shape during growth

(Chapter 4). The goal of this analysis is to further evaluate the hypothesis that corporal morphology is influenced by the development of permanent dentition in the corpus during growth, which will help us understand the significance of corpus shape variation in both living and fossil hominoids. The significance of these three separate analyses to understanding hominoid corpus shape variation and interpreting the fossil record is discussed in Chapter 5.

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Chapter 2

A New Method to Quantify Mandibular Corpus Shape in Extant Great Apes: Implications for Interpreting the Hominid Fossil Record

M. KATHLEEN PITIRRI*, DAVID R. BEGUN

Affiliations: Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2

Correspondence To: M. Kathleen Pitirri, Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2. Email: [email protected]

In review with the American Journal of Physical Anthropology (submitted June 2018)

2.1 Abstract

Objectives

Mandibular corpus robusticity (corpus breadth/corpus height) is the most commonly utilized descriptor of the mandibular corpus in the great ape and hominin fossil records. As a consequence of its contoured shape, linear metrics used to characterize mandibular robusticity are inadequate to quantify the shape of the mandibular corpus. Here, we present an alternative to the traditional assessment of mandibular shape by analyzing the outline of the mandibular corpus in cross-section using landmarks and semilandmarks.

Materials and Methods

Outlines of the mandibular corpus in cross-section between M1-M2 were quantified in a sample of hominoids and analyzed using generalized Procrustes analysis, Procrustes ANOVA, CVA and cluster analysis. Corpus breadth and width were also collected from the same sample and analyzed using regression, ANOVA and cluster analysis.

Results

Analysis of corpus outline shape revealed significant differences in mandibular corpus shape that are independent of size and sex at the genus level across hominoids and at the species level in

African apes. Cluster analysis based on the analysis of corpus outline shape results in almost all specimens grouping based on taxonomic affinity. Comparison of these results to results using traditional measures of mandibular robusticity shows that analysis of the outline of the corpus in cross-section discriminates extant great apes more reliably.

Conclusion

The strong taxonomic signal revealed by this analysis indicates that quantification of the outline of the mandibular corpus accurately characterizes mandibular robusticity and offers the promise of greater power in discriminating among taxa in the hominoid fossil record.

2.2 Introduction

Mandibular fragments are among the most commonly preserved elements in the primate fossil record. The corpus of these specimens is often studied through measurements of mandibular height and breadth. Mandibular robusticity (MR) is a ratio of corpus breadth and height (Figure 1) that is frequently employed to assess the taxonomic affinity and functional significance of fossil hominoid mandibular specimens. MR has been used to describe and interpret fossils representing every hominoid group, including Miocene apes (Brown, 1997;

Ravosa, 2000; Güleç and Begun, 2003; Koufos and Bonis, 2005; McNulty et al., 2015; Fuss et al., 2017), australopithecines (Wood, 1991; Silverman et al., 2001; Ward et al., 2001; Kimbel et al., 2004; Haile-Selassie, 2010; Cofran, 2014; Haile-Selassie et al., 2015; de Ruiter et al., 2013;

Glowacka et al., 2017) and members of the genus Homo (Fabbri, 2006; Skinner et al., 2006;

Lague et al., 2008; Brown and Maeda, 2009; Chang et al., 2015). Extant great ape and human

21 mandibular morphology has also been assessed using MR (Wood et al., 1991; Humphrey et al.,

1999; Ravosa, 2000; Taylor, 2002, 2003, 2006a,b,c, 2009; Antón et al., 2011; Laird et al., 2017).

Additionally, MR and the metrics used to calculate it, have been included as characters in several large-scale cladistics analyses of hominoid evolution (Singleton, 2000; Collard and Wood, 2001;

Strait and Grine, 2004; Berger et al., 2010; Gilbert, 2013).

The mandibular corpus is of interest not only because of its frequency in the fossil record, but also because its shape is hypothesized to be influenced by several factors including diet, allometric scaling, sexual dimorphism, growth, and phylogeny (Ravosa, 1991, 2000; Daegling,

1996; Humphrey et al., 1999; Daegling and Jungers, 2000; Daegling and Grine, 2006; Ross et al., 2012). Therefore, analyses of corpus mandibular shape may provide important insights into the biology of fossil taxa.

Even though MR is often included in interpretations of human and great ape evolution, the ability of this metric to accurately capture the shape of the corpus has been questioned by several researchers (Brown, 1989, 1997; Kimbel et al., 2006; Brown and Maeda, 2009; Kondo et al., 2016). The metrics used to calculate MR, corpus breadth and height, are problematic for several reasons. Both mandibular breadth and height demonstrate allometric scaling in primates

(Smith, 1983; Ravosa, 1991, 2000) and it is possible that these metrics are so influenced by size that they are not useful in interpreting variation among taxa. Additionally, measurements of breath do not provide any information regarding the superioinferior location of maximum mandibular breadth (Figure1A), meaning that the measurements used to calculate MR only account for a portion of the shape of the mandibular corpus. Related to this is that the mandibular corpus is a curvilinear shape, making it a difficult structure to quantify using linear measures

(Scott, 1980). Consequently, it is unlikely that mandibular breadth and height measurements

22 accurately capture the shape of the mandibular corpus. Analyses using MR may actually provide inaccurate depictions of variation in mandibular shape in fossil and living hominoids, possibly resulting in taxonomic and functional misinterpretations of the primate fossil record.

In light of these problems, researchers have attempted to study the shape of the mandibular corpus by examining the outline of the corpus in cross-section (Brown, 1997; Kimbel et al., 2006; Brown and Maeda, 2009; Kondo et al., 2016). However, these studies have been limited by qualitatively assessing features of the corporal outline. Consequently, these assessments are subjective and the comparative power of these studies is low. An alternative method is to use a geometric morphometric approach to study the outline of the mandibular corpus. With the aim of improving how mandibular fossil fragments are studied, the goals of this study are to:

(i) Assess the taxonomic utility of studying the cross-sectional outline of the

mandibular corpus using a geometric morphometric approach.

(ii) Evaluate if this approach results in greater taxonomic discrimination than an

approach using MR.

To achieve these goals, we investigated of the shape of the mandibular corpus using landmarks and semilandmarks to quantify the outline of the corpus at the point between M1 and M2 in a sample of great apes. We then compared the results of this analysis to results from an analysis of corpus shape using the traditional MR approach, with the aim of assessing which method is more informative for studying corporal fragments in the primate fossil record.

2.3 Materials and Methods

2.3.1 Sample

Computerized tomography (CT) scans of mandibular specimens representing Pongo pygmaeus, Pongo abelii, Gorilla beringei, Gorilla gorilla, Pan troglodytes and Pan paniscus were provided for this analysis by Smithsonian Institution - National Museum of Natural History

(USNM) and the Royal Museum for Central Africa (RMCA). RMCA CT scans were obtained using a Siemens Somatom Esprit Spiral CT scanner (slice thickness typically varied among samples but ranged between 0.33 and 0.50 mm). USNM CT scans were obtained using a

2.3.2 MR Data Acquisition and Analysis

Mandibular height (MH) and maximum mandibular breadth (MB) measurements were taken on the right side of the mandible between M1-M2 (Figure 1A). All linear measurements

Table 1: Hominoid taxa used in both analyses.

Taxon Male Female Total Pongo pygmaeus 20 22 42 Pongo abelii 7 7 14 Gorilla gorilla 19 9 28 Gorilla beringei 11 7 18 Pan troglodytes 8 12 20 Pan paniscus 6 10 16

were collected by MKP using ImageJ (Rasband, 1997-2015). Intraobserver error was estimated using technical error of measurement (TEM) (Mueller and Martorell, 1988; Ulijaszek and

Lourie, 1994) to compare 14 (~10% of the sample) randomly selected specimens, which yielded a TEM of 0.102mm and a coefficient of reliability (R) of 0.969. Since the goal of this study is to evaluate the utility of MR to assess corpus shape in fragmentary fossils, geometric mean (GM) was selected as the measurement of size rather than mandibular length because the latter can be unknown in mandibular fragments. Regression of MR and GM was used to assess any relationship between MR and size. We examined the distribution of MR for group separation and compared samples using an ANOVA. Pairwise comparisons were conducted to further assess significant differences found by the ANVOA. The ability of MR to discriminate among taxa was assessed by conducting a cluster analysis (UPMGA), based on Euclidian distances. All statistics were calculated using PAST v. 3.19 (Hammer et al., 2001).

2.3.3 Corpus Outline Data Acquisition and Analyses

To quantify the outline of the mandibular corpus, three landmarks (Table 2) and 60 semilandmarks (Figure 1B) were collected in Amira (Mercury Computer Systems/3D Viz group,

San Diego, CA) on the right side of the mandible between M2-M1. All landmarks and semilandmarks were collected by MKP. Intraobserver error was assessed by collecting all landmarks and semilandmarks from the same specimen ten times on ten different days. These

Table 2: Definitions of landmarks used in geometric morphometric analyses.

No. Landmark Definition 1 BUCM2-M1 Point on alveolar border between M2-M1 on buccal side 2 INFM2-M1 Most inferior point on the corpus between M2-M1 3 LINGM2-M1 Point on alveolar border between M2-M1 on lingual side

data were used to calculate the Procrustes distance (square root of the sum of squares distance between corresponding landmarks after superimposition (Bookstein, 1991)) between the mean for all ten replicates and each replicate, resulting in a range from 0.022 – 0.001. Semilandmarks were resampled to be equidistant along their curves and ‘slid’ via minimizing bending energy.

The resulting homologous landmarks were then subjected to generalized Procrustes analysis

(GPA) (Grower, 1975). GPA translates, scales and rotates the landmark data, producing superimposed Procrustes coordinates (Dryden and Mardia, 1998; Slice, 2007). Shape variation was assessed by conducting a principal component analysis (PCA) on the Procrustes coordinates.

Size was assessed using centroid size (CS) (calculated as the square root of the sum of squared distances of the landmarks from their centroid). Allometric variation (size related shape differences) was examined through a multivariate regression of Procrustes shape coordinates on centroid size. Between group shape differences (inter- and intraspecies) were further examined through a Procrustes ANOVA (Goodall, 1991), considering size, sex and species. Pairwise group comparisons were conducted as post-hoc tests to the Procrustes ANOVA. Because these involved multiple comparisons, the sequential Bonferroni adjustment (Holm, 1979; Rice, 1989) was applied to an a priori significance level of 0.05 to protect against Type I errors associated.

The Bonferroni adjusted alpha was calculated separately for each set of pairwise comparisons.

Distances between groups in shape space were also assessed by conducting a canonical variate analysis (CVA) on the first eight PCs representing 96.1% of the variation in the sample. CVA

26 was conducted in PAST v. 3.19 (Hammer et al., 2001). All other analyses were conducted using the R package ‘Geomorph’ v. 3.0.5 (Adams and Orárola-Castillo, 2017).

2.3.4 Comparison of MR with Analysis of Corpus Outline Shape

Results produced by linear measurements and geometric morphometric analysis were compared by assessing differences between the outputs produced by each of the analyses.

Following Bernal (2007), the degree of association between the two types of data on corpus shape was assessed by calculating the correlation between MR with the specimens’ scores along the most significant PCs (the first eight PCs, accounting for 96.1% % of the total variation). To assess the ability of each method to discriminate shape differences among taxa, the resulting

ANOVAs and pairwise comparisons from each of the analyses were compared. We also conducted a cluster analysis (UPGMA, Euclidian distance) on the CVA results from the outline analysis and a separate cluster analysis derived from MR. Additionally, we also conducted a cluster analysis using a Procrustes distance matrix produced from the analysis of the outline of corpus shape. Correlation analyses and cluster analyses were conducted in PAST v. 3.19

(Hammer et al., 2001).

2.4 Results

2.4.1 Analysis of Corpus Shape from MR Approach

Figure 2A shows that a plot of MR on geometric mean results in separation of Pan from

Gorilla and Pongo, while Gorilla Table 3: ANOVA results from analysis using linear and Pongo display overlap. This measurements testing for mandibular robusticity differences between genus and species and their interactions with sex1. plot also shows that MR and GM Genus Species Genus:Sex Species:Sex are not correlated (RMA: F 15.97 7.782 3.11 2.516 df 2 5 2 5 r2=0.003, slope=1.6455, p 6.14E-07 2.07E-06 0.0479 0.0333 1 p-values < 0.05 in bold

Table 4: P-values obtained from Mann-Whitney pairwise comparison testing for species differences in mandibular robusticity based on linear measurements1.

G. beringei G. gorilla P. abelii P. pygmaeus P. paniscus P. troglodytes G. beringei 1 G. gorilla 0.1345 1 P. abelii 0.0322 0.1491 1 P. pygmaeus 0.0001 0.0006 0.8461 1 P. paniscus 0.0022 0.0174 0.4967 0.7013 1 P. troglodytes 0.0009 0.0064 0.4725 0.6208 0.9873 1 1 Significant at the Bonferroni adjusted alpha of 0.003 in bold

Table 5: P-values obtained from Mann-Whitney pairwise comparisons of all taxonomic groups testing for sexual dimorphism in corpus shape based on linear measurements.1,2

GbM GbF GgF GgM PaF PaM PpyF PpyM PpaF PpaM PtF PtM GbM 1 GbF 0.0571 1 GgF 0.5433 0.1123 1 GgM 0.5186 0.0109 0.1841 1 PaF 0.3191 0.0552 0.2443 0.5249 1 PaM 0.2090 0.0538 0.1407 0.3240 0.6171 1 PpyF 0.0002 0.0002 0.0002 0.0001 0.4576 0.2319 1 PpyM 0.2145 0.0047 0.0856 0.5564 0.7404 0.3272 4.240E-05 1 PpaF 0.0183 0.0029 0.0101 0.0459 0.4068 0.7863 0.2812 0.0643 1 PpaM 0.2913 0.0742 0.2629 0.4643 0.9431 0.5752 0.6618 0.5566 0.6255 1 PtF 0.0605 0.0060 0.0302 0.1184 0.7674 0.7431 0.2099 0.1546 0.8175 0.8883 1 PtM 0.0352 0.0065 0.0184 0.0594 0.6854 0.3329 0.7511 0.1057 0.7558 0.9485 0.6160 1 1 Significant at the Bonferroni adjusted alpha of 0.0008 in bold 2Abbreviations: M = male, F = female, Gb = G. beringei, Gg = G. gorilla, Pa = P. abelii, Ppy = P. pygmaeus, Ppa = P. paniscus, Pt = P. troglodytes.

intercept=9.8715). ANOVA results show a significant relationship between corpus shape, genus

and species and their interactions with sex (Table 3). Pairwise comparison reveals that significant

differences between species are only present between G. beringei and all Pongo and Pan species,

as well as G. gorilla and all Pongo and Pan species, but not between G. beringei and G. gorilla

(Table 4). Pairwise comparisons of corpus shape sexual dimorphism show that significant

Figure 3: Comparison of cluster analyses results showing higher taxonomic discrimination from results produced by analysis of corpus outline relative the results of analysis produced using the mandibular robusticity index. A) Dendrogram produced from cluster analysis of linear assessment of corpus shape (UPGMA, cophen. corr.=0.7887); B) Dendrogram produced from cluster analysis of CVA results from assessment of the outline of corpus shape (UPGMA, cophen. corr.=0.8281); C) Dendrogram produced from cluster analysis of Procrustes distance matrix from assessment of the outline of corpus shape (UPGMA, cophen. corr.=0.7178). POP = P. pygmaeus; POA = P. abelii; GB = G. beringei; GG = G. gorilla; PT = P. troglodytes; PPA = P. paniscus. differences between males and females are only present in P. pygmaeus (Table 5). Cluster analysis (UPGMA) conducted using MR did not result in taxonomic separation (Figure 3A).

2.4.2 Analysis of Corpus Outline Shape

Results of the PCA show a high amount of overlap among genera along the first three

PCs, accounting for 78.7% of the total variance (Figure 2C). At the species level, along PC1

(46.8%) P. paniscus exhibits negative scores and G. beringei exhibits positive scores, with all

30 other taxa falling in the middle (Figure 2C). Positive scores along PC1 are associated with a relatively wider corpus, especially the inferior portion, which buttresses lingually (Figure 2B).

Negative PC1 scores reflect corpora that are relatively tapered inferiorly and extend buccually

(Figure 2B). Gorilla specimens exhibit the largest amount of variation along PC1, while Pan specimens exhibit the least amount of variation (Figure 2C). Along PC2, the majority of specimens exhibit scores near zero, with P. paniscus and G. beringei having the lowest scores and P. abelii having the highest scores (Figure 2C). Negative PC2 scores are associated with a relatively narrower corpus with a lingual buttress at the midpoint. Positive PC2 scores are associated with a corpus shape that is relatively wider throughout the entire cross-section (Figure

2B). All groups demonstrate similar within-group variation along PC2, with P. abelii exhibiting slightly more variation than the other taxa (Figure 2C).

Table 6: Procrustes ANOVA testing for shape and size differences between species and sex, and interactions between them1.

Size Species Sex Size:Species Size:Sex Species:Sex Size:Species:Sex F 2.1465 14.6550 3.6596 1.0033 1.2681 1.9675 0.8506 df 1 5 1 5 1 5 5 p 0.0724 0.0001 0.0004 0.0447 0.0715 0.0001 0.0558 1 p-values < 0.05 in bold

Multivariate regression of Procrustes shape variables on size did not show any relationship between corpus shape and CS (Figure 2D). Results of the Procrustes ANOVA provide additional confirmation that corpus shape is not related to CS (Table 6). The Procrustes

ANOVA results revealed significant shape differences when considering species and sex, but not the interaction between species and sex (Table 6). The Procrustes ANOVA results revealed significant shape differences when considering species and sex, as well as the interaction between species and sex (Table 6). Pairwise comparisons of corpus shape of all taxonomic groups, standardizing for the effects of sex, are provided in Table 7. These show a significant

31 difference among P. paniscus and all other taxa. Both Pongo species show significantly different corpus shapes than G. gorilla, but neither Pongo species are found to demonstrate significant differences from P. troglodytes. Additionally, intraspecies differences are not found to be significant within Pongo nor Gorilla. Pairwise comparisons of corpus shape sexual dimorphism did not result in significant difference between males and females of the same species for any of the great apes (Table 8). These results provide further confirmation of the Procrustes ANOVA result that there is no significant interaction between species and sex in this sample.

Table 7: P-values obtained from pairwise comparisons across great ape species, based on 10,000 permutations of Procrustes distances, after standardising shape data for the effects of sex1.

G. beringei G. gorilla P. abelii P. paniscus P. pygmaeus P. troglodytes G. beringei 1 G. gorilla 0.0078 1 P. abelii 0.0005 0.0105 1 P. paniscus 0.0001 0.0001 0.0001 1 P. pygmaeus 0.0001 0.0001 0.0495 0.0001 1 P. troglodytes 0.0001 0.0018 0.0331 0.0002 0.0407 1 1 Significant at the Bonferroni adjusted alpha of 0.003 in bold

Species mean corpus shape, depicted in Figure 4, shows that the shape differences among species are driven by differences in corpus width located at different points along the corpus in the superior-inferior plane. In overall width, G. beringei exhibits the widest corpus of the entire sample. Relative to all other taxa, Pan species demonstrate narrowing in the inferior portion of the corpus, with P. paniscus corpora narrowing more than P. troglodytes. Compared to P. pygmaeus, P. abelii demonstrates a wider corpus that is relatively more U shaped.

CVA was conducted on the first eight PCs and resulted in five CVs. A plot of the first two axes, accounting for 71.84% of the between group variance, shows four distinct groups with some overlap between P. abelii and Gorilla species (Figure 5). Cluster analysis based on all five

CVs results in almost all specimens grouping based on taxonomic affinity (Figure 3B).

Table 8: P-values obtained from pairwise comparisons of all taxonomic groups, based on 10,000 permutations of Procrustes distances, testing for sexual dimorphism in corpus shape1,2.

GbF GbM GgF GgM PaF PaM PpaF PpaF PpyF PpyM PtF PtM GbF 1 GbM 0.2505 1 GgF 0.1976 0.5974 1 GgM 0.0027 0.0296 0.1395 1 PaF 0.0001 0.0053 0.0076 0.0031 1 PaM 0.0092 0.1527 0.1640 0.5870 0.0989 1 PpaF 0.0001 0.0001 0.0001 0.0001 0.0009 0.0001 1 PpaM 0.0001 0.0001 0.0001 0.0005 0.0014 0.0014 0.5262 1 PpyF 0.0001 0.0001 0.0001 0.0002 0.0750 0.0193 0.0028 0.0087 1 PpyM 0.0001 0.0004 0.0018 0.0129 0.0923 0.2630 0.0004 0.0026 0.1580 1 PtF 0.0003 0.0028 0.0110 0.0226 0.0706 0.0720 0.0025 0.0053 0.0888 0.0831 1 PtM 0.0002 0.0042 0.0207 0.0640 0.1195 0.1401 0.0143 0.0257 0.3206 0.2558 0.9442 1 1 Significant at the Bonferroni adjusted alpha of 0.0008 in bold 2Abbreviations: M = male, F = female, Gb = G. beringei, Gg = G. gorilla, Pa = P. abelii, Ppy = P. pygmaeus, Ppa = P. paniscus, Pt = P. troglodytes.

2.4.3 Comparison of MR Analysis with Analysis of Corpus Outline Shape

Results of the correlation analysis show a very weak relationship between MR and all but

one of the shape variables produced by the geometric morphometric analysis of the outline of

corpus shape (Table 9). The only variable that resulted in a significant p-value when compared to

MR is PC1, which only accounts for 46.8% of the variation in the dataset (Table 9). In addition

to changes in shape in the inferior portion of the corpus, PC1 is associated with large changes in

breadth across the entire cross-section of the corpus (Figure 2B), which is likely driving the

correlation between MR and PC1. Since PCs are mathematically independent of each other and

in light of the strong correlation between PC1 and MR, the lack of correlation between MR and

any of the other PCs is not unexpected.

L B

Figure 4: Species mean corpus shapes warped to mean shape of the entire sample, enhanced by a factor of 2. L = Lingual, B = Buccal.

Comparison of the ANOVA results from each of the analyses of corpus shape show that

both analyses found a significant relationship between corpus shape and species (Tables 3 and

6). Evaluation of pairwise comparisons from each of these analyses reveal differences in the

methods considered here to capture significant differences in corpus shape among great ape

34 species (Tables 4 and 7). The MR method is limited to finding significant shape differences between G. beringei and Pan and Pongo species, as well as between G. gorilla and Pan and

Pongo species. This method does not find any significant differences in the corpus shapes between Pan and Pongo, nor does it identify significant shape differences at the species level

(Table 4). The geometric morphometric approach evaluating the outline of the mandibular corpus is better able to identify significant shape differences among great apes, with all species except P. abelii and P. pygmaeus having significantly different corpus shapes (Table 7).

Figure 5: CVA plot of corpus shape variation in great apes based on the first two CVs, representing 71.84% of the total between species variation. Colors and shapes are the same as in Figure 2.

Table 9: Results of correlation and regression analyses comparing determinants of corpus shape from the MR method and the outline method1.

PC compared with Correlation Coefficient p MR/geometric mean PC1 0.5275 2.9878E-11 PC2 0.0115 0.8933 PC3 0.0313 0.7157 PC4 0.0232 0.7868 PC5 0.0953 0.2660 PC6 -0.0218 0.8000 PC7 -0.1519 0.0753 PC8 -0.0997 0.2445 1 p-values < 0.05 in bold

Additional differences between these methodological approaches are found when results investigating sexual dimorphism in corpus shape are compared. The MR method reveals a significant relationship between MR and sexual dimorphism in P. pygmaeus (Table 5), while the corpus outline method does not find a relationship between corpus shape and sexual dimorphism in any of the great ape species (Table 8).

Cluster analyses based on the shape outputs of each of these methods resulted in dissimilar dendrograms (Figure 3). Comparison of the two dendrograms produced from the analysis of the outline of corpus shape (Figures 3B and 3C) shows that the Euclidian distance matrix based on the CVs results in a higher degree of taxonomic discrimination than the

Procrustes distance matrix. This difference is attributed to the fact that CVA maximizes group differences (Sokal,1995). However, comparison of these cluster analyses reveals that the shape output from derived from MR does not recover taxonomic groups (Figure 3A), while the dendrograms produced from the results of the analysis of the shape of the outline of the corpus does result in taxonomic clusters (Figures 3B and 3C).

2.5 Discussion

The results of the analysis of the outline of corpus shape show that there are significant corpus shape differences among the great apes that are independent of size and sex. These shape differences are significant at the genus level across great apes and at the species level in African apes. Comparison of the MR approach and the outline of the corpus shows that the latter method is better able to identify corpus shape differences among great ape species. We suggest that this is because the mandibular corpus is a curved shape and linear measures do not adequately capture its shape. The evaluation of shape variables produced by each of these approaches provides confirmation for this conclusion. MR displays a relationship with only one of the PCs, which is strongly influenced by maximum corpus breadth, as well as differences in the inferior portion of the corpus (Figure 1). MR does not show a relationship with any of the PCs that are driven by shape differences in the inferior portion of the corpus (Figure 2B). We interpret these results to indicate that shape differences related to the inferior portion of the corpus are important in differentiating among hominoid species and that the metrics used to calculate MR do not capture this aspect of the mandibular corpus. As a result, studying corpus shape as an outline provides improved taxonomic discrimination. Importantly, this approach will allow for the inclusion of fragmentary specimens into analyses of mandibular corpus shape, which has important implications for studying mandibular corpus specimens in the fossil record.

There are additional approaches to studying the outline of biological forms such as elliptical Fourier analysis (EFA) (Kuhl and Giardina, 1982) and Eigenshape analysis (EA)

(Lohmann, 1983). Although these approaches are not assessed in this study, both have been used to analyze the outline of the mandibular symphysis in hominoids (Daegling and Jungers, 2000;

Sherwood et al., 2005; Guy et al., 2008). Studies of hominoid mandibular symphysis shape using

EFA and EA have yielded strong taxonomic discrimination at the genus level, but not at the species level (Daegling and Jungers, 2000; Sherwood et al., 2005; Guy et al., 2008). The low taxonomic discriminatory power of EFA and EA in studies of the mandibular symphysis may reflect that although researchers have advocated the utility of EFA to study complex outlines

(Lestrel, 1989; McLellan, and Endler, 1998), recent empirical evidence suggests that the use of semilandmarks and landmarks is significantly superior to EFA when studying complex outlines

(Van Bocxlaer and Schultheiβ, 2010). Additional critiques of EFA and EA are that the resulting harmonics lack biological significance and that the markers are not homologous, making it difficult to interpret the results (Bookstein et al., 1982) (but see Read and Lestrel, 1986;

MacLeod, 1999 for further discussion on homology in EFA and EA).

Due to the relatively poor performance of EFA and EA in analyses of complex outline shapes (Daegling and Jungers, 2000; Sherwood et al., 2005; Guy et al., 2008; Van Bocxlaer and

Schultheiβ, 2010) and the strong taxonomic discriminatory power shown here in our analysis, we advocate for the use of a semilandmark and landmark approach to study mandibular corporal shape variation in the hominoid fossil record.

2.6 Conclusion

This study shows that MR fails to reveal known taxonomic differences in great apes, which calls into question its reliability for distinguishing taxa in the fossil record. Conversely, the corpus outline method recovers known taxonomic patterns accurately in great apes. The ability of the corpus outline method to accurately assess taxonomic patterns in extant great ape species provides support for the application of this method to mandibular corporal fragments in the hominoid fossil record in which taxonomy is unclear and/or debated. This includes

Sivapithecus (Kay, 1982; Kelley and Pilbeam, 1986; Kelley, 1988, 2005), australopithecines

(Silverman et al., 2001; Haile-Selassie et al., 2016; Wood and Schroer, 2017), early Homo fossils

(Curnoe, 2001, 2010; Antón, 2012; Villmoare, 2018).

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Chapter 3

Functional and Phylogenetic Implications of Cortical Bone Distribution in the Mandibular Corpus of Extant and Fossil Great Apes

M. KATHLEEN PITIRRI*, DAVID R. BEGUN

Affiliations: Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2

Correspondence To: M. Kathleen Pitirri, Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2. Email: [email protected]

To be submitted to the Journal of Human Evolution

3.1 Abstract

Extant great apes are known to differ in their distribution of cortical bone in the mandibular corpus, with Pongo pygmaeus having less cortical bone than African apes. Here, we expand the investigation of the significance of corporal cortical bone distribution (CBD) in hominoids by examining this trait in an expanded sample, including Pan troglodytes, Gorilla gorilla and P. pygmaeus, as well as Pongo abelii, Gorilla beringei and Pan paniscus, for which data on corporal CBD have not been previously published. Additionally, we report the first published results on the shape of CBD in a sample of Miocene hominoids (Rudapithecus hungaricus, Dryopithecus fontani, and Sivapithecus sivalensis). The shape of CBD was quantified using landmarks and semilandmarks in cross-section between M1 and M2 and compared using Generalized Procrustes analysis. The results revealed significant shape differences among all extant species. These shape differences do not match morphological predications based on diet. The results also confirm that P. pygmaeus has relatively thinner CBD throughout the corporal cross-section than all other hominoids and that P. abelii has a corporal

CBD shape that is similar in relative cortical thickness to P. paniscus. Assessment of the phylogenetic signal in extant taxa reveals a phylogenetic signal within hominoids. D. fontani, R. hungaricus and one of the Sivapithecus specimens exhibit corporal CBD shapes that are similar to extant hominines and P. abelii. The second Sivapithecus specimen has a CBD shape that is similar to P. pygmaeus. We conclude that the shape of CBD in the hominoid mandibular corpus has important phylogenetic significance and that the shape of CBD in additional Miocene taxa will help to clarify its importance.

3.2 Introduction

The shape of the primate mandibular corpus is of great interest to paleoanthropologists.

This is due not only to the prevalence of mandibular corpus fragments in the hominoid fossil record, but also because differences in mandibular corpus morphology have been used to help determine taxonomic affinities in Miocene apes (Brown, 1997; Güleç and Begun, 2003; Koufos and Bonis, 2005; McNulty et al., 2015; Fuss et al., 2017), australopithecines (Wood, 1991;

Silverman et al., 2001; Ward et al., 2001; Kimbel et al., 2004; Haile-Selassie, 2010; Haile-

Selassie et al., 2015; Glowacka et al., 2017) and fossil Homo (Fabbri, 2006; Skinner et al., 2006;

Lague et al., 2008; Brown and Maeda, 2009; Chang et al., 2015). In order to develop an understanding of the significance of differences in corpus morphology seen in the fossil record, researchers have focused on mandibular shape in living great apes (Wood et al., 1991; Humphrey et al., 1999; Ravosa, 2000; Taylor, 2002, 2003, 2006a,b,c, 2009; Daegling, 2007). Despite the intense focus placed on studying the hominoid mandible, the significance of taxonomic differences in hominoid mandibular corpus shape remains unclear.

It has been hypothesized that the morphology of the great ape mandible is influenced by several factors including diet, allometric scaling, sexual dimorphism, growth, and phylogeny

(Ravosa, 2000; Humphrey et al., 1999; Daegling and Jungers, 2000; Daegling and Grine, 2006;

Ross et al., 2012). Of these factors, the influence of diet (Ravosa, 2000; Taylor 2003, 2002,

2006a,b,c, 2009; Daegling, 2007; Vogel et al., 2014; Muchlinski and Deane, 2016; Marcé-Nogué et al., 2017) and growth (Daegling, 1996; Boughner and Dean, 2008; Coquerelle et al., 2010;

Boughner, 2011; Singh, 2014; Martinez-Maza et al., 2016) on great ape corpus morphology have received the greatest amount of attention, with relatively less emphasis being placed on phylogeny (Brown, 1997; Singleton, 2000; Collard and Wood, 2001).

The focus on the relationship between diet and mandibular shape in great apes has been largely driven by results from experimental studies on monkeys (Bouvier and Hylander, 1981;

Hylander, 1979a,b,c, 1986; Hylander and Johnson, 1985, 1994; Hylander and Crompton, 1986;

Hylander et al., 1987, 1992, 2000). These studies have shown that the mandibular corpus experiences three important forces during mastication and incision, parasagittal bending, axial torsion and lateral transverse bending. Parasagittal bending acts on the balancing side of the corpus during the power stroke of mastication. This results in tension along the superior aspect of the corpus and compression along the inferior aspect of the corpus. The magnitude of this force is directly proportional to the balancing side muscle force (Hylander, 1979a,b). Axial torsion, which Hylander (1985) argued to be the most important loading regime in the molar region of the corpus, acts on the corpus during both unilateral mastication and incision and is caused by the lateral position of masseter muscle force as well as the bite force acting on the working side of the mandible. These forces evert the inferior aspect of the corpus and invert the superior aspect of the corpus resulting in axial torsion, or twisting of the mandibular corpora about the long axis (Hylander 1979a,b; 1984, 1985). The degree of axial torsion is influenced by muscle force magnitude, masseter muscle orientation and the timing of muscle recruitment (Hylander

1985, Hylander and Johnson, 1994). Lateral transverse bending, or wishboning, occurs due to increased deep masseter muscle activity on the balancing side paired with decreased superficial masseter muscle activity on the balancing side as well as decreased superficial and deep masseter muscle activity on the working side during the power stroke of mastication (Hylander, 1979a,b,

1986; Hylander et al., 1987, 2000; Hylander and Johnson, 1994).

Researchers have used results from experimental studies to develop predictions of optimal corporal morphologies for countering increased forces acting on the mandible due to diet

(Bouvier and Hylander, 1981; Hylander, 1979a,b,c; Hylander, 1984, 1985). In terms of corpus cross-sectional dimensions, parasagittal bending loads are hypothesized to be most effectively resisted by increasing corporal height, axial torsion is optimally countered by a cylindrically shaped corpus, and lateral transverse bending is best countered by increasing corporal width

(Hylander, 1979a,b, Hylander, 1984, 1985). Additionally, the addition of cortical bone throughout the entire cross-section will result in a stronger corpus, however, this may be a biologically expensive approach (Daegling and Grine, 1991; Daegling, 2007). A more economical approach to cortical bone utilization may be distribute cortical bone differentially depending on the forces acting on the corpus (Daegling and Grine, 1991; Daegling, 2007). In terms of cortical bone distribution (CBD), parasagittal bending is optimally countered by increasing cortical bone along the superior and inferior aspects of the corpus, axial torsion is optimally resisted by redistributing cortical bone evenly throughout the corpus, and lateral transverse bending is optimally countered by increasing cortical bone along the lingual and buccal aspects of the corpus (Hylander 1979a,b; Demes et al., 1984; Daegling, 2007).

In addition to identifying the forces acting on the corpus during incision and mastication, and identifying optimal strategies for countering these forces, in vivo analyses in monkeys have

51 shown that increased magnitudes of bone strain in the primate mandible correlate with relatively tougher and/or harder diets (Bouvier and Hylander, 1981; Hylander, 1979a,b,c; Hylander and

Johnson, 1985, 1994; Hylander and Crompton, 1986; Hylander et al., 1987, 1992, 2000). This has led to the hypothesis that primates with relatively harder or tougher diets (such as folivores and hard-fruit eaters) and/or higher amounts of time spent chewing these types of foods, will exhibit mandibular morphologies that reflect their diet (Bouvier and Hylander, 1981; Hylander

1979a,b,c, Hylander, 1984, 1985; Hylander and Johnson, 1994).

Differences in masticatory stress due to dietary differences among great apes make hominoids an ideal group to evaluate this hypothesis. In terms of dietary composition, Gorilla is known to be the most folivorous of the great ape taxa (Tutin and Fernandez, 1993; Rogers et al.,

2004; Masi, 2008; Masi et al., 2009; Head et al., 2011). Within this genus, Gorilla beringei includes more terrestrial herbaceous vegetation (THV) than Gorilla gorilla (Watts, 1984;

Williamson et al., 1990; Remis et al., 2001; Goldsmith 2003; Rogers et al., 2004). Pongo prefers ripe fruit, but during periods of fruit scarcity, this taxon incorporates hard fruits, THV and bark into its diet (Galdikas, 1998; Knott, 1998; Vogel et al. 2008; Vogel et al., 2009; Kanamori et al.,

2010; Vogel et al., 2015). Diet varies between Pongo species, with Pongo pygmaeus masticating increased amounts of hard fruits, THV and bark relative to Pongo abelii (Knott, 1998; Fox et al.,

2004; Wich et al., 2006). Pan is the most frugivorous of the great ape genera. During periods of fruit scarcity, Pan supplements their diet by using strategies that do not result in increased masticatory stress, including tool use and expanded foraging areas (Tutin and Fernandez, 1993;

Conklin-Brittain et al., 1998; Wrangham et al., 1998; Yamagiwa and Basabose, 2009; Head et al., 2011; Watts et al. 2012; McLennan, 2013). Dietary differences between Pan species remain unclear. Research has found that P. paniscus regularly incorporates THV in their diet, while P.

52 troglodytes increases the amount of THV consumed during times of fruit scarcity (Malenky and

Wrangham, 1994). Additionally, P. panicus is known to switch to different types of fruits and

THV when their preferred foods are not available (Serckx et al., 2015).

A growing body of research on the mechanical properties of great ape food items have found that dietary differences translate into differences in dietary toughness and hardness (Vogel et al., 2008, 2014; Coiner-Collier et al., 2016). These data, summarized in Table 10, shows that

P. troglodytes has the least tough diet of all the great apes, P. abelii has the second least tough diet, Pongo pygmaeus wurmbii and Gorilla beringei have similarly tough diets and Pongo pygmaeus morio has the highest levels of dietary toughness (Coiner-Collier et al., 2016). In terms of hardness, the diet of P. troglodytes is significantly less hard than both Pongo species

(Vogel et al., 2008; Coiner-Collier et al., 2016), with P. p. wurmbii having a diet that is relatively harder than that of P. abelii (Coiner-Collier et al., 2016). At this time, mechanical properties of

P. paniscus and G. gorilla diets remain unknown, which makes interpretations of interspecies differences in masticatory stress in African apes difficult to assess. However, since both P. paniscus and P. troglodytes incorporate THV into their diets (Malenky and Wrangham, 1994), it is possible that these taxa do not experience large differences in masticatory stress due to differences in dietary toughness. It is also possible that Pan species may experience differences in masticatory stress due to differences in the amount of time spent chewing relatively tough foods, with P. paniscus eating THV more regularly than P. troglodytes (Malenky and

Wrangham, 1994). Additionally, since G. gorilla masticates lower amounts of THV than G. beringei (Watts, 1984; Williamson et al., 1990; Remis et al., 2001; Goldsmith 2003; Rogers et al., 2004), it is possible that their diets exhibit similar degrees of toughness, but that G. beringei experiences higher levels of masticatory stress due to increased chewing time.

Table 10: Mechanical properties for great ape diets1

Taxon Dietary Toughness Dietary Hardness (weighted2 mean R) (weighted2 mean E) Pongo pygmaeus 1092.6 - 2303.5 7.85 Pongo abelii 671.15 4.26 Gorilla beringei 1112.0 not available Pan troglodytes 224.18 1.03 1Reproduced from Coiner-Collier et al., 2016. 2 Mean values are weighted by the proportion of each food type in dietary composition (see Coiner-Collier et al., 2016 for further explanation)

Hylander, 1984, 1985; Taylor, 2002, 2003, 2006a,b,c; Taylor et al., 2008; Vogel et al., 2014).

Support for these morphological predictions in great ape mandibular corpus shape have been limited to analyses based on linear measurements of external morphology (Ravosa, 2000;

Taylor, 2002; Taylor, 2006a; Taylor, 2009). Additional evidence against the hypothesis that hominoid corporal morphology is primarily determined by diet comes from biomechanical analyses of mandibular cortical bone (Daegling and Grine, 1991; Daegling, 2007). Analysis of corporal cortical bone thickness has found significant differences in extant great apes, with P.

54 pygmaeus specimens characterized by less cortical bone than G. gorilla, P. troglodytes and humans (Daegling and Grine, 1991; Daegling, 2007). These results are counterintuitive to hypotheses of bone utilization and biomechanics and seemingly indicates that there is no apparent connection to diet in CBD patterns in great apes and humans (Daegling, 2007).

However, previous analyses of CBD in hominoids have been limited to P. pygmaeus, G. gorilla,

P. troglodytes and humans (Daegling and Grine, 1991; Daegling, 2007) and the inclusion of additional hominoid species may clarify the relationship between cortical bone distribution and diet in hominoids.

There are two additional factors that may influence the distribution of cortical bone in the great ape mandibular corpus, growth and phylogeny. Of these two factors, growth (Daegling,

1996; Boughner and Dean, 2008; Coquerelle et al., 2010; Boughner, 2011; Singh, 2014;

Martinez-Maza et al., 2016) has received more attention than phylogeny (Brown, 1997;

Singleton, 2000; Collard and Wood, 2001). Investigations into mandibular shape and growth in humans, Pan and Gorilla have found that the shape of the mandible is determined early in development and is not strongly influenced by diet (Coquerelle et al., 2010; Singh, 2013;

Martinez-Maza et al., 2016). An alternative to dietary hypotheses is that corpus shape is constrained by housing developing teeth (Daegling, 1996; Boughner and Dean, 2004, 2008;

Coquerelle et al., 2010; Boughner, 2011; Singh, 2013). In terms of CBD, a comparison of modern human populations has found that cortical bone in the symphysis is constrained by a regulatory role during growth modeling (Fukase and Suwa, 2008). A regulatory mechanism controlling cortical bone during dental development is confirmed by studies of the relationship between tooth root formation and bone growth in mice. These studies have shown that bone

Table 11: Morphological predictions based on diet

Dietary stress due to Dietary stress due to CBD in the inferior region due CBD along lingual and CBD along lingual and repetitive loading increased R and/or E to parasagittal bending buccal sides due to axial buccal sides due to lateral torsion transverse bending Gorilla > Pongo > Pan Gorilla > Pongo > Pan Gorilla & Pongo > Pan Gorilla & Pongo > Pan Gorilla & Pongo > Pan

Gorilla > Pongo Gorilla ≈ P. pygmaeus > P. Gorilla & P. pygmaeus > P. Gorilla & P. pygmaeus > P. Gorilla & P. pygmaeus > P. abelii abelii. abelii. abelii (i) Gorilla ≈ P. pygmaeus (i) Gorilla ≈ P. pygmaeus (i) Gorilla ≈ P. (ii) Gorilla > P. pygmaeus will have similar CBD pygmaeus shapes. (ii) Gorilla > P. pygmaeus (ii) Gorilla > P. pygmaeus

G. beringei > G. gorilla G. beringei > G. gorilla G. beringei > G. gorilla G. beringei > G. gorilla G. beringei > G. gorilla

P. pygmaeus ≈ P. abelii P. pygmaeus > P. abelii (i) P. pygmaeus ≈ P. abelii (i) P. pygmaeus ≈ P. (i) P. pygmaeus ≈ P. (ii) P. pygmaeus > P. abelii abelii abelii (ii) P. pygmaeus > P. abelii (ii) P. pygmaeus > P. abelii

P. troglodytes ≈ P. P. troglodytes ≈ P. paniscus P. troglodytes ≈ P. paniscus P. troglodytes ≈ P. paniscus P. troglodytes ≈ P. paniscus paniscus

morphogenetic proteins work with transcription factors (Msx1 and Msx2) to play an important role in the interactions between epithelial and mesenchymml tissues during early tooth morphogenesis (Vainio et al., 1993; Aberg et al., 1997; Thesleff and Aberg, 1999; Yamashiro et al., 2003).

The genetic link between cortical bone and dental development indicates that CBD in the corpus may be of phyletic significance in great apes. The pattern of thinner corporal cortical bone in P. pygmaeus and thicker corporal cortical bone in G. gorilla and P. troglodytes

(Daegling, 2007) suggests the possibility that CBD in the corpus of great apes reflects an evolutionary pattern distinguishing hominines from pongines. If CBD is constrained by genetic patterning in the mandibular corpus, explorations of the phenotypic expressions of this trait in the great ape fossil record may be useful in understanding the patterns that are present in modern great apes. Here, we investigate cortical bone distribution in the great ape mandibular corpus by expanding the extant sample to include Pan troglodytes, Gorilla gorilla and Pongo pygmaeus, as well as Pongo abelii, Gorilla beringei and Pan paniscus, for which data on corporal CBD have not been previously published. We also evaluate this topic from an evolutionary perspective by assessing the phylogenetic signal in the shape of cortical bone distribution in the corpus in extant great apes. Additionally, we further assess the evolutionary significance of this trait by evaluating the shape of corporal cortical bone distribution in Miocene ape taxa representing hominines (Dryopithecus fontani and Rudapithecus hungaricus) and pongines (Sivapithecus sivalensis) (Kelley, 2002; Begun et al., 2012).

3.3 Materials and Methods

3.3.1 Sample

Computerized tomography (CT) scans of mandibular specimens representing P. pygmaeus, P. abelii, G. beringei, G. gorilla, P. troglodytes and P. paniscus (Figure 6) were provided for this analysis by Smithsonian Institution - National Museum of Natural History

SIEMENS Somatom Emotion CT scanner (110 kV, 70 mA, 1 mm slice thickness, 0.1 mm reconstruction increment, H50 moderately sharp kernel). Specimens used in this study were limited to adults that lacked obvious signs of pathology. Study sample sizes per species and sex are provided in Table 12. The geographic locations, dates and specimen numbers of the Miocene ape sample are provided in Table 13. The hypothesized evolutionary relationship of the fossil taxa relative to each other and to extant hominoids are depicted in Figure 7.

Table 12: Extant hominoid taxa used in this study.

Male Female Total Pongo pygmaeus 20 22 42 Pongo abelii 7 6 13 Gorilla gorilla 19 9 28 Gorilla beringei 11 7 18 Pan troglodytes 8 17 25 Pan paniscus 5 8 13

Table 13: Fossil hominoids used in this study

Specimen number Locality Age (Ma) Rudapithecus RUD 212 Rudabánya, Hungary 10.3-9.81 hungaricus Dryopithecus fontani Gaudry specimen St. Gaudens, France 11-121 Sivapithecus sivalensis YPM 13811 WSW Hasnot 102 Sivapithecus sivalensis YPM 13814 Chinji, SE Hasnot 12.7-11.22 1Casanova-Vila et al., 2012; 2Kelley, 2005.

3.3.2 Data acquisition

To quantify the outline of cortical bone in the mandibular corpus, six landmarks (Table

14) and four curves with 30 semilandmarks each (Table 15) were collected in Amira (Mercury

Computer Systems/3D Viz group, San Diego, CA) on the right side of the mandible between M2-

M1 (Figure 3). Due to damage on the right side of RUD 212, data was acquired from the left side of the mandible and mirror imaged. Unfortunately, the D. fontani specimen is damaged in the inferior portion of the left side of the corpus between M1-M2, as well as along the lingual aspect of the corpus on the right side (Figure 6). To accommodate this, data from the right side and the lingual curves were estimated using the left side as a guide. All landmarks and semilandmarks were collected by MKP. Intraobserver error was assessed by collecting all landmarks and semilandmarks from the same specimen ten times on ten different days. These data were used to calculate the Procrustes distance (square root of the sum of squares distance between corresponding landmarks after superimposition (Bookstein, 1991)) between the mean for all ten replicates and each replicate, resulting in a range from 0.005 – 0.001

Figure 6: Corporal cross-sections of extant and fossil hominoids included in this analysis. Images are not to scale. See methods for explanation of data acquisition for D. fontani specimen.

Table 14: Definitions of landmarks used in geometric morphometric analyses.

No. Landmark Definition 1 LINGM2-M1 Point on alveolar border between M2-M1 on lingual side 2 INFM2-M1 Most inferior point on the corpus between M2-M1 3 BUCM2-M1 Point on alveolar border between M2-M1 on buccal side 4 Internal BUCM2-M1 Point on alveolar border between M3-M2 on buccal side, internal aspect of cortical bone 5 Internal INFM2-M1 Most inferior point on the corpus between M2-M1 on internal aspect of cortical bone 6 Internal LINGM2- Point on alveolar border between M2-M1 on lingual side, M1 internal aspect of cortical bone

Table 15: Definitions of curves used in geometric morphometric analyses.

No. Curve Definition 1 External Lingual Between landmarks 1 and 2 2 External Buccal Between landmarks 2 and 3 3 Internal Buccal Between landmarks 4 and 5 4 Internal Lingual Between landmarks 5 and 6

Figure 7: Depiction of evolutionary relationships of extant and fossil taxa included in this analysis

Figure 8: Depiction of landmarks (red) and semilandmarks (blue) used to quantify the outline of cortical bone in the mandibular corpus in this study.

3.3.3 Geometric Morphometric Analysis

Semilandmarks were resampled to be equidistant along their curves and ‘slid’ via minimizing bending energy. The resulting landmarks were then subjected to generalized

Procrustes analysis (GPA). GPA translates, scales and rotates the landmark data, producing superimposed Procrustes coordinates (Dryden and Mardia, 1998; Slice, 2007). Shape variation was assessed by conducting a principal component analysis (PCA) on the Procrustes residuals.

The resulting principal components (PCs) were then examined. Size was assessed using centroid size (calculated as the square root of the sum of squared distances of the landmarks from their

62 centroid (Zelditch et al., 2004). Allometric variation (size related shape differences) was examined through a multivariate regression of Procrustes shape coordinates on centroid size.

Overall shape variation in the dataset was examined without the effects of allometry by conducting an additional PCA on the multivariate regression residuals (of shape on size).

Between group shape differences (inter- and intraspecies) were further examined through a

Procrustes ANOVA, considering size, sex and species. Pairwise group comparisons were conducted as post-hoc tests to the Procrustes ANOVA. Because these involved multiple comparisons, the sequential Bonferroni adjustment (Holm, 1979; Rice, 1989) was applied to an a priori significance level of 0.05 to protect against Type I errors associated. The Bonferroni adjusted alpha was calculated separately for each set of pairwise comparisons. This protocol was followed for two separate analyses, one that included fossil hominoids and one that only included extant great apes. All geometric morphometric analyses were conducted using the R package ‘Geomorph’ v. 3.0.5 (Adams and Orárola-Castillo, 2017).

3.3.4 Phylogenetic Analysis

The possibility of a phylogenetic signal in the variation of shape among species was assessed through a permutation test (10000 randomization rounds) that simulates the null hypothesis of no phylogenetic signal (Klingenberg and Gidaszewski, 2010). The phylogenetic tree of the extant hominoids used for this analysis was obtained from published data on the

10kTrees website (10ktrees.fas.harvard.edu; Arnold et al., 2010) (Figure 9). The permutation tests were conducted for both shape and centroid size in MorphoJ (Klingenberg, 2011).

Additionally, Kmult, which is a multivariate extension of the K-statistic evaluating phylogenetic signal under a Brownian motion model of evolution (Adams, 2014) was also calculated from the mean shapes of each taxon using the same phylogenetic tree as above (10ktrees.fas.harvard.edu;

Arnold et al., 2010). Kmult was determined using the ‘Geomorph’ v. 3.0.5 (Adams and Orárola-

Castillo, 2017) package in R.

In order to analyze the phylogenetic signal in these data further, a matrix of shape distances was calculated from the Procrustes distances between mean shapes of each species obtained after Procrustes superimposition. Procrustes distances are the square root of the summed squares distances between homologous landmarks (Zelditch et al., 2004).

Figure 9: Phylogenetic tree of extant hominoids used in this study (10ktrees.fas.harvard.edu; Arnold et al., 2010).

The Procrustes distances matrix was then compared to a genetic distance matrix among species that was obtained from the phylogenetic tree from the 10kTrees website

(10ktrees.fas.harvard.edu; Arnold et al., 2010). These matrices were compared by assessing the

64 correlation between the morphometric and genetic distances matrices using a Mantel's test with

10,000 permutations to determine the degree of correspondence between patterns of corpus cortical bone dissimilarities and genetic distances in extant great apes. The reliability of this approach to quantify phylogenetic signal in morphometric data has been demonstrated by numerous studies across several taxa and different anatomical regions (Polly, 2003; Macholan,

2006; Cardini and Elton, 2008; von Cramon-Taubadel and Smith, 2012; Gamarra et al., 2016).

3.3.5 Analysis of Fossil Taxa

In addition to the geometric morphometric analyses described above, the affinities of fossil taxa in shape space were assessed by conducting a CVA on the first 12 PCs, representing

93.34% of the variance found in the PCA. The CVs were used to create a Neighbour joining tree

(NJ) to further assess the affinities of Miocene apes with extant hominoids. CVA and NJ were conducted in PAST v. 3.19 (Hammer et al., 2001).

3.4 Results

3.4.1 Shape of corporal cortical bone distribution in extant great apes

Results of the PCA show grouping of taxa at the species level, with some overlap among them. Along PC1 (39.88%) there is a general taxonomic trend, with G. beringei exhibiting the lowest scores, Pongo species and P. paniscus exhibiting the highest scores and P. troglodytes and G. gorilla overlapping in the middle (Figure 10A & 10B). The lowest PC1 scores are associated with relatively wider, U-shaped corpora with thicker cortical bone distributed fairly

Figure 10: Shape variation in corpus cortical bone distribution in fossil and extant hominoids. A) PCA plot in shape-space showing variation along PC1 and PC2 for analysis of extant hominoids; B) PCA plot in shape-space showing variation along PC1 and PC3 for analysis of extant hominoids; C) Thin plate splines depicting the shape changes along the scores of PC1 and PC2 to illustrate the shape changes from the negative to the positive ends of the axes. Thin plate splines are computed by warping the respective PC scores onto the mean shape of all the specimens in the sample. D) PCA plot in shape-space showing variation along PC1 and PC2 for analysis of fossil and extant hominoids; E) PCA plot in shape-space showing variation along PC1 and PC3 for analysis of fossil and extant hominoids

66 evenly throughout the corpus (Figure 10C). The highest PC1 scores exhibit relatively narrower,

V-shaped corpora with thicker amounts of cortical bone along the superobuccal and inferior aspects of the corpus (Figure 10C). PC2 (14.99%) has a high amount of overlap among taxa, with all taxa exhibiting variation along this PC. Pan exhibits the highest amount of variation, with P. troglodytes having the lowest PC2 scores and P. pansicus having the highest PC2 scores

(Figure 10A). Shape changes along PC2 are related to corporal width, negative scores indicate relatively wider, U-shaped corpora and positive scores have relatively narrower, V-shaped corpora. Overall cortical bone distribution is similar in thickness along PC2 (Figure 10C). PC3

(12.81%) also exhibits a high amount of overlap among taxa, with the exception of P. paniscus, which has lower scores along this PC than all other taxa (Figure 10B). The lowest PC3 scores are associated with corpora that are relatively wider in the superior portion and narrower in the inferior portion with relatively thicker cortical bone than positive PC3 scores (Figure 10C).

Procrustes ANOVA show a significant relationship between size and shape (Table 15).

Multivariate regression of Procrustes shape variables on size shows an allometric trend in the dataset, with mandibular shape scores increasing with centroid size (Figure 11A). This trend also reflects a taxonomic gradient, with Gorilla and Pongo specimens at the two extremes and Pan specimens in the middle. This plot also shows that Gorilla specimens exhibit more variation in centroid size than other taxa in this analysis (Figure 11A). Investigation into allometric shape changes shows that specimens with a large centroid size have a relatively taller corpus with a thinner distribution of cortical bone than specimens with a small centroid size (Figure 11B). To further investigate the effects of allometry, the regression residuals were subjected to PCA.

While the resulting scatterplot of PC1 and PC2 is very similar to the results of the initial PCA, it does show increased variation in P. troglodytes and G. gorilla, as well as decreased variation in

P. pygmaeus along PC1 (Appendix Figure 1). Since the size related shape changes are related to cortical bone thickness (Figure 11B), a central part of our research question, we decided not to remove allometric shape changes from our analysis and to focus our analysis on the original

PCA rather than the PCA on the regression residuals.

Table 16: Procrustes ANOVA for analysis of extant hominoids testing for shape and size differences between species and sex, and interactions between them1.

Size Species Sex Size:Species Size:Sex Species:Sex Size:Species:Sex F 7.981 14.987 3.067 1.620 1.100 1.445 0.777 df 1 5 1 5 1 5 5 p 0.001 0.001 0.002 0.001 0.064 0.001 0.060 1 p-values < 0.05 in bold The Procrustes ANOVA results reveal significant shape differences when considering size, species and sex. Significant shape differences were also apparent when considering the interaction between size and species, as well as the interaction between species and sex (Table

16). Pairwise comparisons of corpus shape of all taxonomic groups standardizing for the effects of sex, provided in Table 17, show a significant difference among all species. Pairwise comparisons of corpus shape sexual dimorphism did not result in significant differences between males and females of the same species for any of the great apes (Table 18). These results indicate that although there is a general trend of sexual dimorphism in the data, it is not significant at the species level.

Species mean corpus shape, depicted in Figure 12, shows that the shape differences among species are driven by distribution of cortical bone as well as corpus height and width.

Relative to all other taxa, Gorilla species have larger amounts of cortical bone throughout their entire corpus. G. gorilla is differentiated from G. beringei by exhibiting a corpus that is relatively narrower in the inferior portion and has a larger amount of cortical bone in the inferiolingual portion of the corpus. Relative to all other hominoids, P. paniscus displays the

Table 17: Analysis of extant taxa p-values obtained from pairwise comparisons across great ape species, based on 10,000 permutations of Procrustes distances, after standardising for the effects of sex1.

G. beringei G. gorilla P. abelii P. pygmaeus P. paniscus P. troglodytes G. beringei 1 G. gorilla 0.012 1 P. abelii 0.001 0.001 1 P. pygmaeus 0.001 0.001 0.002 1 P. paniscus 0.001 0.001 0.001 0.001 1 P. troglodytes 0.001 0.002 0.003 0.001 0.001 1 1 Significant at the Bonferroni adjusted alpha of 0.003 in bold

Table 18: Analysis of extant hominoids, p-values obtained from pairwise comparisons of all taxonomic groups, based on 10,000 permutations of Procrustes distances, testing for sexual dimorphism in corpus shape1,2.

GbF GbM GgF GgM PaF PaM PpyF PpyF PpaF PpaM PtF PtM GbF 1 GbM 0.234 1 GgF 0.175 0.629 1 GgM 0.006 0.031 0.153 1 PaF 0.001 0.004 0.007 0.006 1 PaM 0.002 0.059 0.113 0.078 0.610 1 PpyF 0.001 0.001 0.001 0.001 0.069 0.008 1 PpyM 0.001 0.001 0.001 0.001 0.073 0.035 0.443 1 PpaF 0.001 0.001 0.001 0.001 0.006 0.002 0.001 0.001 1 PpaM 0.001 0.001 0.002 0.004 0.009 0.012 0.003 0.004 0.121 1 PtF 0.001 0.001 0.019 0.002 0.019 0.027 0.001 0.001 0.021 0.002 1 PtM 0.006 0.021 0.115 0.020 0.162 0.181 0.011 0.020 0.003 0.002 0.778 1 1 Significant at the Bonferroni adjusted alpha of 0.0008 in bold 1Abbreviations: M = male, F = female, Gb = G. beringei, Gg = G. gorilla, Pa = P. abelii, Ppy = P. pygmaeus, Ppa = P. paniscus, Pt = P. troglodytes.

most distinctive corpus, which is V-shaped. All of the African apes have thicker cortical bone

distributed in the inferiolingual region than either Pongo species. Compared to all other taxa, P.

pygmaeus has the least amount of cortical bone throughout the entire corporal section. P. abelii

is differentiated from P. pygmaeus by similar amounts of cortical bone as displayed in P.

Figure 12: Thin plate splines of species mean shapes for analysis of extant hominoids. Each species mean shape is warped to the mean shape of the entire sample.

troglodytes along the lingual and buccal aspects of the corpus, but these taxa are differentiated from each other by differences in the inferior region (Figure 12).

3.4.2 Analysis of phylogenetic signal in extant apes

The permutation procedure assessing the phylogenetic signal of the shape of cortical bone distribution in the mandibular corpus of extant great apes resulted in a significant p-value of

0.04. This indicates that cortical shape in the mandibular corpus reflects a phylogenetic signal in the extant taxa, however it does not indicate the strength of the phylogenetic signal (Klingenberg and Gidaszewski, 2010). Regarding size, the permutation procedure resulted in a p-value of

1.000 when assessing the phylogenetic signal of centroid size in extant great apes, indicating that size does not display a phylogenetic signal in this sample. Kmult values for shape and size are

0.5655 (p = 0.131) and 0.26 (p = 0.9465) respectively. Comparison of Procrustes and genetic distance matrices show significant correlation, p=0.0112 (R=0.9983).

3.4.3 Corporal cortical bone distribution in Miocene hominoids

The GPA that included the fossil specimens resulted in all four Miocene ape specimens falling in the area of overlap among extant taxa along PC1 (39.52%), PC2 (15.05%) and PC3

(12.75%) (Figure 10D & 10E). Relative to all other taxa, R. hungaricus is characterized by an even distribution of cortical bone throughout the corpus that is similar in thickness and distribution to that of P. troglodytes (Figure 6). The corpus of D. fontani is characterized by an uneven distribution of cortical bone, with relatively less cortical bone distributed along the lingual aspect compared to the buccal aspect (Figure 6). One of the S. sivalensis specimens,

YPM 13811, has a U-shaped mandibular corpus with a CBD that is thicker than that of P. pygmaeus and is similar to that of P. abelii and P. troglodytes (Figure 6).

YPM 13814, the second S. sivalensis specimen, has a CBD that is distributed evenly throughout the corpus and is similar to P. pygmaeus by having a relatively thin amount of cortical bone.

CVA of the first 12 PC scores results in eight CVs. The plot of CV1 (57.22%) and CV2

(26.55%) results in a separation of extant taxonomic groups with R. hungaricus at the edge between P. troglodytes and P. pygmaeus and D. fontani falling at the edge between P. troglodytes and P. abelii, YPM 13811 falls within P. troglodytes and YPM 13814 falls in the area between P. troglodytes, P. pygmaeus and P. abelii (Figure 13). NJ based on all eight CVs results in D. fontani grouping with P. abelii and G. beringei specimens, R. hungaricus and YPM

13811 falling in a group that consists mostly of P. paniscus and YPM 13814 grouping with P. pygmaeus (Figure 14).

3.5 Discussion

3.5.1 Significance of patterns of corporal cortical bone distribution in extant hominoids

The results of this analysis support Daegling’s (2007) findings that P. pygmaeus has less cortical bone in the corpus than African apes. However, inclusion of P. abelii into our analysis reveals that this pattern does not extend to all Asian apes. P. abelii has a similar shape of CBD as

P. troglodytes in the superior portion of the corpus, but these taxa differ in their distribution of cortical bone in the inferior portion of the corpus. Daegling’s (2007) analysis of cortical bone in extant hominoids also found significant differences in cortical bone distribution between males and females in P. troglodytes at M1. The results of this analysis do not confirm differences in

CBD between males and females in P. troglodytes, which may be the result of an increased number of females in our sample.

The observed taxonomic differences in the shape of corporal CBD among extant hominoids do not follow predictions based on dietary differences. Contrary to predictions based on diet, P. pygmaeus, which has a harder, tougher diet than P. abelii (Table 10), has less cortical bone in the entire cross-section of the corpus between M1 and M2. Among Gorilla species, G. gorilla has relatively more cortical bone in the inferolingual portion of the corpus than G. beringei and similar CBD along the lingual and buccal aspects. The significance of these patterns are difficult to interpret without knowing the mechanical properties of G. gorilla diets. However, if G. beringei experiences increased amounts of masticatory stress related to increased consumption of THV, this may be reflected in the relatively increased amounts of cortical bone located in the inferolingual portion of the corpus in G. beringei but is not reflected in the similar

CBD along the lingual and buccal portions of the corpora in these taxa.

The differences among Pan species also do not seem to correlate with diet, with P. paniscus exhibiting a relatively narrower inferior portion of the corpus that has thicker cortical bone than P. troglodytes. However, the dietary mechanical properties of P. paniscus are currently unknown and interpretations of the functional significance of differences between these taxa are difficult to evaluate. From what we do know about the dietary differences between P. troglodytes and P. paniscus we can infer two scenarios; P. paniscus masticates relatively increased amounts of THV resulting in increased masticatory stresses, or the mechanical properties of P. paniscus and P. troglodytes diets do not differ significantly and they do not experience differences in masticatory stress. In the first scenario, relative to P. troglodytes, P. paniscus would be predicted to have a wider, taller corpus with increased cortical bone along the

75 entire corpus. In the second scenario, P. paniscus and P. troglodytes would be expected to exhibit similarly shaped corpora with similar CBD. Neither of these scenarios are reflected in the shape of corporal CBD in Pan species. The fact that extant hominoids do not match morphological predications based on dietary differences may indicate that these predictions need to be re-evaluated. These predications stem from in vivo analyses of non-hominoid species

(Bouvier and Hylander, 1981; Hylander, 1979a,b,c; Hylander et al., 1987). It is possible that non-hominoid models of the functional significance of mandibular morphology do not apply to hominoids or that in vivo strain gauge experiment results are not correlated with mandibular morphology in a straight-forward way.

One of the arguments used to support the hypothesis that jaw-loading patterns are similar across anthropoids has been that anthropoids share a common pattern of CBD in the post-canine corpus, with a relatively thinner distribution of cortical bone along the lingual aspect and a thicker distribution along the buccual aspect (Demes et al., 1984; Hylander et al., 1988; Daegling and Grine, 1991; Daegling and Hotzman, 2003). To date, whether this pattern is actually ubiquitous among all anthropoids has been unknown (Daegling and Hotzman, 2003). Our study of CBD is the first to include P. paniscus, P. abelii and G. beringei. The mean shapes of the taxa in this analysis (Figure 13), do not support the hypothesis that there is a common pattern of CBD in the anthropoid post-canine corpus. We find that the corpus of Gorilla species has a relatively equal distribution of cortical bone along their lingual and buccal aspects of the corpus (Figure

13). Additionally, P. troglodytes does not demonstrate uniformly ‘thin’ cortical bone along the lingual aspect of the corpus, it is only relatively thinner beginning at almost the mid-point of the lingual aspect of the corpus (Figure 13). Daegling (2002) has suggested that jaw loading patterns are not conservative across anthropoids. Our results provide additional support that non-

76 hominoid models predicting the relationship between diet and mandibular morphology may not apply to hominoids. Hominoids demonstrate longer periods of growth and dental development than non-hominoid anthropoids (Smith, 1989; Godfrey et al., 2003) and it is possible that CBD in the mandibular corpus of hominoids is either not influenced or less influenced by diet than by other factors such as growth and/or phylogeny. However, jaw adductor muscle activity patterns are similar across anthropoids (Vinyard et al., 2008), which supports the hypothesis that jaw- loading patterns are similar across anthropoids. Further research into this area is needed to help develop an understanding of the possibility of hominoids exhibiting different jaw-loading patterns than other anthropoids.

An important potential issue in this study (and any study that utilizes museum samples) is that the localities that these specimens come from do not overlap with existing field data on diet.

Since Pongo and Pan are known to exhibit geographic variation in dietary composition (Russon et al., 2009; Hohmann et al., 2010) it is possible that the specimens used in this study may not provide an accurate reflection of dietary variation present in extant great apes. This may have important implications for our understanding of the relationship between diet and CBD.

3.5.2 Phylogenetic signal of corporal cortical bone in extant hominoids

Results of analysis of the phylogenetic signal of the shape of corporal CBD in extant hominoids are mixed. Assessing phylogenetic signal in morphometric data is complex

(MacLeod, 2002; Revell et al., 2008; Münkemüller et al., 2012). The analysis of the phylogenetic signal in this data set provides an example of using multiple lines of evidence to understand the complexity of phylogenetic signaling in morphometric datasets. Kmult values range from 1 to 0, with values closer to 1 indicating a strong phylogenetic signal and values closer to 0 indicating no phylogenetic signal (Adams, 2014). Kmult values in the middle of 0 and 1, such as the one

77 produced here (Kmult=0.5655) are more difficult to interpret. Analysis of the phylogenetic significance of the shape of CBD using the permutation procedure yielding a p-value of 0.04, a significant result indicating that there is some phylogenetic signal in the dataset. Comparison of the Procrustes distance and genetic distance matrices provides the strongest evidence of phylogenetic signal in this dataset (p=0.0112). These results support the hypothesis that CBD shape differences among hominoids exceeds those expected for closely related species.

3.5.3 Affinities of Miocene hominoid corporal cortical bone distribution

In order to understand the evolutionary significance of the shape of corporal CBD in extant hominoids, the pattern of this trait was examined in Miocene hominoids. Three of the

Miocene hominoid specimens included in this study (R. hungaricus, D. fontani and YPM 13811) have CBDs that are more similar to African apes and P. abelii than they are to P. pygmaeus. The last Miocene specimen considered in this analysis, YPM 13814, has relatively thin cortical bone throughout the corporal cross-section and is more similar to P. pygmaeus specimens than any of the other hominoids.

The taxonomy of Sivapithecus has been debated for over 30 years (Kay, 1982; Kelley and

Pilbeam, 1986; Kelley, 1988, 2002, 2005). Unfortunately, the results presented here do not resolve this issue. The distance between both Sivapithecus specimens is lower than the maximum within-species distances among extant specimens, indicating that a single species attribution cannot be excluded (Figure 13). In Figure 14, YPM 13814 is nested in a cluster that includes only Pongo, with all but one specimen belonging to P. pygmaeus. YPM 13811 occurs in a cluster consisting mostly of P. paniscus, with one specimen each of P. troglodytes and P. pygmaeus.

However, in Figure 14, although most of the P. pygmaeus specimens cluster to the left, six specimens fall within the cluster to the right, which includes both YPM 13811 and Rudapithecus.

Thus, within our sample of extant P. pygmaeus the spread of specimens encompasses the differences between the two specimens of Sivapithecus and Rudapithecus. Having said that, the

Sivapithecus specimens are less separated in morphometric space than many specimens of different extant great ape genera, so that a two species, or even two genus hypotheses cannot be excluded either.

Though all the fossil specimens included in this analysis fall within the spread of

Euclidean distances of CVA exhibited by P. pygmaeus (Figure 14), no one would suggest that these specimens all belong to the same species. Sivapithecus is distinguished from Rudapithecus and Dryopithecus by numerous attributes of the dentition, cranium and postcranium (Kelley,

2002; Alba, 2012; Begun, 2014). Rudapithecus and Dryopithecus are also distinguished by dental attributes (Alba, 2012; Begun, 2014). Each of the fossil specimens occupies a small portion of the range of variation of the taxon they represent. At these sample sizes it is not possible to determine where the convex hulls of each would fall.

3.6 Conclusions

Analysis of the shape of CBD between M1-M2 in extant hominoids identifies significant shape differences among species. These shape differences do not reflect predictions based on differences in diet, suggesting that the shape of CBD in hominoids is not strongly influenced by diet. This study provides the first published CBD data for several extant hominoids (P. paniscus,

G. beringei and P. abelii) and fossil hominoids (D. fontani, R. hungaricus and S. sivalensis). The expanded sample analysed here allows for the first comprehensive test of the hypothesis that all anthropoids have relatively thin cortical bone along the lingual border of the corpus and thick cortical bone along the buccal border of the corpus (Demes et al., 1984; Hylander et al., 1988;

Daegling and Grine, 1991; Daegling and Hotzman, 2003). Results of the analysis of the shape of

CBD between M1-M2 in extant hominoids do not provide support for this hypothesis and raise questions regarding the hypothesis that jaw-loading patterns are constrained across anthropoids

(Demes et al., 1984; Hylander et al., 1988; Daegling and Grine, 1991; Daegling and Hotzman,

2003).

Analysis of the phylogenetic signal of the shape of CBD in extant hominoids shows that the shape differences among hominoids exceeds those expected for closely related species.

Investigation of the shape of CBD in fossil hominoids reveals that three of the four Miocene apes studied here exhibit CBD shapes more similar to extant P. troglodytes than to any other extant great ape. Although the two Sivapithecus specimens included in this analysis demonstrate separation from each other in some aspects of this analysis, these specimens are less separated in morphometric space than many specimens of different extant great ape genera. Meaning that a two species, or even two genus hypotheses of Sivapithecus taxonomy cannot be excluded. The results of this study suggest that CBD in the mandibular corpus of hominoids is influenced by phylogeny, growth or both and is less reflective of diet. Further research into corporal CBD in fossil hominoids is necessary to provide additional insight into the influence of phylogeny on the shape of CBD in hominoids. Furthermore, the relationship between corporal CBD and the regulatory mechanisms linking cortical bone with tooth root formation and dental growth requires further attention in hominoids.

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Chapter 4

The Relationship Between Growth, Dental Development and Mandibular Corpus Shape in Hominoids.

M. KATHLEEN PITIRRI*, DAVID R. BEGUN

Affiliations: Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2

Correspondence To: M. Kathleen Pitirri, Department of Anthropology, University of Toronto, 19 Russell St. Toronto, ON, M5S 2S2. Email: [email protected]

Submitted to the American Journal of Physical Anthropology (September 17, 2018)

4.1 Abstract

The influence of growth on the shape of the mandibular corpus has important implications for interpreting the significance of corpus shape variation in the primate fossil record. Here, we quantify and compare the cross-sectional shape of the mandibular corpus at M1-

M2 during growth in Pan paniscus, Pan troglodytes and Pongo pygmaeus, and assess the hypothesis that the shape of the corpus is influenced by development of permanent molars in their crypts. Landmarks and semilandmarks were utilized to quantify ontogenetic changes in the shape of the outline of the mandibular corpus in cross-section and measurements of length, width and height were used to quantify molar crypts. Ontogenetic changes in corpus growth from the eruption of M1 to the eruption of M3 were evaluated for each species through Generalized

Procrustes analysis and PCA in shape-space and form-space. The relationship between corpus shape and molar crypt form was investigated at three different developmental stages using two- block partial least squares (2B-PLS) analysis. The results show clear differences in growth patterns among all three species and provide evidence that species level differences in

95 mandibular corpus growth occur prior to the emergence of M1. The results of the 2B-PLS analysis reveal that significant covariance between corpus shape and molar crypt form is limited to the developmental stage marked by the emergence of M1, with covariance between corpus shape and M2 crypt width. Corpora that are relatively narrower in the inferior portion of the cross-section covary with relatively narrower M2 crypts. The results also show a lack of significant covariance between corpus shape observed later in development (after the emergence of M2) and M3 crypt form, indicating that the shape of the corpus at the cross-section of M1-M2 is not influenced by the development of permanent molars after the emergence of M2.

4.2 Introduction

The mandibular corpus plays an important role in interpretations of the hominoid fossil record (Miocene apes (Brown, 1997; Güleç and Begun, 2003; Koufos and Bonis, 2005; McNulty et al., 2015; Fuss et al., 2017), australopithecines (Wood, 1991; Silverman et al., 2001; Kimbel et al., 2004; Haile-Selassie, 2010; Cofran, 2014; Haile-Selassie et al., 2015; Glowacka et al., 2017) and members of the genus Homo (Fabbri, 2006; Skinner et al., 2006; Lague et al., 2008; Brown and Maeda, 2009; Chang et al., 2015)). However, the underlying factors influencing taxonomic differences in mandibular corpus shape in hominoids remains poorly understood (Humphrey et al., 1999; Taylor, 2002, 2003; Ross et al., 2012). Understanding the factors influencing mandibular corpus shape in extant hominoids has important implications for interpreting corporal shape variation in the hominoid fossil record. Several researchers have suggested that the hominoid mandibular corpus is influenced by diet (Bouvier and Hylander, 1981; Hylander,

1979a,b, 1984, 1985, Taylor, 2006a,b) and/or by factors related to growth (Dageling, 1996;

Boughner and Dean, 2004, 2008; Boughner, 2011). Although covariation of mandibular corporal morphology and dental development has been suggested by previous researchers, this

96 relationship has been understudied in hominoids (Daegling, 1996; Boughner and Dean, 2004,

2008; Boughner, 2011). This study aims to provide exploratory results on the relationship between dental development and the ontogeny of mandibular corpus shape in extant hominoids.

4.2.1 Previous studies of mandibular corpus ontogeny

While growth and development of great ape crania has been the focus of numerous studies (Giles, 1956; Shea, 1983, 1985; Leutenegger and Masterson, 1989; Bromage, 1992;

Lieberman and McCarthy, 1999; Lieberman et al., 2000; Bruner and Manzi, 2001; Williams et al., 2003; Bastir and Rosas, 2004; Berge and Penin, 2004; Mitteroecker et al., 2004, 2005;

Ackermann, 2005; Cobb and O’Higgins, 2007; Lieberman et al., 2007; Mitteroecker and

Bookstein, 2008; Singh et al., 2012; Pérez-Claros et al., 2015; Zollikofer et al., 2017), relatively less research has been conducted on mandibular growth and development (Daegling, 1996;

Taylor and Groves, 2003; Taylor, 2002, 2003; Boughner and Dean, 2004, 2008; Singh, 2014;

Terhune et al., 2014; Martinez-Maza et al., 2016). Of these studies, very few have quantified the shape of the mandibular corpus in great apes by including measures of corpus breadth as well as height, and data are limited to Pan and Gorilla species (Taylor and Groves, 2003; Taylor, 2002,

2003; Boughner and Dean, 2004, 2008; Martinez-Maza et al., 2016). These analyses have provided evidence of developmental decoupling in the mandible, with the corpus developing separately from the symphyseal and ramus regions of the mandible (Daegling, 1996, Singh,

2014, Martinez-Maza et al., 2016). Ontogenetic analyses of the entire mandible have found that

Pan paniscus and Pan troglodytes have parallel allometric trajectories and that species-specific patterns of adult mandibular shape are established prior to the eruption of deciduous teeth

(Boughner and Dean, 2008; Singh, 2014). Comparisons of mandibular growth in African apes have shown that Pan and Gorilla share parallel allometric trajectories and that decrease in the

97 rate of the growth of corpus width coincides with M1 eruption (Taylor, 2002, Taylor and Groves,

2003). However, recent analysis of mandibular growth and bone modelling in Gorilla gorilla and

P. troglodytes has shown that greatest amount of corpus growth occurs on the lingual aspect along the portion from the canine to the corpus ramus junction and that the rate of growth in this portion is highest during the period between the eruption of M1 and M3 (Martinez-Maza et al.,

2016).

4.2.2 Previous studies on the relationship between mandibular corpus growth and dental development

Research on the relationship between dental development and mandibular corporal morphology in hominoids has been limited to P. paniscus, P. troglodytes (Boughner and Dean,

2004, 2008; Boughner, 2011) and modern humans (Coquerelle et al., 2010; Coquerelle et al.,

2011). Boughner and Dean’s (2004, 2008) analyses of mandibular shape and the timing of dental development in chimpanzees did not find a relationship between corpus shape and the timing of dental development. However, Coquerelle and colleagues (2010, 2011) have found a strong correlation between mandibular shape and dental development during the first two years of life in humans. Boughner and Dean’s (2004, 2008) approaches did not quantify the mandibular corpus using the same methodology as Coquerelle et al. (2010, 2011), which might explain why

Boughner and Dean (2004, 2008) did not find a similar relationship between dental development and corpus shape in chimpanzees as Coquerelle et al. (2010, 2011) did in humans. Additionally,

Boughner’s (2011) analysis of mandibular growth in P. troglodytes and P. paniscus provides some support for covariation between mandibular growth and dental development. However, it should be noted that the data used in Boughner (2011) do not quantify corpus height or width. To date, the relationship between dental development and mandibular corpus shape has not been

98 evaluated in Pongo and Gorilla species. Consequently, the relationship between the shape of the corpus and dental development requires further investigation in all of the extant hominoids.

4.2.3 Homology in the mandibular corpus during growth

Studying ontogenetic shape variation of the mandibular corpus is confounded by problems surrounding homology during growth and development. The adult permanent dentition is absent during early stages of growth. In hominoids, the deciduous premolars function as permanent molars but are replaced by permanent premolars.

In light of the problems surrounding homology along the postcanine row during growth, several studies have excluded points along the corpus and maxilla that are associated with the deciduous dentition (Bruner and Manzi, 2001; Berge and Penin, 2004; Singh et al., 2012; Pérez-

Claros et al., 2015). This approach is problematic because it does not allow for research into ontogenetic variation along the postcanine row, which may be important for interpreting the significance of shape variation of the masticatory apparatus in living and extinct hominoids.

Several studies have considered the deciduous premolars to be functional homologues to M1 and

M2 in primates (Cole, 1992; Taylor, 2002; Taylor and Groves, 2003). Another approach to this problem has been to limit analyses of growth to specimens that have already erupted M1 (Cole,

1992; Daegling, 1996; Daegling et al., 2014). This approach is also problematic because it does not allow for investigation into ontogenetic variation prior in the earliest stages of development.

Despite this limitation, this approach is presently the only approach that allows for comparison of the corpus that are homologous in the sense that they follow the same developmental pathways.

Here, we evaluate ontogenetic changes in the shape of the mandibular corpus after the emergence of M1 in a sample of extant hominoids. The goals of this study are to:

(i) Provide the first analysis of mandibular corpus growth in P. pygmaeus

(ii) Compare the results of P. pygmaeus corpus shape growth to P. paniscus and P.

troglodytes

(iii) Investigate the relationship between mandibular corpus growth and dental

development in hominoids

4.3 Materials and Methods

4.3.1 Sample

Computerized tomography (CT) scans of mandibular specimens representing P. pygmaeus, P. troglodytes and P. paniscus were provided for this analysis by Smithsonian

Institution - National Museum of Natural History (USNM) and the Royal Museum for Central

Africa (RMCA). RMCA CT scans were obtained using a Siemens Somatom Esprit Spiral CT scanner (slice thickness typically varied among samples but ranged between 0.33 and 0.50).

USNM CT scans were obtained using a SIEMENS Somatom Emotion CT scanner (110 kV,

70 mA, 1 mm slice thickness, 0.1 mm reconstruction increment, H50 moderately sharp kernel).

Specimens used in this study were limited to specimens that lacked obvious signs of pathology.

Since chronological ages were not available for these specimens, the sample was divided up into age groups based on dental development. Following Singh (2014), we divided our sample into

Table 19: Samples used in this analysis.

Taxon DS 1 DS 2 DS 3 M1 in occlusion M2 in occlusion M3 in occlusion Pongo pygmaeus 5 3 42 Pan troglodytes 4 2 20 Pan paniscus 2 3 16

100 three developmental stages: Developmental Stage (DS) 1 includes individuals with M1 fully erupted, DS2 includes specimens with M2 fully erupted and DS3 contains adults that have fully erupted M3. Study sample sizes per species and DS are provided in Table 19.

4.3.2 Data Acquisition and Analyses

Following the same methodology as our previous analysis on great ape mandibular corpus shape (Pitirri and Begun, 2019), the outline of the mandibular corpus in cross-section was quantified using three landmarks and two curves, comprised of 30 semilandmarks per curve

(Table 20) (Figure 15A). These data were collected using Amira (Mercury Computer

Systems/3D Viz group, San Diego, CA). All data were collected on the right side of the mandible at different points along the corpus depending on DS. For DS1, data were collected at the cross-section distal to M1 and for both DS2 and DS3, data were collected at the cross-section between M1-M2.

Table 20: Definitions of landmarks used in geometric morphometric analyses.

Semilandmarks were resampled to be equidistant along their curves and ‘slid’ via minimizing bending energy (Bookstein 1997; Bookstein et al. 1999; Gunz et al. 2005). The resulting landmarks were then subjected to generalized Procrustes analysis (GPA). GPA translates, scales and rotates the landmark data, producing superimposed Procrustes coordinates

(Dryden and Mardia, 1998; Slice, 2007). This approach allows for analysis of changes in shape during ontogeny (development) without the influence of changes in size during ontogeny

(growth) (Mitteroecker et al., 2004, 2005). Shape refers to geometric properties that are

101 independent of rotation, translation and scale, while form refers to geometric properties that are only independent of rotation and translation (Mitteroecker et al., 2013).

Figure 15: Depiction of data used in this study. A) Coronal cross section of Pongo pygmaeus specimen between M1 and M2 depicting the three landmarks (blue circles) and 60 semilandmarks (red circles) used to quantify corpus shape in this analysis. B) Coronal cross section of M2 crypt of P. pygmaeus CT scan depicting measurements of crypt height (solid line) and crypt width (dotted line). C) Transverse cross section of M2 crypt of P. pygmaeus CT scan depicting measurement of crypt length (dashed line). L = Lingual, B = Buccal

To visually examine and summarize variation in shape-space, the landmarks were subjected to principal components analysis (PCA). Shape changes during ontogeny were examined by calculating the mean landmark configuration for each DS per species. Thin plate spline analysis (TPSA) was conducted for each species by warping the mean DS shapes consecutively (DS2 was warped to DS1 and DS3 was warped to DS2). Allometric trajectories were assessed by conducting a form-space PCA (FPCA). PCA in form-space uses Procrustes registration, but includes the natural logarithm of centroid size (CS) as a variable in the PCA.

This method has been used in other studies, comparing ontogenetic trajectories in both the

102 cranium and mandible (Bastir et al., 2007; Mitteroecker et al., 2004, Singh 2014, Coquerelle et al., 2010, 2011).

The relationship between mandibular corpus shape during ontogeny and dental development was investigated by comparing the DS shapes for each species to aspects of dental development. To do this, we measured permanent molar crypt height, width and length for each

DS (Figure 15B & C). These metrics were chosen to quantify the crypts because as three- dimensional structures, molar crypts are irregular in shape that change during growth and do not exhibit morphologies that meet the requirements of homologous points necessary for landmarks

(see Zelditch et al., 2012 for discussion of homology in landmark data collection). The relationship between crypt height, width and length with mandibular corpus shape of relevant

DSs were assessed using Two-block Partial Least Squares (2B-PLS) analysis (Rolf and Corti,

2000). Separate analyses were conducted for each DS and for each stage in crypt development.

2B-PLS analyses were conducted using tps-pls (v. 1.18) (Rolf and Corti, 2000). Permutation tests (999 permutations) were conducted to assess the statistical significance of the results of each of the 2B-PLS analyses.

4.4 Results

4.4.1 Ontogeny of mandibular corpus shape variation – PCA in shape-space

Scatterplot of PC 1 and PC 2, accounting for 46.72% and 19.95% of the total variance respectively, does not result in taxonomic separation of the sample along PC1 (Figure 16A).

However, this plot results in some separation of specimens by DS (Figure16B). Most of the DS1 and DS2 specimens have relatively lower PC 1 scores than DS3 specimens, although a few DS1 and DS2 P. paniscus specimens have PC1 scores within the range of adult P. paniscus specimens. Shape changes along PC 1 are associated with differences in overall corpus height

103

104 and width. Specimens with low PC 1 scores demonstrate shorter corpora that are relatively wider throughout the entire corpus. High PC 1 scores are associated with relatively taller corpora that are narrower, especially in the inferior region (Figure 16A). Along PC 2, many of the P. paniscus specimens have higher scores than both P. troglodytes and P. pygmaeus specimens. Shape changes along PC 2 are associated with differences in corpus width. Lower PC 2 scores are associated with corporal cross-sections that are superiorly wider and inferiorly narrower than specimens with higher PC 2 scores (Figure 16A).

Results of the TPSA of mean corpus shape for each DS per species are depicted in Figure

17. Examination of the ontogenetic shape changes in P. troglodytes shows that between DS1 and

DS2 the inferior portion (base) of the corpus narrows buccolingually, the lingual aspect of the corpus flattens from inferior to the occlusal margin to the base, and the superiobuccal portion of the alveolar process widens or flared buccally. Between DS2 and DS3 alveolar process narrows both lingually and buccally. In P. paniscus between DS1 and DS2 there is an even more substantial narrowing of the corpus below the alveolar margin lingually, with the development of a pronounced sublingual fossa and a broadening of the superior-buccal aspect of the alveolar process buccally. Between DS2 and DS3 both the alveolar and base portions of the corpus become narrower. In P. pygmaeus ontogenetic shape changes between DS1 and DS2 include, a widening of the alveolar portion buccally without the flattening or concavity development seen in Pan. Between DS2 and DS3 the corpus becomes narrower.

4.4.2 Mandibular corpus ontogenetic trajectories – PCA in form-space

Figure 18 shows the first two components of the form-space PCA. The first axis, FPC 1, summarizes 99.98% of the total variance and its correlation with LnCS is r = 0.9998, which means that almost all of the variance is driven by size related shape change of the mandibular

105

Figure 17: Thin plate splines showing ontogenetic shape change within each species.

corpus. The second axis, FPC 2, represents 0.008% of the total variance within the sample.

Relative to both Pan species, P. pygmaeus demonstrates the greatest amount of variation along

FPC 1. DS1 and DS2 specimens of each species have lower FPC 1 scores than DS3 specimens of

106 the same species. P. paniscus is separated from P. troglodytes along FPC 1, while P. troglodytes demonstrates overlap with P. pygmaeus along this axis.

4.4.3 Relationship between ontogeny of mandibular corpus shape and dental development

Descriptive statistics for the linear measurements used to quantify M2 and M3 crypts for each of the 2B-PLS analyses are provided in Table 21. The singular values and the correlations for the first two dimensions of the 2B-PLS analyses are provided in Table 22.

Figure 18: Bivariate plot of the first two PCs in form–space. Pongo pygmaeus are represented in blue, Pan troglodytes in purple and Pan paniscus in red.

Covariation of DS1 corpus shape and M2 Crypt Form

The analysis of M2 crypt form and DS1 corpus shape included 12 specimens. The A1 singular value, 0.13189, explains 91.865% of the total covariance with a correlation of 0.903.

The A2 singular value, 0.03513, explains 6.527% of the total covariance with a correlation of

107

0.073. Permutation test of the singular covariance values for the first dimension found that only

4% of the random samples were equal to or larger than the observed values. The second dimension was found to be exceeded or equaled randomly in 92% of the 999 samples.

The plot of the first axis for both datasets shows the relationship between DS1 corpus shape and M2 crypt form (Figure 19A). The loadings for M2 crypt measurements show that A1 is strongly influenced by M2 crypt width and to a lesser extent M2 crypt length and height (Figure

19B). Specimens with high A1-Block 1 scores have the smallest values for M2 crypt width.

Higher A1-Block 2 scores are associated with corpora that have relatively narrower inferior portions and relatively wider superior portions than lower A1 scores (Figure 19C).

Table 21: Descriptive statistics for measurements of M1, M2 and M3 crypts in mm per DS.

DS 1 – M2 DS 1 – M2 and M3 crypts DS 2 – M3 crypt n = 6 crypts n = 12 n = 16 M2 crypt x̅ = 13.95 x̅ = 13.46 length max. = 17.32 max. = 15.42 min. = 11.27 min. = 11.27 M2 crypt x̅ = 12.11 x̅ = 12.41 width max. = 15.52 max. = 13.67 min. = 8.38 min. = 11.12 M2 crypt x̅ = 11.79 x̅ = 11.54 height max. = 14.01 max. = 13.81 min. = 9.29 min. = 9.39 M3 crypt x̅ = 10.99 x̅ = 12.08 length max. = 15.01 max. = 16.99 min. = 8.36 min. = 8.36 M3 crypt x̅ = 7.79 x̅ = 9.69 width max. = 10.11 max. = 14.45 min. = 6.67 min. = 6.67 M3 crypt x̅ = 10.33 x̅ = 11.16 height max. = 12.07 max. = 14.56 min. = 8.47 min. = 8.47

108

Covariation of DS1 corpus shape and M2 and M3 Crypt Form

A total of six specimens were included in the analysis of M2 and M3 crypt form with DS1 corpus shape. The A1 singular value of the 2B-PLS analysis is 0.19537, which explains 94.783% of the total squared covariance with a correlation of 0.752. The A2 singular value, 0.043422, explains 4.68% of the total covariance with a correlation of 0.654. Permutation test of singular

109 covariance values for the first and second dimensions found that only 5% and 2% respectively of the random samples were equal to or larger than the observed values (999 permutations).

The plots of A1 and A2, provided in Figures 20A and 20B, show the relationship of M2 and M3 crypt form with DS1 corpus shape. The loadings for the linear data indicate that all of the crypt variables contribute to A1 and M3 length has the highest influences on A1 (Figure 20C).

A2 loadings show that A2 is strongly influenced by M2 and M3 crypt length as well as M3 crypt height (Figure 20D). Specimens with higher A1 scores have the lowest M3 crypt lengths, widths and heights. Specimens with the highest A2 scores have the relatively higher M2 and M3 crypt lengths than specimens with the lowest A2 scores. In terms of corpus shape, specimens with higher A1 scores have corpora that are relatively wider in the midsection with narrower inferior portions (Figure 20E). Specimens with higher A2 scores have corpora that are relatively wider, especially in the inferior portion than specimens with lower corpus shape A2 scores (Figure

20F).

Table 22: Results for the first two dimensions of the 2B-PLS analyses.

Axis 1 Axis 2 DS 1 – M2 Crypt Singular value 0.13189 0.03513 Correlation 0.903 0.073 p 0.04 0.92 DS 1 – M2 M3 Crypt Singular value 0.19537 0.043422 Correlation 0.752 0.654 p 0.05 0.02 DS 2 – M3 Crypt Singular value 0.12606 0.03033 Correlation 0.410 0.533 p 0.095 0.891

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Covariation of DS2 corpus shape and M3 Crypt Form

A total of 6 specimens are included in the analysis of the relationship between DS2 corpus shape and M3 crypt form. The singular value of A1, 0.12606 explains 99.81% of the total covariance with a correlation of 0.410. The singular value of the A2 is 0.03033 and explains

0.175% of the covariance between DS2 corpus shape and M3 crypt form with a correlation of

0.533. Permutation tests of singular covariance values for the first and second dimensions found that 9.5% and 89.1% respectively, of the random samples were equal to or larger than the observed values. The results of the permutation tests suggest that there is not a significant relationship between DS 2 corpus shape and M3 crypt form.

4.5 Discussion

4.5.1 Ontogenetic shape variation in the great ape mandibular corpus

The youngest specimens of each taxon have corpus shapes that are distinct from adult specimens, however there is considerable intraspecies variation in this sample, especially within

DS 2. The variation between developmental stages in each taxon could indicate that these developmental categories are fairly broad with samples in each DS representing a range of actual chronological ages. Intraspecies variation within DSs may also be driven by differences in growth between males and females. Singh (2014) and Taylor (2002) have found differential mandibular growth rates between males and females. Due to the small sample sizes and the nature of the sample (the sex of several of the younger specimens is unknown), it is not possible to evaluate growth differences between males and females in this study. However, the variation between developmental stages may be reflective of the wide range of variation that is independent of sex is known to be present in adult hominoid corporal shape (Pitirri and Begun,

2019).

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Previous research has shown that differences in mandibular morphology among great apes occur early in development, prior to the emergence of M1 (Boughner and Dean, 2008;

Singh, 2014; Taylor, 2002,2003; Terhune et al., 2014). Here, the form-space (including size) results in our analysis finds that species level differences in corpus shape are present at DS1, further supporting the hypothesis that species level differences occur prior to the emergence of

M1 in great apes. However, the shape-space (independent of size) results in our analysis do not show species level differences at DS1, indicating that DS1 species level differences are driven by form (size and shape) and not by shape alone.

The primary shape changes that occur during ontogeny are an overall narrowing of the mandibular corpus. Relative to older specimens, DS1 specimens exhibit wider corpora. Pongo specimens are differentiated from Pan specimens by exhibiting corporal cross-sections that are wider at the base lingually, especially at DS2, and this trend continues into adulthood. Within

Pan species, P. paniscus is differentiated from P. troglodytes by having a narrower corpus inferiorly at DS1, which also continues through development into adulthood.

4.5.2 Relationship between molar crypt form and corpus shape during growth

Analyses of the relationship between permanent molar crypt form and mandibular corpus shape provide evidence for significant covariation between M2 crypt form as well as M2 and M3 crypt form with corpus shape at DS1. Specimens with the widest M2 crypts have corpora that are relatively wider inferiorly. Conversely, specimens with the narrowest M2 crypts have corpora that are relatively narrower inferiorly. The relationship between DS1 corpus shape and M2, as well as M2 and M3 crypts provides support for the hypothesis of integration between the shape of the mandibular corpus and the development of permanent molars in great apes. The overall trend in this analysis indicates that relative to Pan, Pongo specimens have inferiorly wider corpora to

113 accommodate wider M2 crypts during development of the second permanent molar. However, the lack of covariance between development of permanent molars and corpus shape later in developmental may indicate that this relationship is not uniform throughout growth.

The lack of covariance between DS2 and M3 development provides evidence that development of the permanent molars within the corpus does not influence corpus shape at M1-

M2 after the emergence of M2. This suggests that the corpus shape differences observed between

DS2 and DS3 are not the result of the development of M3 within the corpus. However, the changes in corpus shape between DS2 and DS3 may be influenced by M1 and M2 root formation occurring during this growth period. Molar root development is known to vary among tooth positions and among taxa (Dean and Vesey, 2008). Interspecies variation in root formation times are difficult to assess. Molar tooth roots grow in a nonlinear manner in great apes, with differences in peak formation times occurring at different stages of growth (Dean and Vesey,

2008). From the limited data that is currently available, it seems that relative to Pan, Pongo has greater molar root extension rates (Dean and Vesey, 2008). However, how differences in molar root formation times among great apes relates to corpus shape remains unknown and requires further investigation.

One of the limitations of this analysis is that the vertical placement of the molar crypt is not accounted for in the measurements used to quantify M2 and M3 molar crypts. It is possible that the relationship between DS1 corpus shape and molar development is influenced not only by the height, width and length of the crypts, but also by the vertical placement of the crypts during development. Although we found that Pongo specimens have both inferiorly wider corpora and molar crypts than Pan specimens, it is possible that the molar crypts are located lower in the

114 corpus in Pongo than in Pan. This is a hypothesis that requires further investigation with data that accounts for the vertical placement of the molar crypts in the corpus during growth.

4.5.3 Implications for interpreting mandibular corpus shape in the fossil record

The relationship between corpus shape and M2 crypt form during development has important implications for interpreting mandibular corpus shape in the fossil record. The shape of the mandibular corpus has been shown to have strong taxonomic utility in extant great apes

(Pitirri and Begun, 2019). However, the significance of the taxonomic differences in corpus shape at M1-M2 have been unclear. The results presented here, indicate that there is a strong relationship between M2 crypt width and the shape of the mandibular corpus, especially the inferior portion of the corpus. Consequently, comparison of corporal shapes of fossil taxa may provide insight into differences and similarities in growth and development of the permanent dentition.

4.6 Conclusion

The results of this study provide evidence supporting the hypothesis that the shape of the mandibular corpus is related to the development of the permanent molars in the crypt in hominoids. Our analysis finds that after the emergence of M1, the shape of the mandibular corpus covaries with M2 as well as M2 and M3 crypt form. The trends in these results indicates that specimens with corpora that are relatively wider inferiorly, such as in P. pygmaeus, also have wider M2 crypts. The results also show a lack of significant covariance between corpus shape observed later in development (after the emergence of M2) and M3 crypt form, indicating that the shape of the corpus at the cross-section of M1-M2 is not influenced by the development of permanent molar crowns after the emergence of M2. In terms of ontogenetic shape changes, corpus shape is found to change throughout growth, with species level differences in the inferior

115 portion of the corpus continuing into adulthood. Furthermore, this study also provides the first examination of cross-sectional corpus growth in P. pygmaeus, P. troglyodytes and P.paniscus, which show clear differences in growth patterns among all three species and provide evidence that species level differences in mandibular corpus growth occur prior to the emergence of M1.

The results of this analysis have important implications for the interpretation of the significance of mandibular corpus shape in the hominoid fossil record.

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Chapter 5

Conclusion

This chapter provides a review of the three separate analyses of mandibular corpus shape variation in hominoids by discussing this body of work in the context of evaluating the significance of hominoid corpus shape variation and provides a framework for interpreting corporal fragments in the primate fossil record. Here, the research goals presented in Chapter 1 are revisited and discussed in light of the results presented throughout this dissertation.

5.1 Measuring the Mandibular Corpus

The first goal of this dissertation was to develop and test an alternative method to the traditional approach using mandibular robusticity (MR). In Chapter 2, a geometric morphometric approach to quantifying the shape of the cross-sectional outline of the mandibular corpus using landmarks and semilandmarks was compared to the MR approach. The results of this analysis demonstrate that the outline approach more accurately recovers known taxonomy than does MR.

This indicates that the outline approach provides a more accurate quantification of mandibular corpus shape than the approach using MR. Furthermore, because studying corpus cross-sectional shape as an outline provides enhanced taxonomic discrimination that are independent of sex and size, I suggest that this approach is a preferred alternative to MR to study mandibular corpus shape in the primate fossil record. Finally, I also suggest that application of the outline approach to extant hominoids is necessary to help understand the factors influencing hominoid mandibular corpus shape variation.

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5.2 Factors Influencing Mandibular Corpus Shape

5.2.1 Diet

The relationship between diet and extant hominoid mandibular corpus shape was evaluated in Chapter 3, where the relationship between cortical bone distribution and diet was assessed. Although the results presented in Chapter 3 reveals significant shape differences among most extant hominoids, these shape differences do not appear to meet morphological predications based on dietary differences. Taxa with relatively tougher and/or harder diets are predicted to have taller, wider corpora with increased amounts of cortical bone (Bouvier and

Hylander, 1981; Hylander, 1979a,b,c; Hylander, 1984, 1985; Daegling, 2007). The results of both analyses did not find that extant hominoids have corporal shapes that are expected based on their dietary differences. Taxa with relatively tougher diets, such as P. pygmaeus and G. beringei, did not exhibit wider, taller corpora with increased cortical bone than taxa with less tough diets, such as P. troglodytes. Rather, the corporal shape differences that distinguish among hominoids are largely driven by variation in the inferior portion of the corpus. Therefore, the current evidence does not support the hypothesis that mandibular corpus shape is strongly influenced by diet.

5.2.2 Growth

The influence of growth and development of the permanent dentition in the corpus on mandibular corpus shape were investigated in Chapter 4. Chapter 4 provided an evaluation of ontogenetic changes in the shape of the mandibular corpus after the emergence of M1 in a sample of extant hominoids. The goals of Chapter 4 were to provide the first analysis of mandibular corpus growth in P. pygmaeus and compare the results to P. paniscus and P. troglodytes and

126 investigate the relationship between mandibular corpus growth and dental development in hominoids. The form-space results of this analysis show that species level differences in corpus shape are present at DS1, further supporting the hypothesis that species level differences occur prior to the emergence of M1 in great apes (Boughner and Dean, 2008; Singh, 2014; Taylor,

2002,2003; Terhune et al., 2014). However, the shape-space results presented in Chapter 4 do not show species level differences at DS1, indicating that DS1 species level differences are driven by form (size and shape) and not by shape alone.

This analysis also found that the primary shape changes that occur during ontogeny are an overall narrowing of the mandibular corpus. Relative to older specimens, DS1 specimens exhibit wider corpora. Pongo specimens are differentiated from Pan specimens by exhibiting corporal cross-sections that have wider inferiolingual regions, especially at DS 2, and this trend continues into adulthood. Within Pan species, P. paniscus is differentiated from P. troglodytes by having an inferiorly narrower corpus at DS 1 than P. troglodytes, which also continues through development into adulthood.

The results of this analysis provided evidence that mandibular corpus shape is strongly influenced by development of the permanent molars in their crypts prior to the emergence of M2.

The overall trend in this analysis indicates that relative to Pan, Pongo specimens have inferiorly wider corpora to accommodate wider M2 crypts during development of the second permanent molar. The relationship between DS1 corpus shape and M2, as well as M2 and M3 crypts provides support for the hypothesis of integration between the shape of the mandibular corpus and the development of permanent molars in great apes. However, the lack of covariance between development of permanent molars and corpus shape later in developmental may indicate that this relationship is not uniform throughout growth.

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The results presented in Chapter 4 indicate that development of M3 within the corpus does not influence corpus shape at M1-M2 after the emergence of M2. This suggests that the corpus shape differences observed between the developmental stages marked by the emergence of M2 and the emergence of M3 are not the result of the development of M3 within the corpus.

However, the changes in corpus shape between these two developmental stages may be influenced by M1 and M2 root formation occurring during this growth period. From the limited data that is currently available, it seems that relative to Pan, Pongo has greater molar root extension rates (Dean and Vesey, 2008). However, if differences in molar root formation times among great apes relates to corpus shape remains unknown and requires further investigation.

Another aspect of the relationship between ontogenetic changes in mandibular corpus shape and development of the permanent dentition that requires further investigation is the inferiosuperior location of the crypts during development. I think that interspecies variation in the vertical placement of the crypts is an important factor in the relationship between mandibular corpus shape and development of permanent dentition. However, a different methodological approach to quantifying the molar crypts is necessary in order to capture the vertical placement of the crypts within the corpus. Future research on this topic should focus on including this important aspect of molar crypt development.

5.2.3 Phylogeny

The possibility of a phylogenetic signal in the shape of corporal cortical bone distribution

(CBD) in hominoids was investigated in Chapter 3. Previous analyses have identified a possible phylogenetic pattern in hominoid CBD, with P. pygmaeus having less cortical bone in the mandibular corpus than African apes (Daegling and Grine, 1991; Daegling, 2007), possibly indicating a difference between Asian and African hominoids. This hypothesis was further

128 investigated in Chapter 3 by expanding the sample to include all of the extant hominoid species, as well as Miocene fossil taxa. The results of this analysis confirm that P. pygmaeus has relatively thinner CBD throughout the corporal cross-section than all other hominoids, however

P. abelii has a corporal CBD shape that is similar in relative cortical thickness to P. paniscus.

Assessment of the phylogenetic signal in extant taxa reveals a phylogenetic signal within hominoids. Analysis of the Miocene fossils found that D. fontani, R. hungaricus and one of the

Sivapithecus specimens exhibit corporal CBD shapes that are similar to extant hominines and P. abelii. The second Sivapithecus specimen has a CBD shape that is similar to P. pygmaeus. These results indicate that the shape of CBD in the hominoid mandibular corpus has phylogenetic significance and that further investigation into the shape of CBD in additional Miocene taxa will help to clarify its importance.

5.3 Interpreting Mandibular Corpus Shape in the Primate Fossil Record

The results of this dissertation have significant implications for studying and interpreting mandibular corpus shape variation in the primate fossil record. The first relates to how corpus shape of fossil primates is measured. In Chapter 2, I show that quantification and comparison of the shape of mandibular corpus in cross-section results in higher taxonomic discrimination than the traditional MR in extant hominoids. These results indicate that implementing this methodological approach to studying mandibular corpus fragments in the primate fossil record may results in higher taxonomic discrimination.

Additionally, the results presented in Chapter 4 provide support for the hypothesis that mandibular corpus shape is influenced by the development of permanent molars in their crypts, help us understand the significance of hominoid corpus shape variation. Consequently, we are

129 able to hypothesize that mandibular corpus shape variation in the hominoid fossil record is linked to variation in growth and development of the permanent dentition.

5.4 Literature Cited

Boughner, J.C. and Dean, M.C. 2008. Mandibular shape, ontogeny and dental development in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Evolutionary Biology, 35, 296-308.

Bouvier, M., Hylander, W.L., 1981. Effect of bone strain on cortical bone structure in Macaques

(Macaca mulatta). Journal of Morphology, 167, 1-12.

Daegling, D.J. 2007. Relationship between bone utilization and biomechanical competence in hominoid mandibles. Archives of Oral Biology, 52, 51-63.

Daegling, D.J., Grine, F.E. 1991. Compact bone distribution and biomechanics of early hominid mandibles. American Journal of Physical Anthropology, 86, 321-339.

Dean, M.C., Vesey, P. 2008. Preliminary observations on increasing root length during the eruptive phase of tooth development in modern humans and great apes. Journal of Human

Evolution, 54, 258-271.

Hylander WL. 1979a. The functional significance of primate mandibular form. Journal of

Morphology 106:223-240.

Hylander WL. 1979b. Mandibular function in Galago crassicaudatus and Macaca fascicularis: an in vivo approach to stress analysis of the mandible. Journal of Morphology 159:253-296.

Hylander WL, 1979c. An experimental analysis of temporomandibular joint reaction force in macaques. American Journal of Physical Anthropology, 51:433-456.

Hylander WL. 1984. Stress and strain in the mandibular symphysis of primates: a test of competing hypotheses. American Journal of Physical Anthropology 64:1-46.

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Hylander WL. 1985. Mandibular function and biomechanical stress and scaling. American

Zoologist 25:315-330.

Singh, N. 2014. Ontogenetic study of allometric variation in Homo and Pan mandibles. The

Anatomical Record, 297, 261-272.

Taylor, A.B. 2002. Masticatory form and function in the African apes. Am. J. Phys. Anthropol.

117, 133-156.

Taylor, A.B. 2003. Ontogeny and function of the masticatory complex in Gorilla: functional, evolutionary, and taxonomic implications. Gorilla biology: a multidisciplinary perspective. pp.132-189.

Terhune, C.E., Robinson, C.A., Ritzman, T.B. 2014. Ontogenetic variation in the mandibular ramus of great apes and humans. Journal of morphology, 275, 661-677.

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Appendix A

List of Abbreviations by Order of Appearance

THV terrestrial herbaceous vegetation MR mandibular robusticity CT computerized tomography MH mandibular height MB mandibular breadth GM geometric mean GPA generalized Procrustes analysis PCA principal component analysis CS centroid size CVA canonical variate analysis USNM Smithsonian Institution - National Museum of Natural History RMCA Royal Museum for Central Africa BUCM2-M1 point on alveolar border between M2-M1 on buccal side INFM2-M1 most inferior point on the corpus between M2-M1 LINGM2-M1 point on alveolar border between M2-M1 on lingual side RMA reduced major axis L lingual B buccal F female M male Gb/GB Gorilla beringei Gg/GG Gorilla gorilla Pa Pongo abelii Ppy Pongo pygmaeus Ppa/PPA Pan paniscus Pt/PT Pan troglodytes POP Pongo pygmaeus POA Pongo abelii PC1 first principal component PC2 second principal component EFA Elliptical Fourier analysis EA Eigenshape analysis CBD cortical bone distribution RUD 212 Rudapithecus hungaricus specimen #212 YPM 13811 Sivapithecus sivalensis specimen #13811 YPM 13814 Sivapithecus sivalensis specimen #13814 Internal point on alveolar border between M3-M2 on buccal side, internal aspect of BUCM2-M1 cortical bone Internal most inferior point on the corpus between M2-M1 on internal aspect of INFM2-M1 cortical bone Internal point on alveolar border between M2-M1 on lingual side, internal aspect of LINGM2-M1 cortical bone

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D Dryopithecus fontani R Rudapithecus hungaricus S1 Sivapithecus sivalensis specimen #13811 S2 Sivapithecus sivalensis specimen #13814 NJ neighbour joining PC3 third principal component 2B-PLS two-block partial least squares DS developmental stage

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Appendix B

Full Research Sample

Specimen Number Genus Species Sex Developmental Stage usnm239883 Gorilla beringei Male Adult usnm395636 Gorilla beringei Male Adult usnm396934 Gorilla beringei Male Adult usnm396935 Gorilla beringei Female Adult usnm396936 Gorilla beringei Female Adult usnm396942 Gorilla beringei Male Adult usnm397351 Gorilla beringei Male Adult usnm397356 Gorilla beringei Female Adult usnm505429 Gorilla beringei Female Adult usnm545027 Gorilla beringei Male Adult usnm545026 Gorilla beringei Female Adult usnm545028 Gorilla beringei Male Adult usnm545030 Gorilla beringei Female Adult usnm545031 Gorilla beringei Female Adult usnm545032 Gorilla beringei Male Adult usnm545034 Gorilla beringei Male Adult usnm545035 Gorilla beringei Male Adult usnm545036 Gorilla beringei Male Adult usnm252575 Gorilla gorilla Female Adult usnm252576 Gorilla gorilla Female Adult usnm252579 Gorilla gorilla Female Adult usnm220060 Gorilla gorilla Female Adult usnm220380 Gorilla gorilla Female Adult usnm252577 Gorilla gorilla Female Adult

134 usnm252581 Gorilla gorilla Female Adult usnm271347 Gorilla gorilla Female Adult usnm582726 Gorilla gorilla Female Adult usnm176207 Gorilla gorilla Male Adult usnm176209 Gorilla gorilla Male Adult usnm176210 Gorilla gorilla Male Adult usnm176211 Gorilla gorilla Male Adult usnm176216 Gorilla gorilla Male Adult usnm176213 Gorilla gorilla Male Adult usnm176225 Gorilla gorilla Male Adult usnm176215 Gorilla gorilla Male Adult usnm154553 Gorilla gorilla Male Adult usnm154554 Gorilla gorilla Male Adult usnm174712 Gorilla gorilla Male Adult usnm174713 Gorilla gorilla Male Adult usnm174714 Gorilla gorilla Male Adult usnm174715 Gorilla gorilla Male Adult usnm174716 Gorilla gorilla Male Adult usnm174722 Gorilla gorilla Male Adult usnm174717 Gorilla gorilla Male Adult usnm176205 Gorilla gorilla Male Adult usnm176206 Gorilla gorilla Male Adult usnm143597 Pongo abelii Female Adult usnm143601 Pongo abelii Female Adult usnm270807 Pongo abelii Female Adult usnm134602 Pongo abelii Female Adult usnm143596 Pongo abelii Female Adult usnm144598 Pongo abelii Female Adult

135 usnm143587 Pongo abelii Male Adult usnm143588 Pongo abelii Male Adult usnm143593 Pongo abelii Male Adult usnm143594 Pongo abelii Male Adult usnm267325 Pongo abelii Male Adult usnm143600 Pongo abelii Female Adult usnm143590 Pongo abelii Male Adult usnma49898 Pongo pygmaeus Unknown Juvenile usnm197665 Pongo pygmaeus Male Juvenile usnm302052 Pongo pygmaeus Female Juvenile usnm142171 Pongo pygmaeus Female Juvenile usnm021987 Pongo pygmaeus Female Juvenile usnm145303 Pongo pygmaeus Male Subadult usnm142183 Pongo pygmaeus Male Subadult usnm145322 Pongo pygmaeus Female Subadult usnm578647 Pongo pygmaeus Male Adult usnm142169 Pongo pygmaeus Female Adult usnm142170 Pongo pygmaeus Female Adult usnm142190 Pongo pygmaeus Female Adult usnm142202 Pongo pygmaeus Female Adult usnm145302 Pongo pygmaeus Female Adult usnm145308 Pongo pygmaeus Female Adult usnm145309 Pongo pygmaeus Female Adult usnm153832 Pongo pygmaeus Female Adult usnm197664 Pongo pygmaeus Female Adult usnm142182 Pongo pygmaeus Female Adult usnm142191 Pongo pygmaeus Female Adult usnm145300 Pongo pygmaeus Female Adult

136 usnm145306 Pongo pygmaeus Female Adult usnm145315 Pongo pygmaeus Female Adult usnm145320 Pongo pygmaeus Female Adult usnm145321 Pongo pygmaeus Female Adult usnm153805 Pongo pygmaeus Female Adult usnm153808 Pongo pygmaeus Female Adult usnm153812 Pongo pygmaeus Female Adult usnm153814 Pongo pygmaeus Female Adult usnm153819 Pongo pygmaeus Female Adult usnm153822 Pongo pygmaeus Female Adult usnm142188 Pongo pygmaeus Male Adult usnm142189 Pongo pygmaeus Male Adult usnm142200 Pongo pygmaeus Male Adult usnm145301 Pongo pygmaeus Male Adult usnm145304 Pongo pygmaeus Male Adult usnm145310 Pongo pygmaeus Male Adult usnm145318 Pongo pygmaeus Male Adult usnm145319 Pongo pygmaeus Male Adult usnm153815 Pongo pygmaeus Male Adult usnm142194 Pongo pygmaeus Male Adult usnm142195 Pongo pygmaeus Male Adult usnm142196 Pongo pygmaeus Male Adult usnm142197 Pongo pygmaeus Male Adult usnm142198 Pongo pygmaeus Male Adult usnm142199 Pongo pygmaeus Male Adult usnm145305 Pongo pygmaeus Male Adult usnm153806 Pongo pygmaeus Male Adult usnm153807 Pongo pygmaeus Male Adult

137 usnm153810 Pongo pygmaeus Male Adult usnm153823 Pongo pygmaeus Male Adult rmac88041M16 Pan paniscus Unknown Juvenile rmac88041M01 Pan paniscus Unknown Juvenile rmac84036M03 Pan paniscus Male Subadult rmac88041M15 Pan paniscus Unknown Subadult rmac88041M11 Pan paniscus Female Subadult rmac29060 Pan paniscus Female Adult rmac29065 Pan paniscus Female Adult rmac84036m10 Pan paniscus Female Adult rmac84036m4 Pan paniscus Female Adult rmac88041m5 Pan paniscus Female Adult rmac88041m12 Pan paniscus Female Adult rmac88041m13 Pan paniscus Female Adult rmac8841m17 Pan paniscus Female Adult rmac29064 Pan paniscus Male Adult rmac84036m9 Pan paniscus Male Adult rmac88041m2 Pan paniscus Male Adult rmac29063 Pan paniscus Male Adult rmac88041m10 Pan paniscus Male Adult usnm174709 Pan troglodytes Unknown Juvenile usnm174708 Pan troglodytes Unknown Juvenile usnm220067 Pan troglodytes Male Juvenile usnm236972 Pan troglodytes Female Juvenile usnm220066 Pan troglodytes Male Subadult usnm176236 Pan troglodytes Male Subadult usnm174700 Pan troglodytes Female Adult usnm174701 Pan troglodytes Female Adult

138 usnm176243 Pan troglodytes Female Adult usnm2200062 Pan troglodytes Female Adult usnm2200064 Pan troglodytes Female Adult usnm174702 Pan troglodytes Female Adult usnm174710 Pan troglodytes Female Adult usnm2200063 Pan troglodytes Female Adult rmac88041m3 Pan paniscus Female Adult rmac9338 Pan paniscus Female Adult rmac29066 Pan paniscus Male Adult usnm176229 Pan troglodytes Female Adult usnm599173 Pan troglodytes Female Adult usnm176229 Pan troglodytes Male Adult usnm220327 Pan troglodytes Male Adult usnm395820 Pan troglodytes Male Adult usnm599172 Pan troglodytes Male Adult usnm176228 Pan troglodytes Male Adult usnm176244 Pan troglodytes Male Adult usnm220065 Pan troglodytes Male Adult usnm220326 Pan troglodytes Male Adult usnm084655 Pan troglodytes Female Adult usnm282763 Pan troglodytes Female Adult

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Appendix Figures

Appendix Figure A1: PCA on regression residuals showing very minimal change between and within extant hominoids (see Figure 10A) when size is regressed out of the analysis. Colors and shapes are the same as those in Figure 10.

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