John D. Kingston Stratigraphy, age and environments of the Department of Anthropology, late Miocene Mpesida Beds, Tugen Hills, Emory University, Kenya 1557 Pierce Drive, Atlanta, Georgia 30322, U.S.A. E-mail: [email protected] Interpretations of faunal assemblages from the late Miocene Mpesida Beds in the Tugen Hills of the Central Kenyan Rift Valley have figured prominently in discussions of faunal turnover and establish- Bonnie Fine Jacobs ment of the modern East African communities. These faunal changes Environmental Studies have important implications for the divergence of the human lineage Program, Southern Methodist from the African apes ca. 8–5 Ma. While fossil material recovered University, Box 750395, from the Mpesida Beds has traditionally been analyzed collectively, Dallas, Texas 75275, U.S.A. accumulating evidence indicates that Mpesida facies span the 7–6 Ma E-mail: [email protected] interval and are scattered more than 25 km along the eastern flanks of the Tugen Hills. Stratigraphic distinctions between Mpesida facies Andrew Hill and younger sediments in the sequence, such as the Lukeino Department of Anthropology, Formation, are not yet fully resolved, further complicating temporal Yale University, Box 208277, assessments and stratigraphic context of Mpesida facies. These issues New Haven, Connecticut are discussed with specific reference to exposures of Mpesida facies at 06520, U.S.A. Rurmoch, where large fossil tree fragments were swept up in an E-mail: [email protected] ancient ash flow. Preserved anatomical features of the fossil wood as well as estimated tree heights suggest a wet, lowland rainforest in this portion of the rift valley. Stable isotopic analyses of fossil enamel and Alan Deino paleosol components indicate the presence of more open habitats Berkeley Geochronology locally. Overlying air-fall tuffs and epiclastic debris, possibly associ- Center, 2455 Ridge Road, ated with the ash flow, have yielded an assemblage of vertebrate Berkeley, California 94709, fossils including two teeth belonging to one of the earliest colombines U.S.A. E-mail: [email protected] of typical body size known from Africa, after the rather small Microcolobus. Single-crystal, laser-fusion, 40Ar/39Ar dates from a Received 1 March 2001 capping trachyte flow as well as tuffs just below the lava contact Revision received indicate an age of greater than 6·37 Ma for the fossil material. 15 June 2001 and  accepted 19 June 2001 2002 Academic Press

Keywords: late Miocene, paleovegetation, fossil wood, Mpesida beds, Tugen Hills, paleoenvironment, Journal of Human Evolution (2002) 42, 95–116 colobines, rift valley, stable doi:10.1006/jhev.2001.0503 isotopes. Available online at http://www.idealibrary.com on

Introduction elephantids and earliest African leporids more representative of contemporary East The 8–6 Ma interval in sub-Saharan Africa African communities (Hill, 1999; Winkler, represents an interesting and pivotal period 2002). By 6 Ma, faunas were quite different in the establishment of modern East African not only from older ones in East Africa, but ecosystems. Faunal assemblages during also from contemporaneous faunas in south- this time are interpreted as transitional, ern Asia (Kalb et al., 1982; Barry et al., reflecting replacement of more archaic 1985; Hill, 1985, 1987, 1995, 1999; Hill middle Miocene communities containing et al., 1985, 1986). These faunal changes creodonts, Climacoceras, caprines, and bose- have been linked to migratory exchange with laphines by advanced tribes of bovids, Eurasia (Bernor et al., 1996) as well as the

0047–2484/02/010095+22$35.00/0  2002 Academic Press 96 . .  ET AL. notion of gradual replacement of subtropical the village of Kisitei (Figure 3)(Chapman, forested environments by more seasonal, 1971). This nomenclature was subsequently open country ‘‘savanna-mosaic’’ habitats extended to include all volcaniclastic lenses considered characteristic of the latest or wedges within the Kabarnet Trachyte Miocene (Leakey et al., 1996; Cerling et al., sequence, as well as sedimentary units con- 1997). Fossiliferous sediments from this formably overlying trachytic lavas at some interval, however, are poorly known and the localities. Mpesida sediments are generally tempo and mode of this transition remains lithologically distinctive within the upper unclear. Vertebrate localities in Africa and portion of the Tugen Hills succession, typi- Arabia broadly correlative in age to this cally comprised of coarse, primary pyro- interval include the Lothagam Nawata clastic flows and air-fall tuffs interbedded Formation (Leakey et al., 1996), the with volcaniclastic material reworked by Manonga Valley, Tanzania (Harrison, fluvial and lacustrine processes. Exposures 1997), the Sinda Basin, Republic of are yellow/buff to heavily oxidized red and Congo (Yasui et al., 1992), the Baynunah are frequently characterized by significant Formation in Abu Dhabi (Hill & Whybrow, primary structural, textural and lateral 1999), the Nkonodo area in Uganda facies disruption, most likely related to (Pickford et al., 1988), the Middle Awash, emplacement during high energy eruptive Ethiopia (Kalb & Mebrate, 1993), events on topographically complex volcanic Langebaanweg, South Africa (Hendey, landscapes. Chapman (1971) and others 1970, 1974), and possibly the upper portion (Pickford, 1975b; Williams & Chapman, of the Namurungule Beds at Samburu 1986) have noted lithological similarities (Nakaya, 1993) and Nakali (Aguirre & with older Ngorora Formation type-section Leakey, 1974). Hominoids are consistently facies rather than younger sediments of the rare to absent at these localities and the only Lukeino and Chemeron Formations in the known specimens from this interval are the eastern Tugen foothills, and attributed these Lukeino molar from the Tugen Hills Suc- differences to a significant shift in the geo- cession (Pickford, 1975a; Hill & Ward, chemistry of associated eruptive rocks. This 1988), hominoid material attributed to contrast has been utilized to differentiate Orrorin tugenensis (Senut et al., 2001), and a Mpesida facies from Kaperyon, Lukeino or lower incisor and an upper molar from the ‘‘Kaparaina subbasalt’’ sediments at sites upper Nawata Member at Lothagam where no intervening trachyte exists. This (Leakey et al., 1996). The Mpesida Beds, approach is problematic at sites where there documented here, contain fossiliferous sedi- is no apparent unconformity between ments that provide additional faunal, floral, Mpesida deposits and overlying lacustrine and stable isotopic information about this facies typically associated with the Kaperyon critical time period. or Lukeino Formations. The Mpesida Beds are volcaniclastic Almost all Mpesida deposits occur in out- horizons associated with and interbedded liers east of the main Kamasia (Saimo) Fault between trachytic flows of the Kabarnet escarpment scattered over a roughly N–S Trachyte Formation in the Tugen Hills transect spanning 25–30 km (Figure 3). succession, exposed west of Lake Baringo in Vertebrate fossils have been recovered pri- the Central Kenya Rift Valley (Figures 1 marily from three main exposures of and 2). The name was initially used to Mpesida Beds to the north, Kasigoryan describe epiclastic and pyroclastic sediments west of Yatya, and the Cheparain/Rurmoch exposed adjacent to and immediately north area southwest of Yatya (Figure 3) of the Mpesida River about 3 km north of (Chapman, 1971; Hill et al., 1986).   97

Figure 1. Location map of the Tugen Hills within the eastern branch of the East African Rift System.

horizons producing fossil plant material have been described at the site of Kapturo (BPRP#133) (Jacobs & Deino, 1996), and are currently being investigated at a site north of Cheboit (BPRP#172) and a locality on the western flank of the Tugen Hills, northwest of Kabarnet (BPRP#148). In addition, nonfossiliferous facies attributed to the Mpesida Beds have been described at Sumet, Mukur, Kokwomur, Mukhungo, and various localities along the Yatya River drainage west of Yatya (Chapman, 1971). These exposures are, in general, structurally, stratigraphically, and geographically isolated Figure 2. Composite stratigraphic section of the Tugen and there are no obvious spatial or temporal Hills Succession. Sedimentary rocks ; volcanic rocks ; metamorphic rocks . relationships among them. Chapman (1971) recognized that stratigraphic corre- lation between outcrops is ‘‘dubious’’ and Pickford (1975b) lists additional vertebrate suggested that it would be more appropriate localities at Chemoi, Chepkoyom, Kisitei, to refer to the sediments as the ‘‘Mpesida Kokwonyeswa, and Koitugum. Mpesida facies’’. Up to six flows have been recorded 98 . .  ET AL.

Figure 3. Distribution of major exposures of Mpesida Bed sedimentary facies along the eastern foothills of the Tugen Hills, west of Lake Baringo. in the Kabarnet Trachytes (Martyn, 1969; third flows of the Kabarnet Trachyte Lippard, 1972; Chapman et al., 1978) and it Formation, but this definition excludes a is unclear as to the relative stratigraphic number of previously mapped exposures position of the various Mpesida exposures and contrasts with Chapman’s initial assess- within the series of trachytic flows. Pickford ment of the unit. Abrupt and ubiquitous (1975b) suggests that the term ‘‘Mpesida lateral facies changes within the Mpesida Beds’’, by definition, is confined to sedi- exposures as well as structural discon- ments deposited between the second and tinuities preclude regional or even local   99 correlation based on lithology. Radiometric and sedimentary processes, and differential dating, discussed below, indicates Mpesida erosion or nondeposition of small-scale facies are not temporally equivalent, ranging eruptive horizons within the local sequence. in age from about 7·0 Ma to 6·2 Ma (Hill As part of a large-scale geological mapping et al., 1985; Jacobs & Deino, 1996). Based effort by the East African Geological on these data and the wide geographic dis- Research Unit in the 1960s and 1970s tribution of Mpesida Beds, it is unlikely that (Martyn, 1969; Bishop et al., 1971; the isolated exposures represent erosional Chapman, 1971; King & Chapman, 1972; remnants of what was a large pyroclastic Chapman et al., 1978), this specific basin sheet or genetically related eruptive event. was originally mapped as both in situ and The source(s) of Mpesida pyroclastic sedi- ‘‘landslipped’’ Mpesida Beds in unspecified, ments are unknown, but locally extensive queried contact with juxtaposed Lukeino exposures including primary air-fall tuffs Formation sediments which were inter- and pyroclastic flows in the Yatya area preted to be stratigraphically overlain by suggest that at least these deposits reflect Kaparaina Basalts ringing the base of proximity to a source. Rurmoch Peak (Chapman, 1971). These Recently, the Baringo Paleontological interpretations reflect the difficulty in resolv- Research Project (BPRP) has been actively ing specific stratigraphic relationships in this investigating Mpesida sediments deposited area. Our opportunity to reconnoiter this in a basin adjacent to Rurmoch that have basin more thoroughly has revealed that: (1) produced vertebrate fossils, including pri- Mpesida sediments in the basin are entirely mates, and silicified wood fragments. We in situ, (2) different facies of Mpesida are describe here the geological context of these distributed irregularly throughout the basin, fossils, 40Ar/39Ar dates on the uppermost (3) intrabasinal sediments are penecontem- tuffs in the basin as well as the base of the poraneous with no evidence of a major overlying trachyte, and paleoenvironmental unconformity and are allocated entirely to interpretations based on preliminary analy- the Mpesida Beds, and (4) the sediments are ses of fossil wood anatomy and isotopic overlain by Kabarnet Trachytes (rather than biochemistry of fossil enamel and paleosol Kaparaina Basalts) comprising the upper components. flanks of Rurmoch as well as the slopes west of the basin. Geological setting of Mpesida Beds at Pyroclastic flow facies Rurmoch The base of the section is exposed along the Immediately southwest of the Rurmoch southwestern portion of the basin [Figures 4 complex (Figures 3, 4 and 5) is a depo- and 6(a)] and is made up of pyroclastic flow sitional basin infilled with an intricate deposits containing silicified wood frag- sequence of pyroclastic and epiclastic debris ments (Figure 7). In general, this yellow/ interbedded at the top of the section with buff bed is massive and unsorted, consisting lacustrine facies. Stratigraphic and struc- entirely of ash-sized material with abundant tural relationships among these sediments lapilli up to 10–15 cm. The chaotic nature of and surrounding lavas, predominantly tra- this deposit more likely reflects the high chytes, are complicated by varied local particle concentration rather than turbu- predepositional topography, differential lence, with the dominant flow mechanisms deposition and onlapping of basal pyro- being laminar or plug flow or both (Cas & clastic flow unit(s), rapid local facies Wright, 1987). Diffuse layering, evident changes, complex interaction of volcanic primarily through differential weathering 100 . .  ET AL.

Figure 4. Generalized geologic sketch map of the Mpesida depositional basin west of Rurmoch. processes, is occasionally observed within mantles the trachyte provides the only this lower unit and is most likely due to evidence indicating that the ash, at least internal shearing during transport. Cross- locally, was emplaced on lava. The base of bedding is only weakly developed and rare. the flow unit was nowhere clearly exposed in It is difficult to distinguish unequivocally the area and it is unknown whether there primary pyroclastic flows from epiclastic may be other volcaniclastic facies interven- debris flow deposits of volcanic material. ing between underlying trachytes and the Evidence of zones of partial welding and ash unit. At the locality of Cheparain, the carbonization of wood fragments (subse- southern exposures at Rurmoch, Chapman quently silicified) indicate relatively high (1971) describes ‘‘Mpesida facies’’ (silts, temperatures of emplacement for this flow. sandstones, feldspathic tuffs, and argil- Distribution of this flow within the paleo- laceious pumice tuffs) as underlying non- basin was controlled by topography and bedded primary pumice-tuffs containing deposits are well exposed in paleo-drainages abundant remains of trees. The flow facies with evidence of draping against trachyte is clearly overlain by trachyte along the canyon walls. Areas where the ash flow western basin margin. Chapman & Brook   101

Figure 7. Silicified fossil wood fragment (diameter of 28 cm) in massive pyroclastic flow facies. Note abun- dant dark lapilli within yellow/buff coarse matrix.

(1978) note that exposures at Kasigoryan provide the only clear evidence that Mpesida facies are stratigraphically bracketed be- tween trachyte flows and at other major sites the exact relationship is consistently obscured by overstep of younger rocks, tec- tonism, and landslip. Volcaniclastic material which filled the basin and associated drain- Figure 5. View of a portion of the Rurmoch Mpesida age during the late Miocene is preferentially depositional basin looking south–southwest from the western flank of Rurmoch. Kabarnet Trachytes form being eroded, resurrecting the ancient pre- the dark exposures while yellow–buff sediments in the Mpesida trachyte topography, including foreground hill represent vertebrate fossil-bearing drainages to the south. This massive ash pyroclastic ash falls and epiclastic facies. Light colored sediments just behind this hill to the south below expo- facies can be traced in modern drainages sures of Kabarnet Trachytes are the fossil wood-bearing 2–3 km to the north along the western ash-flow facies. In the far background are the Tugen Hills exposures of sediments where the fossil uplifted along the Saimo Fault Escarpment.

Figure 6. Stratigraphic sections of Mpesida volcaniclastic sediments west of Rurmoch. Section locations indicated in Figure 4. (a) Fossilized wood-bearing pyroclastic flow unit overlain by Kabarnet Trachyte along the southwestern basin margin; (b) pyroclastic and epiclastic facies section including primary air-fall tuffs and epiclastic debris at the primate locality; (c) lacustrine facies overlying massive ash flow at north terminus of depositional basin. 102 . .  ET AL. wood fragments become rare and the tuff Intercalated epiclastic sediments below is eventually overlain by lacustrine facies the upper tuff sequence range from well including fine-grained, bedded tuffaceous sorted, oxidized red, fine-grained sand- sandstones, siltstones, and clays as well as stones, siltstones, and mudstones with lami- diatomite (Figure 4). Wood fragments are nations and high angle cross-bedding to also rare in exposures of the ash flow(s) in coarse conglomerate lenses with lapilli and the central and eastern portion of the basin rounded lava clasts. Transport and rework- where the unit is well exposed in deeply ing by fluvial processes was minimal as incised N–S trending modern drainages. At indicated by the high content of juvenile one locality just south of the main vertebrate vitric material such as lapilli as well as lack fossil site, portions of fossil trees appear to of rounding of grains, and it is likely that be preserved in growth position including epiclastic debris is derived almost directly possible in situ root structures. from pyroclastic eruptions rather than by weathering and erosion of consolidated vol- Pyroclastic ash falls and epiclastic facies canic rocks. It is difficult to distinguish Exposures of Mpesida pyroclastic and epi- unequivocally exclusively fluvial processes clastic facies occur along the eastern basin throughout this section from water- margin, primarily along the relatively steep supported subaerial mass flows resulting flanks of Rurmoch, topographically and from remobilization of piles of loose stratigraphically above the massive ash pyroclastic sediments, or possibly even flow(s). These volcaniclastic materials are parallel-bedded pyroclastic surge deposits. over 30 m thick and are highly resistant, Infrequent block impact structures (mafic typically forming rugged, near-vertical bomb sags) up to 15 cm are present in slopes. This sequence is comprised pre- the sequence indicating proximity to the dominantly of medium- to well-bedded eruptive center. Limited exposure prevents (10–30 cm) light grey to buff/yellow primary lateral tracing of beds along the steep sides air-fall tuffs interbedded with reworked grey of Rurmoch, but lithofacies variation to orange coarse volcaniclastic sandstones between adjacent exposures separated by and minor conglomerate horizons [Figures 4 100–200 m indicates rapid lateral as well as and 6(b)]. These sediments are overlain by a vertical facies changes. Well-defined bed- columnar-jointed flow of Kabarnet Trachyte ding of nontuffaceous siltstones, mud- at least 15 m thick. stones, and sandstones below the upper tuff Tuff beds include (1) fine-grained, sequence in the Mpesida Beds suggest that bedded to massive yellow/buff ash with accumulation was much slower than the occasional black pumice fragments, (2) grey overlying tuff sequences. lapilli tuff bearing crystal-poor angular Studies of pyroclastic flow sequences pumice fragments, and (3) minor amounts indicate that many flows separate into a of crystal tuff. These ash units dominate the dense basal avalanche or underflow and an upper portion of this facies and may repre- overriding dilute, turbulent ash cloud sent relatively near-vent accumulation of (Sheridan, 1979). Up to half the erupted tephra and possibly surge deposits related mass may be contained in the ash cloud at least to one flow unit in the Kabarnet (Cas & Wright, 1987) and the basal flow, Trachyte. Deposition of the uppermost the pyroclastic flow deposit proper, is 30 cm of this Mpesida unit was probably overlain in many cases by a significant quite rapid, perhaps on the order of days, sequence of ash-fall or ash-cloud surge and was terminated apparently without deposits. The well-bedded, laminated or lacunae by the Kabarnet Trachyte flow. cross-bedded ash deposits represented by   103 the middle unit may reflect this type of ash Lacustrine facies fallout, associated here with emplacement At least 15 m of buff to tan, fine-grained of the massive flow comprising the lower lacustrine and minor fluvial volcaniclastic Mpesida facies at Rurmoch. These ash facies stratigraphically overlie the lower clouds frequently separate completely from pyroclastic flow unit in the northwestern the basal avalanche and tend to be less portion of the Rurmoch depositional basin. confined by topography and can mantle the These strata reflect a hiatus, at least locally, landscape, covering a much larger area. At in volcanic activity associated with the Rurmoch, the middle unit pyroclastics is Kabarnet Trachytes. These facies include confined to the eastern basin margin but well-bedded clays, silts, and fine-grained these lithofacies occur in Mpesida expo- sandstones interbedded with massive sures 2–3 km east of Rurmoch indicating a diatomite horizons exceeding 2 m, which relatively greater extent than the underlying form low-lying exposures that extend north- flow. Lack of these bedded ashes in ward to the present drainage divide and sequences 0·5 km to the west may reflect beyond [Figures 4 and 6(c)]. These massive differential erosion prior to extrusion of to laminated horizons comprise essentially the overlying trachyte. Paleosols or any pure diatomites with minimal terrigenous indications of pedogenic processes were input and generally lacking structures rare in the sections, suggesting only brief indicative of lake margin bioturbation or hiatuses between erosional and depositional pedoturbation. Preliminary analyses of sequences. diatom taxa (Robert Edgar, personal com- Orientations of the long axes of fossil munication) also suggest the development wood fragments measured at a number of of substantial lake systems and not just localities in the basin indicate that although ephemeral ponding in this high energy vol- flow orientations were relatively consistent canic landscape. A complete lack of bedding within a site there was significant intersite orientation in the uppermost massive ash differences. Most probably these varying unit precludes precise determination of the orientations reflect extensive localized shifts specific nature of the contact between this in the flow as it moved down meandering ash and the overlying lake depoits but there paleo-drainages dissecting the underlying is no evidence of an unconformity. Bedding trachytes. In general, the long axes are orientations in the ash flow 4 m below the oriented E–W to NW–SE. contact and within the lacustrine strata Vertebrate fossil localities within this above are consistent and indicate no struc- basin are confined to exposures of pyro- tural disruptions. The stratigraphic relation- clastic ash falls and epiclastic facies. There ship of the lacustrine facies to the air-fall and are two main concentrations of fossils epiclastic deposits is unknown, as nowhere (Figure 4) which occur at the base of steep are exposures of the two facies juxtaposed. slopes. Surface fossil fragments have been It is possible that the two are in part recovered up to 15 m above the contact with lateral facies equivalents, although bedding the underlying ash flow facies but as in situ orientation projections of the lacustrine specimens are rare, it remains unclear as to facies eastward towards the western flank the number of horizons yielding fossils. of Rurmoch indicate that these deposits Large fragments of fossil wood are absent structurally overlie the tuffs. These lacus- from these deposits although small (<1 cm trine facies with the distinct diatomite hori- diameter) in situ wood fragments have been zons can be traced northeast around the observed in some of the more massive ash northern base of Rurmoch, along a bench horizons. forming cliffs that extend down to the Yatya 104 . .  ET AL.

River drainage (Figure 4). There is no Mpesida tuffs. Phenocrysts up to 3 cm long evidence of any significant local faulting of generally cloudy, occasionally iron- within this basin and along the basin stained anorthoclase constitute about 15% margins. of the rock. Anorthoclase phenocrysts were Lithologically similar lacustrine sedi- extracted for age determination from ments, including diatomites, occur at a essential pumice of a lapilli tuff sample number of exposures of the Mpesida Beds approximately 5 m below the contact with including a number of sites in the Mpesida the overlying Kabarnet Trachyte (ROR-4) River and the Yatya areas as well as along in the upper tuffaceous portion of the pyro- the northern and southeastern flanks of clastic and epiclastic facies, and from one Rurmoch. Unlike the Rurmoch basin, sample of the lower part of the trachyte flow lacustrine sediments at other localities (ROR-3). These grains were analyzed by the include vertebrate fossil assemblages single-crystal laser-fusion 40Ar/39Ar dating composed of both terrestrial and aquatic method (Deino & Potts, 1990; Deino et al., fauna including large concretions (<35 cm) 1990). containing skeletal fragments of fish tenta- 40Ar/39Ar analytical results are shown in tively identified as Tilapia. Intercalated Table 1. Ten single-crystal analyses were with these lake deposits at Rurmoch are obtained for each sample. The distributions minor fluvial, pyroclastic and paleosol hori- of the age populations are illustrated in zons. To the north, these deposits are Figure 8, using age-probability spectra. stratigraphically overlain by Kabernet These spectra were generated statistically as Trachytes forming small hills along the the sum of the errors of the individual single- divide between the Rurmoch basin and crystal analyses for a sample, assuming a the Cheboit drainage area to the north gaussian shape to the probability curve of (Figure 4). the run uncertainties (Deino & Potts, 1992). Mpesida lacustrine facies, especially ROR-4 shows a symmetrical, unimodal where lakes are forming above the pyro- distribution with a mode of 6·36 Ma, in clastic flow and ash deposits, may arise agreement with a weighted mean age calcu- from caldera topography. This scenario is lation of the population of 6·360·03 Ma suggested by evidence of greatly disturbed (1
standard error of the mean, incor- deposition including entrained breccias of porating error in the neutron flux cali- older rock, disrupted and convoluted bration parameter, J, of about 0·5%). stratigraphy, and silicification of some of the ROR-3 also yielded a smooth, symmetrical diatomite horizons forming porcellanite. distribution with the exception of a single anomalously young apparent age (4744D- 05). The origin of this anomalous age is Rurmoch 40Ar/39Ar dating probably geologic, given the turbid nature Samples for radiometric dating were col- and iron staining of much of the lected stratigraphically above the main fossil anorthoclase in the original rock. However, locality where the pyroclastic and epiclastic the results are the same whether or not this facies are overlain by Kabarnet Trachyte grain is excluded from the statistical analy- forming steep cliffs along the western flank sis: the weighted mean population age is of Rurmoch summit (Figure 4). The base of 6·370·03 Ma and the mode is also the trachyte incorporates a carpet breccia 6·37 Ma. The 40Ar/39Ar ages of the upper approximately 1 m thick indicating emplace- part of the Mpesida Beds and the capping ment as a surface flow. A slight reddish Kabarnet Trachyte cannot be distinguished baking was imparted to the underlying statistically.   105

Table 1 40Ar/39Ar analytical data, Mpesida Beds and Kabarnet Trachyte

Age (Ma) Lab ID No. Ca/K 36Ar/39Ar 40Ar*/39Ar %40Ar* 1

Sample ROR-4, tuff within Mpesida Formation: 4748D-06 0·096 0·00012 1·653 98·2 6·330·03 4748D-02 0·117 0·00009 1·656 98·6 6·340·03 4748D-03 0·123 0·00017 1·656 97·4 6·350·03 4748D-07 0·046 0·00012 1·658 97·9 6·350·03 4748D-08 0·115 0·00055 1·659 91·3 6·350·03 4748D-04 0·091 0·00010 1·660 98·4 6·360·03 4748D-10 0·086 0·00011 1·661 98·3 6·360·03 4748D-09 0·103 0·00011 1·663 98·2 6·370·03 4748D-01 0·116 0·00010 1·663 98·5 6·370·03 4748D-05 0·092 0·00011 1·667 98·3 6·390·03 Weighted average, 1
error without error in J= 6·360·01 1
error with error in J= 0·03 Sample ROR-3, Kabarnet Trachyte: 4744D-05 0·052 0·00031 1·645 94·8 6·300·04 4744D-04 0·251 0·00028 1·658 95·8 6·350·04 4744D-08 0·091 0·00016 1·658 97·4 6·350·03 4744D-03 0·053 0·00010 1·661 98·4 6·360·03 4744D-10 0·190 0·00040 1·663 93·7 6·370·04 4744D-02 0·113 0·00016 1·664 97·5 6·380·03 4744D-01 0·197 0·00022 1·665 96·7 6·380·04 4744D-09 0·092 0·00021 1·666 96·6 6·380·04 4744D-06 0·096 0·00010 1·666 98·4 6·380·04 4744D-07 0·087 0·00016 1·669 97·4 6·390·04 Weighted average, 1
error without error in J= 6·370·01 1
error with error in J= 0·03

Notes: Errors in age quoted for individual runs are 1
analytical uncertainty. Weighted averages are calculated using the inverse variance as the weighting factor (Taylor, 1982), while errors in the weighted averages are 1
standard error of the mean (Samson & Alexander, 1987). Ca/K is calculated from 37Ar/39Ar using a multiplier of 1·96. 40Ar* refers to radiogenic argon. =5·5431010 y1. Isotopic interference 36 37   4 39 37   4 corrections: ( Ar/ Ar)Ca=(2·58 0·06) 10 ,( Ar/ Ar)Ca=(6·7 0·3) 10 , 40 39   2   3 ( Ar/ Ar)K=(2·08 0·13) 10 . J=(2·114 0·010) 10 . Sanidine from the Fish Canyon Tuff was used as the monitor mineral, with a reference age of 28·02 Ma (Renne et al., 1998).

Age of the Mpesida Beds Baker et al., 1971). From the Yatya area Previous age determinations relevant to Chapman & Brook (1978) obtained an age the Mpesida Beds unit have not been of 7·00·4 Ma on a flow just beneath determined on elements of the sediments Mpesida sediments, and a date of themselves, but on samples from flows of 6·90·3 Ma on a flow near Morop in the Kabarnet Trachytes that bound the sedi- main Tugen Hills succession. These dates mentary lenses above and below (Hill et al., have been corrected for new constants using 1985, 1986). Three analyses of a single the formula of Ness et al. (1980). Trachytes trachytic flow specimen from near the base above the sedimentary sequence in the of the volcanic succession near Kabarnet Yatya are dated to 6·200·19 Ma, produced ages of 7·30·3 Ma, 7·5 6·200·26 Ma, 6·310·20 Ma and 6·36 0·3 Ma, and 7·50·3 Ma (Walsh, 1969; 0·08 Ma (Hill et al., 1985, 1986). More 106 . .  ET AL.

200 Upper Mpesida Formation (ROR-4) Mode = 6.36 Ma Weighted Mean = 6.36 0.01 ( 0.03) Ma 180 Kabarnet Trachyte (ROR-3) Mode = 6.37 Ma Weighted Mean = 6.37 0.01 ( 0.03) Ma 160

140

120

100

80 Probability

60

40 Includes relatively young analysis 20 4744D-05 Excludes 4744D-05 0 6.24 6.26 6.28 6.30 6.32 6.34 6.36 6.38 6.40 6.42 6.44 6.46 6.48 Age (Ma) Figure 8. Age-probability spectra for samples ROR-4 (upper Mpesida Beds) and ROR-3 (Kabarnet Trachyte). Uncertainties in the weighted means are expressed as 1
standard error of the mean. The uncertainty quoted in parenthesis incorporates error in the neutron fluence parameter, J.

recently 40Ar/39Ar dates have been obtained The Rurmoch forest from a site to the north (BPRP#133), that may be within the Mpesida Beds as currently Sediments of the Mpesida facies exposed defined (Jacobs & Deino, 1996). These west of Rurmoch also contribute to our provide a lower estimate for the unit of knowledge of the vegetation at this time. 6·7–7·2 Ma. The basal massive pyroclastic unit preserves The ages reported here for the Kabarnet a forest of abundant silicified wood Trachyte conformably overlying the sedi- (BPRP#87) which provides the opportunity mentary sequence and of the uppermost to reconstruct the nature of the plant com- ash levels at Rurmoch indicate that most munity present at the time of the volcanic or all sediment immediately surrounding eruption and to document the paleo- Rurmoch should be considered Mpesida botanical history of the modern African facies. This exposure of Kabarnet Trachyte flora. This massive flow is characterized by was originally interpreted as Kaparaina features suggesting emplacement as a high Basalt which defines the top of the younger energy flow deposit, possibly associated with Lukeino Formation (Chapman, 1971; the eruption of the Kabarnet Trachytes. Martyn, 1969). On the basis of these new Silicified wood fragments up to 1·2 m in data, Lukeino Formation sediments to the diameter and 2–3 m long are common in north and west should in part be considered these flow deposits over an area of at least contiguous with the Mpesida Beds, and 4km2. The orientation of the fossil wood in may be as much as 0·5–0·7 Ma older than the tuff matrix indicates that some trees previously interpreted. were swept up and transported by the ash   107

Table 2 Comparative data for specimen KNMP-TH 18763

Possible generic affinity, Flacourtiaceae Modern distribution* Habitat†

Camptostylus (4) Tropical West Africa Forest‡§ Caloncoba (10) Tropical Africa Lower story rain forest, forest margin, riverine forest, second growth, wooded grassland‡§ Dasylepis (6) Tropical Africa Understory tree in dense forest, gallery forest, upland rain forest‡§0 Erythrospermum (4) Madagascar, Sri Lanka, Malaysia Primary Forest‡§0 Hydnocarpus (40) Malaysia, Indo-China Substage shrubs or trees in evergreen, lowland rain forests‡§0 Rawsonia (2) Tropical Africa Understory and shrub layer of lowland and upland rain forest, dry evergreen forest, semi-swamp and riverine forest‡§ Scottellia (3) Tropical Africa Swamp forest to forest‡§0

*Mabberley, 1993. †van Steenis, 1950; Keay et al., 1954; Sleumer, 1975. Comparative sources: ‡Wheeler et al., 1986. §Miller, 1975. 0Harvard University Herbaria. Number of species in the genus is shown in parentheses. Fossils were compared with published data (Miller, 1975; Wheeler et al., 1986) and prepared slides from the Harvard University Herbaria (HUH).

Table 3 Summary of describable wood specimens from the Mpesida Forest at Rurmoch

Specimen Possible affinity Environmental information

KNMP-TH 18763 Flacourtiaceae (Table 2) Montane or lowland forest trees or shrubs KNMP-TH 18760 Olacaceae—Coula edulis, Diogoa zenkeri, Forest trees of West and Central Africa Strombosiopsis tetrandra KNMP-TH 18757 Lauraceae, Anacardiaceae KNMP-TH 18758 KNMP-TH 18762 Lauraceae MP-84F 84E Unknown MP-344A Unknown

More detailed information about specimen KNMP-TH 18763 is given in Table 2. Three genera in the Olacaceae, possibly related to KNMP-TH 18760, are monotypic.

flow. At present it is impossible to has diagnostic characters identifiable to a discriminate between upland or lowland small group of genera in the Flacourtiaceae, forest, though this will come with further another to the Olacaceae (Tables 2 and 3). identifications. Specimen KNMP-TH 18763 shares Twenty-two specimens of fossil wood anatomical characteristics with the family have so far been sectioned for study. Of Flacourtiaceae. It has diffuse porous vessel these, preservation of eight specimens distribution with 25% solitary vessels. The was fine enough for detailed anatomical vessels average 132/mm2 with a mean description. Preliminary study indicates that tangential diameter of 68·5 m, and vessel the specimens represent at least six taxa; one elements are long, averaging 912 m. 108 . .  ET AL.

Perforation plates are scalariform, inter- The remaining five species in the fossil vessel pits are alternate and 7–10 m wide; assemblage are not securely identified to vessel-ray pits are 15–20 m. Fibers are family. Assignment to Lauraceae or Anacar- septate and long (>1000 m). Axial diaceae must be verified through additional parenchyma is absent, ray parenchyma is comparative study. The unknowns in Table heterocellular. The fossil compares favor- 3 are well preserved enough for description, ably with genera in group I of Miller (1975) but possess characters that are shared with a which, in addition to general features of the number of extant families. Flacourtiaceae, all have exclusively scalari- form perforation plates, medium to very Paleohabitat reconstructions large intervascular pits (7–>15 m) and Structural and lithologic aspects of the medium to coarse (7–>10 m) vessel-ray massive pyroclastic facies indicate that the pits. Genera from within this group include fossil wood has not traveled far from its those shown in Table 2, with the exception point of growth, and that it is unlikely that of Camptostylus and Caloncoba, which can more than one plant community is repre- have simple or scalariform perforation sented by the fossil assemblage. Thus, the plates. One genus from outside the fairly narrow environmental range tolerated Flacourtiaceae, Rinorea (Violaceae), has by the living representatives of the identified been described as sharing some features fossils can be used as an indication that the with Scottellia, but it can be distinguished trees were growing originally in a lowland or from it by the smaller intervascular pits upland forest having floristic affinities with and smaller pore diameter (Normand & West and Central Africa, and possibly with Paquis, 1976). Thus, the Mpesida fossil fits Indo-Malaysia. well within Flacourtiaceae and compares Minimum tree heights can be determined favorably with the genera in Table 2. from measurements of fossil stem diameters, Additional study may limit further the which, in some cases exceed 90 cm. The possible modern affinities of this fossil and estimated height of these large trees is place it more firmly in a taxonomic and upwards of 50 m (Rich et al., 1986), in evolutionary context. agreement with what would be expected for Specimen KNMP-TH 18760 is character- a wet or moist forest community; drier, ized by vessels often in radial groups of four, deciduous forests of Africa characteristically alternate intervessel pitting, scalariform per- reach a maximum height of 25 m (Menaut foration plates, abundant tyloses and diffuse, et al., 1995) and montane forest trees rarely predominantly apotracheal, parenchyma; exceed 37 m (Richards, 1996). rays are heterocellular. Preliminary compari- sons using Wheeler et al. (1986) indicate Stable isotopes of fossil herbivore teeth affinities with the Olacaceae, particularly the and paleosols genera Coula, Diogoa and Strombosiopsis, all of which are monotypic West and Central Stable carbon isotopic analysis of fossil African endemics (Table 3). Although initial material have contributed to reconstructions comparisons are favorable overall, Strombosi- of past ecosystems. The relative abundance opsis tetrandra has opposite, rather than alter- of two naturally occurring stable isotopes of nate, intervessel pitting. Further study must carbon (12C and 13C) in fossil tooth enamel resolve this discrepancy, and whether the fos- apatite and paleosol carbonates has proved sil compares well in all anatomical features particularly useful in reconstructing aspects with Coula and Diagoa to the exclusion of of the vegetation. The premise underlying other genera within the family. this approach is that the tissue of plants   109

Table 4 Stable carbon isotopic analyses of Mpesida herbivore enamel and paleosol components

ca. % 13 Sample no. Locality Material/taxa  C(‰) ca.%C4* dietary C4†

Material MP263c E. Rurmoch Pedogenic carbonate nodule 7·99 14–34 (0–49) MP263o E. Rurmoch Paleosol disseminated bulk organic 22·28 32 (0–57) MP265c E. Rurmoch Pedogenic carbonate nodule 7·53 17–38 (0–62) MP265o E. Rurmoch Paleosol disseminated bulk organic 18·72 57 (19–78) MP265no E. Rurmoch Paleosol nodular bulk organic 22·0 34 (0–59) MP266 E. Rurmoch Pedogenic carbonate nodule 7·42 18–39 (0–62) Herbivore taxa MP311h BPRP#085a Hippopotamidae, gen. & sp. 1·85 76 (50–94) MP311b BPRP#085a Bovidae, gen. & sp. 6·62 43 (11–66) JK3 BPRP#085a Hipparioninae, gen. & sp. indet. 0·02 89 (66–100)

13   *Based on an average  C values of about 27‰ for C3 plants and 12·5‰ for C4 vegetation (combined NADP-me and NAD-me). Range shown for pedogenic carbonate reflects 14–17‰ enrichment due to combination of diffusion and equilibrium fractionation factors and seasonal variation of vegetation. The difference in the 13C values of sample MP265 bulk disseminated organic component (MP265a) and the associated pedogenic carbonate (MP265c) is outside of the conventionally accepted 14–17‰ range. More sequestered nodular organic residue (MP265na) places the carbonate–organic difference within this range suggesting alteration of the disseminated organic component. Values in parentheses indicate maximum range of estimates that take into account the 13  variability of  C associated with the C3 and C4 photosynthetic pathways—a carbon isotopic range of 22‰ to    13 32‰ and 15‰ to 10‰ for C3 and C4 plants, respectively. Anthropogenic alteration of atmospheric  C by 1·5‰ over the last 150 years not entered into these calculations. †Assuming a physiological carbon isotope enrichment of 14·1‰ between herbivore enamel and diet (Cerling & 13   Harris, 1999) and an average  C values of 26·5‰ and 12·5‰ for C3,C4 diets respectively. Range of values 13 in parentheses span estimates of dietary C4 taking into account the variability of  C associated with the C3 and C4 photosynthetic pathways. utilizing alternative photosynthetic pathways plants can be used as a measure of how ff can be di erentiated on the basis of the ratio ‘‘open’’ past ecosystems were with C4 domi- of 13C/12C(Craig, 1953; Smith & Epstein, nated habitats reflecting Serengeti-type

1971). This isotopic signature can be grasslands, C3 dominated ecosystems indi- retrieved from the fossil record, either by cating bushland, woodland, or forested direct analysis of ancient organic residues conditions, and mixed C3–C4 signals repre- or from inorganic material that formed in senting open woodland or heterogeneous isotopic equilibrium with paleovegetation forest-grassland environments. (Cerling, 1984; Ambrose & Sikes, 1991; Isotopic analyses of paleosol carbonates Kingston, 1999). The two pathways of sig- and herbivore tooth enamel from the nificance here, the C3 (Calvin-Benson) and Mpesida Beds at Rurmoch, previously C4 (Hatch-Slack or Kranz) are associated reported as part of an isotopic study of the with different environmental conditions and entire Tugen Hills succession (Kingston often vegetation physiognomy. Specifically, et al., 1994; Morgan et al., 1994; Kingston, the link between C4 metabolism and warm- 1999), are summarized in Table 4. climate grasses provides a means of recog- Although paleosols were not documented in nizing and characterizing plant communities the depositional basin where fossil wood was in the tropics that include varying propor- recovered, exposures of Mpesida beds tions of grasses such as open woodlands, 0·5 km east of Rurmoch included poorly savannas, and grasslands. In tropical developed tan/light brown paleosols con- regions, relative proportions of C3 and C4 taining pedogenic carbonate nodules. 110 . .  ET AL.

Analyses of the carbon isotopic signature of Anancus kenyensis, and Sanders (1999) has disseminated organic residues, nodular recently recognized Stegodon, a rare taxon in organics, and carbonate nodules indicate Africa. There is also the first known occur- either a minor C4 (grassy) component or rence of a modern genus of rhinoceros, Cera- 13 water stressed C3 vegetation with  C totherium praecox. Some new species arise values on the extreme positive range of within already established genera, such as modern C3 vegetation (Table 4). Although Brachypotherium cf. lewisi, and within Hippa- these horizons could not be traced laterally rion. There is also the chalicothere Chemosi- to the exposures west of Rurmoch, struc- tia tugenensis, a hippopotamid, a giraffid, and tural and lithologic aspects indicate that they bovids, which include Tragelaphus, Kobus, are correlative with the pyroclastic and epi- Madoqua, and Gazella or Raphiceros. clastic facies and hence stratigraphically In view of their rarity it is worth noting a above the fossil wood-bearing ash-flow unit. couple of new primate specimens from the Of fossil herbivore enamel sampled from Mpesida Beds. Among primates there has the Mpesida Beds for isotopic analyses, previously only been an indeterminate talus three specimens were from horizons known from the unit (KNM-MP 227). within the Rurmoch basin. Carbon isotopic However, recently BPRP has discovered two signatures of fossil taxa all indicate a signifi- colobine specimens from site BPRP#85 at 13 cant C4 dietary component.  C values of Rurmoch. Both are molar teeth (KNM-TH equid and hippopotamus enamel are con- 23169; KNM-TH 30775). The Mpesida sistent with C4 dominated if not exclusive colobine is larger than the earlier Microcolo- C4 grazing diets and indicate the presence of bus from Ngeringerowa in the Tugen Hills grass-dominated habitats. Fossil enamel was succession. collected from BPRP#85a, within the pyro- The early record of cercopithecoids is clastic and epiclastic facies. relatively poor. The earliest colobines in Africa, probably in the world, come from Ngeringerowa, also in the Tugen Hills suc- Mpesida fauna cession, and other cercopithecoids, slightly This time period represented by the later than this, come from the sites of Mpesida facies, absent or poorly represented Nakali, Kenya, and the Nkondo Formation, elsewhere in Africa, is a most interesting one Uganda (Gundling & Hill, 2000). The for understanding African faunal evolution. colobines from Ngeringerowa in the Tugen The Mpesida Beds show the earliest evi- Hills are Microcolobus tugenensis which, as dence of a major change from the under- the name implies, is a small species (Benefit lying Ngorora assemblages. The latter & Pickford, 1986). Recently BPRP has have a typical late Miocene character, redated this locality to between 9 and even those that contain equids (Barry et al., 8·5 Ma (Deino, unpublished). Benefit & 1985; Hill, 1985, 1987, 1999; Hill et al., Pickford also described another specimen of 1986). cercopithecoid found by Meave Leakey at Work on definitive faunal lists for indi- Nakali (Benefit & Pickford, 1986) as an vidual Mpesida fossil sites is underway, indeterminate colobine. Benefit later mis- but following are some more general com- takenly equates Nakali with site BPRP#38 ments. In the Mpesida Beds whole new in the Tugen Hills (Benefit, 1999; see Hill families appear, such as the Elephantidae et al., 2002). In fact Nakali is a locality on (Stegotetrabelodon orbus), and the first the eastern side of the Rift Valley, first leporids in sub-Saharan Africa (see Winkler, investigated by Aguirre and Philip Leakey 2002). Among new proboscidean genera is (Aguirre & Leakey, 1974), and later by   111

Richard Leakey and Alan Walker (un- While these distinctions are based on struc- published). Nakali is an interesting site tural, sedimentologic, lithologic, geochrono- faunally, probably just a little younger than logic or faunal evidence, no unequivocal the Ngeringerowa locality (Hill, 1987; criteria for these distinctions have been Gundling & Hill, 2000). Among monkey formally articulated or recognized. For fossils from sediments in the western branch example, while the vertebrate fauna from of the East African Rift, along the Uganda/ the Lukeino Formation is more diverse Congo border, are two colobine M3s than assemblages attributed to the Mpesida reported by Senut (1994) from the Nkondo Beds, including new taxa such as Hystrix, Formation. They were assigned to cf. several carnivores, the suid Nyanzachoerus, Paracolobus, and dated between 6·5 and Ancylotherium, and the elephantid Prime- 6·2, based on comparison with Tugen Hills lephas gomphotheroides, this most probably faunas (Pickford et al., 1993). Among other reflects the more extensive collections of fossil colobine teeth from Africa are some known Lukeino fossil material (Hill et al., from Marceau in Algeria that may be as old 1986) rather than significant differences in as 8 Ma (see Gundling & Hill, 2000). composition. Faunal data are not yet com- plete enough to resolve stratigraphic or chronologic relationships among these Discussion sediments associated with the Kabarnet Stratigraphy Trachytes. Earlier researchers in the Tugen Hills The nature of the contact between sedi- region (Martyn, 1969; Chapman, 1971; ments and volcanic flows have in part been McClenaghan, 1971) recognized the diffi- used to differentiate the sedimentary culties in formalizing the Mpesida Beds as units. The basal horizons of the Lukeino an operational geologic unit, even as a mem- Formation have been described as resting on ber of the Kabarnet Trachyte Formation. weathered Kabarnet trachyte (Chapman This was due primarily to difficulty in cor- et al., 1978), lying on faulted trachytes with relating the widespread isolated exposures a slight disconformity (Chapman, 1971), temporally or genetically. Typically, all incorporating trachytic weathering products sediments associated with the Kabarnet (Martyn, 1969; Williams & Chapman, Trachytes, whether intercalated within tra- 1986), consisting of reworked paleosols chytic flows or in conformable contact with developed on the Kabarnet Trachyte the base or top of flow units, have tradition- (Pickford, 1975b, 1978) or accumulated on ally been categorized as Mpesida Beds. subaerially weathered and downwarped Most of the difficulties involved in this Kabarnet Trachyte (Martyn, 1969). Specific designation relate to the epiclastic and exposures of the Kaperyon Formation have lacustrine facies which stratigraphically also been described as resting with pro- overlie trachytes and have been allocated nounced unconformity (Chapman & Brook, to either the Mpesida Beds, Kaperyon 1978) or with paraconformity (Chapman, Formation, Lukeino Formation, Riwo Beds, 1971) on the Kabarnet Formation or Kaparaina ‘‘sub-basalt’’ sediments or some Mpesida facies (Chapman, 1971). Pickford combination of the above. Volcaniclastic (1975b) has suggested that these older deposits interpreted as Mpesida Beds are Kaperyon sediments should be allocated to alternatively described as being overlain at the Lukeino Formation based on aspects of certain localities by sedimentary facies of the the fauna, specifically proboscidean taxa. Kaperyon Formation (Chapman, 1971)or Martyn (1969) describes up to 47 m of the Lukeino Formation (Pickford, 1975b). Riwo Bed deposits unconformably lying on 112 . .  ET AL.

Kabarnet trachytes. Mpesida Beds, in con- by volcanic formations as depicted on the trast, are typically assumed to be conformable schematic, composite Tugen Hills section on flows of the Kabarnet Formation. How- (Figure 2). ever, given the time span allocated to this Sediments exposed along the flanks of formation (>1 Ma) and the episodic nature of Rurmoch and to the southwest highlight deposits resulting from eruptive events, it is some of the problems discussed above. reasonable to assume that there are uncon- These deposits were previously mapped as formities between Mpesida facies and strati- ‘‘landslipped’’ Mpesida facies in the southern graphically adjacent flows in some areas. portion of the basin in contact with overlying Limited exposures of Mpesida sediment on Lukeino sediments to the north and north- lava as well as the difficulty in establishing east (Martyn, 1969; Chapman, 1971). The stratigraphic relationships among sediments flow overlying the ‘‘Lukeino deposits’’ and and massive, nonbedded volcanic units forming the base of Rurmoch was interpreted complicates these interpretations. to be Kaparaina basalt. Our investigations of Lithologic aspects of the various sedimen- this area indicate that most if not all sedi- tary units have provided the primary means ments immediately surrounding Rurmoch for identifying sedimentary facies associated should be considered Mpesida facies as they with Kabarnet Trachytes. Mpesida Beds comprise a conformable sequence and are tend to be composed of coarse, poorly overlain by 6·3 Ma Kabarnet Trachytes sorted yellow/buff epiclastic and pyroclastic rather than Kaparaina Basalt. Facies associ- horizons and exposures are frequently highly ations are complex, reflecting deposition on a disrupted structurally with contorted and deeply dissected topography consisting of sheared bedding. However, lacustrine and trachyte and subsequent differential erosion to a limited extent the fluvial facies of the prior to the capping of the sequence by later Mpesida Beds are difficult to distinguish trachytic flows. These observations have sig- from basal Lukeino and Kaperyon clastics nificance for interpreting the age of a number and confound specific allocation of isolated of fossil localities to the north such as the exposures of sediment. Abrupt and ubiqui- Cheboit area which includes the site where tous lateral and vertical facies transitions, the Lukeino molar was recovered (Pickford, localized depositional basins, and syn- and 1975b). These sediments, typically allocated postdepositional structural deformation fur- to the Lukeino Formation, should in part be ther complicate unraveling stratigraphic considered contiguous with the Mpesida relationships among late Miocene sediments beds and may be as much as 0·5–0·7 Ma in the Tugen Hills. Developing type sections older than previously interpreted. Ongoing of these formations or units is not neces- investigations by the BPRP are focusing on sarily informative as there is substantial this issue. inter- and intrabasinal variation in facies associations and there are no consistent Paleoenvironment marker beds or sedimentary packages to Paleobotanical material, especially when guide correlations. The complex interplay autochthonous, provide the most robust between volcanic extrusions, tectonic evidence of paleovegetation. Preliminary activity, and sedimentary deposition indi- studies of wood fragments collected from cated by a scrutiny of the Mpesida Beds as a the massive ash flow west of Rurmoch indi- geologic unit is characteristic of the Tugen cates that moist forests were present in the Hills succession in general and it can be rift valley ca. 6·3 Ma. While fossils of large misleading to assume that sedimentary units colobines from overlying strata are consist- are universally bracketed and delineated ent with these interpretations, isotopic proxy   113 data indicate more open habitats extensive tion of Kabarnet Trachytes, span a critical enough to support large-bodied obligate interval of time relevant to the evolution of grazers. Although a number of explanations East African fauna, including hominids. could be developed to account for these This 7–6 Ma period is poorly represented in apparent discrepancies in interpretation, the the known fossil record of East Africa and following two scenarios are most compel- empirical data from Mpesida deposits pro- ling. As the fossil wood derives from strata viding evidence of faunal and floral commu- underlying the epiclastic, fossiliferous hori- nities are extremely valuable. As a member zons, it is plausible to consider that the within the Kabarnet Trachyte Formation, interval of time represented by the inter- the Mpesida facies have lacked clear strati- vening sediments or an unrecognized depo- graphic definition other than association sitional hiatus is sufficient for vegetational with or juxtaposition within or above lava change. Forests, at least locally, may have flows of the Kabarnet Trachyte. Deposits been replaced by more open woodland habi- attributed to this member are scattered tats as a response to changes in climatic widely throughout the Tugen Hills, primar- conditions, tectonically controlled geo- ily east of the Saimo Fault, and the temporal graphic and altitudinal factors, or extreme and genetic relationships among these habitat disruption/destruction by volcanic patches of sediment are diverse. Where no eruptive processes. Alternatively, evidence trachytic flows intervene, distinctions of a forest penecontemporaneous with between lacustrine and fluvial Mpesida exclusive open-country grazers may reflect facies and sediments of the overlying habitat heterogeneity resulting from topo- Lukeino, Kaperyon, and Riwo Beds remain graphic variations and/or localized moisture equivocal. These unresolved issues have differences. Based on interpretations of implications for assuming dates of overlying relatively rapid deposition of the fossil- fossiliferous strata including the Lukeino iferous horizons and the underlying ash molar site and horizons yielding Orrorin flow containing the wood fragments, the tugenensis. latter scenario is favored. In addition, the To help resolve some of the stratigraphic colobines are associated with the grazing relationships within the Mpesida Beds, three fauna suggesting the presence of forested main facies associations of Mpesida volcani- patches juxtaposed with open areas region- clastic deposits have been recognized west of ally if not locally. Presently the Tugen Rurmoch along the eastern margin of the Hills, with topographic relief of over Tugen Hills. The lowermost is a pyroclastic 3000 m, supports a wide range of eco- flow facies, formed by the gravitational col- systems in anthropogenically undisturbed lapse of a discrete eruptive column, which areas ranging from open acacia bushland includes a massive flow unit that swept complex/grassland to lake margin/riverine through the rugged volcanic landscape, gallery forests to montane rainforest habi- incorporating and transporting large tree tats. It is reasonable to consider that por- fragments in a fluidized gas/ash matrix. Sub- tions of the rift valley in the past may have sequent silicification has preserved cellular similarly been characterized by vegetational structures within the wood fragments and heterogeneity. taxonomic identification based on analyses of anatomical features indicate a lowland or upland forest having floristic affinities with Summary West and Central Africa, and possibly with Sediments of the Mpesida Beds, consisting Indo-Malaysia. Estimated tree heights based of volcaniclastic debris associated with erup- on stem diameters exceed 50 m suggesting a 114 . .  ET AL. wet lowland forest community. Overlying References pyroclastic ash fall and epiclastic facies have Aguirre, E. & Leakey, P. (1974). Nakali: nueva fauna yielded vertebrate fossils including two de Hipparion del Rift Valley de Kenya. Estud. Geol. large-bodied colobine molars. Isotopic 30, 219–227. analyses of herbivore enamel and paleosol Ambrose, S. H. & Sikes, N. E. (1991). Soil carbon isotope evidence for Holocene habitat change in the carbonates from these strata and correlative Kenya Rift Valley. Science 253, 1402–1405. horizons indicate more open habitats. Col- Baker, B. H., Williams, L. A. J., Miller, J. A. & Fitch, lective interpretations of the fossil wood and F. J. (1971). Sequence and geochronology of the Kenya Rift volcanics. Tectonophysics 11, 191–215. the isotopic data indicate habitat hetero- Barry, J., Raza, S. M. & Jacobs, L. L. (1985). Neogene geneity within this portion of the rift. mammalian faunal change in southern Asia: corre- Environmental data for late Miocene rift lations with climatic, tectonic, and eustatic events. Geology 13, 637–640. valley sites are rare and interpretations of Benefit, B. R. (1999). Victoriapithecus:thekeytoOld the fossil wood, isotopes and fauna from World Monkey and Catarrhine origins. Evol. the Mpesida Beds provide a unique Anthrop. 7, 155–174. Benefit, B. R. & Pickford, M. (1986). Miocene fossil window into potential early hominid cercopithecoids from Kenya. Am. J. phys. Anthrop. habitats. In the northern portion of the 69, 441–461. depositional basin, the ash-flow facies is Bernor, R. L., Fahlbusch, V. & Mittmann, H.-W. (1996). The evolution of western Eurasian Neogene overlain by fine-grained lacustrine sedi- mammal faunas. Columbia University Press, New ments, which are lithologically and faunally York. indistinguishable from previous interpret- Bishop, W. W., Chapman, G. R., Hill, A. & Miller, J. A. (1971). Succession of Cainozoic vertebrate ations of the Lukeino or Kaperyon For- assemblages from the northern Kenya Rift Valley. 40 39 mations. Ar/ Ar single-crystal dates on Nature 233, 389–394. anorthoclase from a pumice tuff within the Cas, R. A. F. & Wright, J. V. (1987). Volcanic Successions—Modern and Ancient. London: Allen & Mpesida Beds at Rurmoch, and from the Unwin. overlying trachyte flow, indicate an age of Cerling, T., MacFadden, B. J., Leakey, M. G., Quade, about 6·37 Ma for the upper part of the J., Eisenmann, V. & Ehleringer, J. R. (1997). Global sequence. vegetation change through the Miocene/Pliocene Boundary. Nature 389, 153–158. Cerling, T. E. (1984). The stable isotopic composition of modern soil carbonates and its relationship to Acknowledgements climate. Earth Planet. Sci. Lett. 71, 229–240. Cerling, T. E. & Harris, J. (1999). Carbon isotope This work is part of the Baringo Paleonto- fractionation between diet and bioapatite in ungulate logical Research Project, based at Yale mammals and implications for ecological and paleo- University, operating jointly with the ecological studies. Oecologia 120, 347–363. Chapman, G. R. (1971). The geological evolution of National Museums of Kenya. We thank the the northern Kamasia Hills, Baringo District, Kenya. Government of the Republic of Kenya for Ph.D. Dissertation, London University. permission to carry out research in Kenya Chapman, G. R. & Brook, M. (1978). 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