Andrew Hill Department oJAnthropology, Yale linirwsi&, R'erer Ham, CT 06520, 1I.S.A.

Kay Behrensmeyer Department ~~Paleobiology, MRC-.VHB 121, National Museum of Natural History, Smithsonian Institution, Wash%zgton, DC 20506, l_S.A.

Barbara Brown Department of Zoological and Biomedical Sciences. College of the Arts and Sciences, and Basic Science l,rnit, College 01 Osteopathic Medicine, Ohio L’niuersi@. Athens: OH 45701. C!.S.A.

Alan Deino (~eo~hmnolog Center, Institute OS Human &gins, 2453 Ridge Road, Berkelq, CA .94709, 1 IS.A.

Michael Rose Department of :inatom_v. ,Veur Jersq .\leditai aSchool, l~‘niuersi~y oj .\ledicine and Dentistry of Nerer

Jersq. ,Vmark, NJ 0710.7. 1iS.A.

Jeffrey Saunders Illinois Sfatr .llusmn. Sprin+$eld, II> 62706. 1:. s. A.

Steven Ward Department of .Smiologv and Anfhropologv. Kent .Stafe Uniuersi&, Kent, OH 4KV2. and Human Anatomy Progmm, Northeastern Ohio 1 izir&ities College oj‘hledicine, Rootstown. OH d-/272, 1I.S.A.

Alisa Winkler

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Kipsaramon: a lower Miocene hominoid site in the Tugen Hills, Baring0 District, Kenya

Journal qf Human Rrmlution (1991) 20, 67-75

Introduction

Kipsaramon, a fossil site complex in the northern Tugen Hills, Baring0 District, is possibly the richest local site in Kenya. Fauna1 remains are very numerous, with a diverse array of

0047-2484/91/010067 + 09 $03.00/O 0 1991 Academic Press Limited

68 CURRENT EVENTS

taxa. Hominoid specimens have already been found, and the site promises an abundance of further, taxonomically varied hominoid material. The mammalian fossils are associated with well-preserved plant remains, and the site appears well-dated at 15.5 Ma. The general impression of Africa in the lower Miocene derives from localities in the west of Kenya, but more recently discovered sites, such as Buluk and Kalodirr, hint at the possibility of a different and distinct northern fauna1 area. If this northern-west Kenyan dichotomy really exists it may have been influenced by the developing Rift system. This influence may be monitored by Kipsaramon. Our site is situated within the Rift and geographically intermediate between these two regions (Figure 1).

Context and geology

Martin Pickford and Kiptalam Cheboi originally discovered the Kipsaramon sites in the mid-1970s and referred to them as Poi. They collected surface specimens, and Pickford (1988) has recently published his earlier observations, but essentially the sites remained uninvestigated in detail until 1987, when the Baring0 Paleontological Research Project conducted exploratory work there (Hill, 1989). The main Tugen Hills sequence known until now spans the period from over 13 Ma to 1.6 Ma fairly continuously, and crops out over a 60 km range of hills trending north-south along the north Kenyan rift (Hill et al., 1985, 1986). The Kipsaramon site is at the northern end of these hills, in sediments that have been called the Muruyur Beds (Chapman, 1971; Bishop et al., 1971; Pickford, 1975, 1988), but which are so far little known. They occur beneath our lowest well-documented sedimentary unit, the Ngorora Formation, dated back to 13 Ma (Hill et al., 1985, 1986; Tauxe et al., 1985; Deino et al., 1990). Pickford (1988) describes a type section for the unit and provides other geological and paleontological details.

When first discovered, what is now the main site (BPRP #K089A) appeared as a number of bones densely packed in the wall ofa gulley. There was the possibility that it was the infilling ofa small channel. However, test excavations conducted by us reveal that it is a very substantial and extensive bone bed. The bone bed occurs within a sequence of lacustrine clays (Figure 2), but appears to represent an influx of fluvially derived elastic material. Bone is solidly packed over a thickness of about 20 cm, the matrix consisting of poorly consolidated fine sand and clay clasts. It is clear that the bed covers a considerable area, of at least 2000 m’. This whole laterally continuous unit is labelled BPRP #K089, separate test excavations being indicated by suffixed letters. There are other separate sites nearby, some of which may be outcrops of this same unit; others may be slightly higher or lower in the sequence.

The bone bed is underlain by a phonolite lava flow, and is overlain by an ignimbrite (Figure 2). Single-crystal laser fusion ““Ar/3gAr dating has been carried out on samples of these volcanic units at the Berkeley Geochronology Laboratory. Preliminary determinations produce 15.6 Ma for the flow beneath the fossiliferous horizon, and 15.4 Ma for the overlying ignimbrite. These early results suggest that the age of the bone bed is very close to 15.5 Ma. Although consistent, these are older dates than previously envisioned (Pickford, 1988; Hill, 1989) from inferred stratigraphic relations to dated Tiim Phonolite flows in the Ngorora Formation (Hill et al., 1985, 1986; Deino et al., 1990). And the Atimet Trachyphonolite, which is supposed to underlie the Muruyur Beds, has been dated by conventional K-Ar methods at 14.7 + 0.5 Ma (corrected for new constants)

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UGANDA ,.,z

..’ ‘.

,t+J&iobi ,.

:’

,: : . ..j

3.6’ : 5

il. -) ‘, :.

...

Figure 1. Map of western and central Kenya and eastern Uganda showing Miocene primate-bearing

localities referred to in the text.

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lgnimbrite 15.4 Ma

PLANT BED

BONEBED 9KO89

Phonolite 15.6 Ma

I 1111

CZSG

Figure 2. Stratigraphic section showing relationship ofbonebed (#K089) to dated volcanic units. Grain size scale below sections applies only to non-volcanic sediments; C = clay, Z = silt, S = sand, G = gravel.

(Chapman, 1971; Chapman & Brook, 1978; Pickford, 1988). This matter will probably be resolved by more detailed stratigraphic work.

Paleontology

Our preliminary work in the area was confined simply to exposing and mapping the bone bed surface in a number of test excavations, and lifting some bones, rather than conducting a major excavation. The fauna so far includes animals ranging in size from proboscideans to rodents. In general, bones are well preserved, not usually articulated, although mandibles and maxillae with dentitions occur. The most conspicuous of these surface- exposed bones belonged to proboscideans. We recovered many skeletal parts, including complete maxillae and mandibles with dentitions of an animal similar to Choerolophodon,

and also remains of deinotheres. There are rhinoceros, probably Acerutherium. The fossil material is not confined to large taxa. In the matrix between the large bones are the remains of smaller animals. There are turtles, crocodiles, bovids and other small artiodactyls, possibly paleomerycids. At least seven families of rodents are represented including a squirrel, one or two taxa of springhares, a small scaly-tailed flying squirrel, several genera of cane rats, an afrocricetodontine, and several indeterminate muroids. The cane rat Diamantoqs is not yet known from the main site complex, but a specimen was collected by Pickford from an adjacent site not yet tied into the main sequence. The assemblage includes a mixture of unique taxa (new scaly-tailed flying squirrel) and typical

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lower Miocene taxa also known from west Kenya [Diamantomys, Elmerimys (Rusinga only), Notocricetodon] and west Turkana (ElmerimyJ). It also includes taxa with similarities to those from the west Kenya middle Miocene Maboko site (new springhare and derived cane rat). The presence of springhares suggests some open habitat, while the scaly-tailed flying squirrel suggests extensive tropical forest.

In the context of most east African fossil vertebrate sites, this one is unusual, being a very dense and extensive bone bed. It is not yet exactly clear what the site means taphonomically or how it originated.

The bone bed is immediately overlain by a silty unit with compressed leaves and fruits, which have not yet been identified.

Primates

At least three and possibly five hominoid specimens are known from Kipsaramon. One was found previously by Pickford’s expedition, from a nearby locality in the site complex (BPRP #K091), possibly the same age. We found three or four additional primate specimens in the last 2 days of our season when performing a more detailed test excavation from the exposed bone surface at K089A, about 3 m2 in extent, to a depth of about 3 cm.

Pickford’s specimen (KNM-MY 34) is a talus, not described until recently (Pickford, 1988; Hill & Ward, 1988). I p n reserved morphology and size it is comparable with those attributed to Proconsul major, although it also has features in common with an Afropithecus

specimen from Kalodirr. The 1987 primate specimens consist of the crown of a large hominoid right M2 (KNM-

TH 18690) ; and M’ or M” of a smaller species of hominoid (KNM-TH 18689); a tooth of as yet indefinite attribution (KNM-TH 18691); and a phalanx (KNM-TH 18692). All of the 1987 specimens come from one small arbitrarily chosen area, and from a small amount of sediment. They promise the recovery of a relatively large quantity of hominoids from this site in the future.

KNM-TH 18690 is a mandibular right second molar (Figure 3A). The specimen is a complete crown, preserved to the cervix. The complete lack of roots suggests that the specimen was unerupted, but a distinct wear facet on the apex of the protoconid indicates that at least the mesial part of the tooth had undergone gingival eruption. In all likelihood, given its minimal state of wear, this tooth had undergone some root formation prior to deposition. The crown itself is undistorted and undamaged. Its breadth length index of 93 (Table 1) places it within the limits defined by Harrison (1982) for P. major. However, it is somewhat unusual, when compared to other P. major second mandibular molars, in its expanded breadth dimensions, especially across its talonid. The trigonid is slightly broader than the talonid primarily due to basal swellings on the cervical margins of the protoconid and metaconid. The protoconid is the largest cusp, followed by the metaconid, hypoconid, entoconid, and hypoconulid. The latter is centrally placed on the distal marginal ridge of the crown. As is the case for most Proconsul molars, marginal/cingular elements are strongly developed. The mesial fovea and mesial marginal ridge are prominent, and the buccal and disto-buccal cingular are augmented by discrete stylids. The lingual base of the metaconid is likewise imbricated with two small stylar elements. The central fovea is large and somewhat lingually displaced by a prominent mesiolingual extension of the hypoconid. There is no evidence of a deflecting wrinkle. #NM-TH 18690 is similar in almost all respects to other P. major M~s, except for its expanded talonid breadth. It is most

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I cm I cm

I I

A B

Figure 3. A: KNM-TH 18690. Mandibular right second molar attributed to Proconrul cf. mujor. Occlusal view, distal to the right. R: KNM-TH 18689. Maxillary kft first molar attributed to Knlepithecus cf. songhorenris. Occlusal view, distal to the right.

similar to the Chamtwara R/1, (KNM-CA 1298) in details of cusp morphology and development of the mesial marginal elements. However, we recognize that, on current evidence, the Chamtwara specimen is 4 or 5 million years older than the Kipsaramon tooth, and KNM-TH 18690 does present a rather discordant occlusal shape when compared to the total sample of P. major second molars. For this reason we assign KNM-TH 18690 to Proconsul cf. major.

KNM-TH 18689 is a maxillary left first molar of an adult small-bodied hominoid (Figure 3B). The crown is undistorted and is preserved to the cervix. The two buccal roots are also present, lacking only their apices. They are fused along their length. The lingual root is broken immediately below its origin at the base of the crown. The crown itself has undergone either mechanical or chemical alteration that has slightly blurred occlusal surface elements. The crown of KNM-TH 18689 is essentially square in occlusal view. It is clearly a very broad tooth for its size (Table 1). The protocone is deflected medially,

Table 1 Measurements of KNM-TH 18689 and KNM-TH 18690

Length Breadth LIBx 100 R/I. x 100

KNM-TH 18689 6.0 7.5 80 125 KNM-TH 18690 14.9 13.8 108 93

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accentuating the width of the prominent lingual cingulum. The trigone cusps are therefore

rather compressed together, with the concomitant effect of compressing the mesial fovea. Collectively these morphological features, as well as the metric dimensions noted above, are concordant with the diagnosis developed by Harrison (1989) for Kule@thecus

songhorensis. However, several other features of the Kipsaramon small hominoid differ from Harrison’s diagnosis. The protoconule is poorly developed rather than being fairly large and prominently expressed, the hypocone is equal in height to the other cusps, rather than smaller as in Kalepithecus. A short and ill-defined crest connects the hypocone and protocone, rather than the long and sharp crest reported by Harrison. Despite these differences, some of which could be attributed to the alteration noted above, we propose to refer KNM-TH 18689 to Kalepithecus cf. songhorensis.

KNM-TH 18691 is a fragmentary tooth. One suggestion is that it is a deciduous molar of a small hominoid; another, that it belongs to the Cercopithecidae. KNM-TH 18692 is the distal portion of a proximal phalanx. It is morphologically similar to those of Proconsul and Afropithecus.

Significance

In addition to the intrinsic interest of fossils from the site, Kipsaramon is important in extending the geographic range sampled by lower Miocene localities. The many sites in Africa ranging from 23 Ma to 15 Ma give the impression that African early Miocene hominoid diversity, evolution and environments are well understood. But nearly all of these localities are situated in a small area of western Kenya in the vicinity of Rusinga, Maboko, and Ft. Ternan (Figure 1). So the picture we have of this time period of hominoid evolution, like later phases (Hill, 1987; Hill & Ward, 1988), has derived from a highly restricted region not necessarily representative of Africa as a whole.

Also the geographic location of Kipsaramon is a particularly interesting one. Recently other sites have been found outside of the west Kenya area, such as Buluk on the Ethiopian border of Kenya, dated at >17 Ma (Harris & Watkins, 1974; Leakey & Walker, 1985; McDougal & Watkins, 1985). The large hominoid from there, originally referred to Sivapithccus, was later seen to belong to the new genus named Afiopithecus turkanensis from Kalodirr, west Turkana (Leakey & Leakey, 1986a; Leakey et al., 1988a), and dated between 17.7 Ma and 16.8 Ma (Boschetto, 1988). The taxon is unknown at the west Kenyan sites, and is associated with two other new hominoid taxa also unknown in western Kenya; the mid-sized Turkanapithecus kalakolensis (Leakey & Leakey, 1986b; Leakey et al.,

1988b), and the small Simiolus enjiessi (Leakey & Leakey, 1987). They seem to suggest a distinct, hitherto unsampled environment in the African lower Miocene, or perhaps a distinct fauna1 region. This idea is supported by the possible generic identity of the Saudi Arabian Heliopithecus leak& with Afropithecus (Andrews & Martin, 1987a), and some of the other Buluk and Kalodirr fauna show north African rather than west Kenyan affinities (Delson, 1985). The hominoid from Moroto, another northern site of about the same age (Pickford et al., 1986), may also belong to Afropithecus (Andrews & Martin, 1987a,b; Andrews et al., 1987). Other possible explanations for these differences include discrepancies in ages sampled at different sites. And differences in altitude, about which practically nothing is known for the east African Miocene, would also have significant effect.

These two Kenyan regions, the west and the north, are today separated by the western

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fault of the Rift Valley, parts ofwhich probably became active over 20 Ma ago. Differential accumulation of plateau phonolites of different age within and outside the rift suggest that the structure had significant relief in the lower Miocene (Lippard, 1973). The developing rift may therefore have had a role in the biological separation of the two areas, either simply physically, or mediated through climatic effects. In any case, any new lower Miocene sites, such as Kipsaramon, in this intervening area are particularly interesting in this context.

The presence at Kipsaramon of hominoids which may be attributable to Proconsul and Kalepithecus, along with some of the other fauna, seems to ally it provisionally with the west Kenya occurrences, rather than with the more northern Lake Turkana region. Nevertheless, some fauna does resemble material from Kalodirr. Further hominoids, and other fauna from this site, which we will collect in future seasons, should clarify some of these issues. The potential of the hominoids, coupled with the abundant remains of proboscideans and other fauna and plants, provides an excellent opportunity for a more comprehensive assessment of the lower Miocene hominoid environments of east Africa than has been possible before.

Acknowledgements

This work is part of the Baring0 Paleontological Research Project, based at Yale University, operating jointly with the National Museums of Kenya. It is funded by grants to A. H. from the National Science Foundation (BNS 88-02629), the Louise Brown Foundation, the Ingalls Foundation, and the Yale University Social Sciences Faculty Research Fund; and to A. H. and S. W. from the L. S. B. Leakey Foundation. S. W. also receives support from NSF grant BNS 87-18856. A. D. acknowledges support from NSF grants BNS 88-66106 and BNS 86-16256. M. R. acknowledges the support of NSF grant BNS 90-04502 and NIH grant 2S07RR05393 (B iomedical Research Support Grant). A. W. received support from Sigma Xi and the Institute for the Study of Earth and Man, SMU. Thanks are due to the field team in 1987. We also thank the Government of the Republic of Kenya for research permission and the National Museums of Kenya for their help.

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