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Hominoid Teeth with Chimpanzee- and Gorilla-Like Features from the Miocene of Kenya: Implications for the Chronology of Ape-Huma

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Hominoid Teeth with Chimpanzee- and Gorilla-Like Features from the Miocene of Kenya: Implications for the Chronology of Ape-Huma ANTHROPOLOGICAL SCIENCE Vol. 000, 000-000, 2004 Hominoid teeth with chimpanzee- and gorilla-like features from the Miocene of Kenya: implications for the chronology of ape-human divergence and biogeography of Miocene hominoids MARTIN PICKFORD1*, BRIGITTE SENUT2 1Chaire de Paléoanthropologie et de Préhistoire du Collège de France, GDR 983 and UMR 5143 du CNRS, 8, rue Buffon, 75005 Paris 2Département Histoire de la Terre du Muséum National d’Histoire Naturelle, Paris Received 30 June 2003; accepted 16 March 2004 Abstract One of the major lacunae in our knowledge of African hominoid evolution concerns the origins of the chimpanzee and gorilla. Several thousand specimens from the Plio–Pleistocene of Africa have been attributed to Hominidae (sensu stricto) of which only a few, including Ardipithecus ramidus, have been re-interpreted by some authors as possibly representing an ape rather than a hominid (Senut, 1998). Four recently discovered ape-like specimens from the late Middle Miocene (12.5 Ma) and Late Miocene (5.9 Ma) of Kenya partly fill the gap in the fossil record of African apes, and show some mor- phological and metric affinities with teeth of chimpanzees and gorillas. If these few specimens from Kenya are indeed more closely related to chimps and gorillas than to hominids, then this implies that the dichotomy between African apes and hominids occurred several million years earlier than is cur- rently estimated by most researchers. Furthermore these ape teeth from Ngorora and Lukeino suggest that extant African apes evolved in Africa, and did not immigrate into the continent from Europe or Asia. Key words: Miocene, Kenya, Orrorin, Pan, Gorilla Introduction trally directed crests which close off a low walled mesial fovea. Anterior crests from these same cusps meet mesially This paper concerns four ape-like teeth from Kenya, one to wall off the anterior side of the mesial fovea, which is from the Ngorora Formation (12.5 Ma), and three from the buccolingually wide and mesiodistally short. The hypoconid Lukeino Formation (5.9 Ma), collected by the Kenya Palae- has low crests descending from its apex mesially and distally ontology Expedition. but not crossing the grooves, which separate the cusp from In this paper we restrict the term ‘hominid’ to encompass its neighbors. There is a basal cingular enamel fold between only those hominoids that possess skeletal morphology the protoconid and hypoconid and a tiny fold between the indicative of habitual or obligate bipedal locomotion. We do hypoconid and hypoconulid. The hypoconulid has a crest not employ it in the much wider sense that has recently been descending obliquely anteriorly into the talonid basin, but it used by some authors, in which even the genera Pan and is low and does not separate the basin from the distal fovea. Gorilla are included in the family Hominidae. The postcristid reaches lingually towards the low cusplet present between the hypoconulid and the entoconid. The Ngorora Lower Molar The apices of the metaconid and entoconid are strongly buccolingually compressed and peripherally positioned, a Morphological description (Figure 1A) disposition that results in a voluminous sunken talonid basin Bar 91’99 is an unworn right lower molar, probably the bordered by cusps that are somewhat trenchant in appear- second (11.4 mm mesiodistal P 9.4 mm buccolingual), ance. The posterior crest of the metaconid descends towards although a case could be made for it being a third molar. The a low, peripherally positioned cusplet between the meta- apices of the protoconid and hypoconid are buccolingually conid and entoconid. The hypoconid extends beyond the compressed and not very voluminous. They are located midline of the crown and reaches this small cusplet. As a towards the buccal margin of the tooth and they express consequence, the metaconid and entoconid do not touch minor buccal flare. The apices of the protoconid and meta- each other. A postentoconid crest descends obliquely conid are 5.2 mm apart and the tooth is 9.2 mm broad at this towards another low cusplet located between the entoconid level. The protoconid and metaconid have low-relief, cen- and hypoconulid from which it is separated by narrow but short vertical grooves on the posterolingual surface. * Corresponding author. e-mail: [email protected] The occlusal basin is wide, deep, and long. The wall phone: +33-1-40-79-30-03; fax: +33-1-40-79-35-80 between the anterior fovea and the talonid basin is low and Published online 13 July 2004 the posterior fovea is not clearly demarcated from it, and in J-STAGE (www.jstage.jst.go.jp) DOI: 10.1537/ase.04S014 thus extends for almost the entire length of the tooth. The © 2004 The Anthropological Society of Nippon 1 2 M. PICKFORD AND B. SENUT ANTHROPOLOGICAL SCIENCE Figure 1. Selected hominoid teeth from Africa and Europe (scale bar, 10 mm). (A) Bar 91’99, right lower molar (cast); (A1) stereo occlusal view; (A2) mesial view to show buccal flare. (B) Pan paniscus right m/2, occlusal view. (C) Pan troglodytes, cast of right m/2, occlusal view. (D– F) Dryopithecus brancoi casts of lower molars from Europe, occlusal views (D, Trochtelfingen; E, Salmendingen; F, Ebingen). (G) Gorilla gorilla lower m/2, occlusal view. occlusal surface of the enamel in the basin is wrinkled. chtelfingen, and Ebingen (Figure 1D–F). It is not impossible that the Ngorora species is related to the genus Dryop- Comparisons with Pan ithecus. The main differences from molars of Pan paniscus (Fig- ure 1B) concern the buccal cusps, which are slightly more Comparisons with Miocene African hominoids internally positioned in the fossil. In P. paniscus, the buccal Comparison of the Ngorora tooth (Figure 1A) with lower cusps are even more peripherally located than they are in the molars of African fossil hominoids reveals that it does not Ngorora tooth. Another difference between the Ngorora belong to any of the described Miocene forms, none of tooth and P. paniscus lies in the height of the entoconid, which possess the large occlusal basin and peripheralized which is taller and more trenchant in P. paniscus. The ante- cusps that distinguish this tooth. The only specimen with rior fovea is buccolingually narrower in the fossil than in which it accords in morphology is the Ngorora upper molar Pan. However, these are relatively minor differences. (KNM-BN 1378), which has wide occlusal fovea and a large The Ngorora molar resembles lower molars of Pan trogl- protocone, but this tooth is appreciably larger than what odytes from Mahale, Tanzania (Figure 1C) more closely than would be expected to occlude with Bar 91’99. it does those of P. paniscus. This is mainly because the buc- cal cusps are slightly more internally positioned in the com- Implications of the Ngorora lower molar mon chimpanzee, and they thus have moderate buccal flare Until now, no fossil chimpanzees and gorillas have been of the cusp walls. A minor difference is the extent of the reported and there has thus been a lack of evidence to test the mesial fovea, which is greater in P. troglodytes than it is in molecular clock estimates of the dichotomy between African the Ngorora specimen, and most chimpanzee molars do not apes and hominids. If the derived characters of the Ngorora have any trace of a buccal cingulum. molar represent homologies shared with chimpanzees, then it would indicate that the Pan clade has its roots in the latter Comparison with molars of Dryopithecus part of the Middle Miocene, sometime prior to 12.5 Ma. Lower molars of the European genus Dryopithecus (Fig- The dichotomy between chimpanzees and humans is usu- ure 1D–F) have peripheralized cusps, and broad but shallow ally estimated by molecular biologists to be more recent than occlusal basins (Begun, 2002). They also possess slight buc- 6 Ma (Gagneux et al., 1999; Stauffer et al., 2001) and the cal cingula and some specimens have a small accessory cus- split between chimpanzees and gorillas has been estimated plet separating the metaconid and entoconid in the lingual at 8–9 Ma by Wrangham and Pilbeam (2001) and 7.7 Ma by notch. The most significant differences between the Kenyan Gagneux et al. (1999). The only molecular biologists who and European specimens are the shallower occlusal basins have proposed an earlier age for African ape origins are and the less buccolingually compressed lingual cusps that Arnason and co-workers (Arnason et al.,1996, 1998, 2000; occur in Dryopithecus. In particular, the Ngorora tooth Janke and Arnason, 2002), but their results are usually con- resembles some of the central European D. brancoi, espe- sidered suspect by others who appear to favor appreciably cially specimens such as those from Salmendingen, Tro- later divergence times (Bailey et al., 1992; Adachi and Vol. 112, 2004 HOMINOID TEETH FROM THE MIOCENE OF KENYA 3 Hasegawa, 1995; Gagneux et al., 1999; Stauffer et al., 2001; are the entire paracone and metacone, part of the hypocone Pilbeam, 2002). and the distobuccal extremity of the protocone (the crista The Ngorora specimen thus runs counter to the recent obliqua). The distal fovea is complete. The tooth is 14 mm ideas of Pilbeam (1996) who wrote that “the common ances- mesiodistal. It is low crowned (7.3 mm from cervix to tip of tor of humans and chimpanzees was probably chimpanzee- metacone) and the cusps are endowed with low but broad like, a knuckle-walker with small thin-enamelled cheek wrinkles. teeth,” and Wrangham and Pilbeam (2001) who postulated The paracone and metacone are separated from each other that the “6 mybp ancestor…would have been thin- by a narrow slit-like buccal groove (Figure 2B). The part of enamelled, knuckle-walking, and females would have had the trigon basin preserved is relatively large and open, the black body coats.” It is already known that 6 Ma hominids main cusps being peripherally positioned and the cusps not such as Orrorin tugenensis had thick-enamelled molars with bulbous.
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