Jour. Geol. Soc. Japan, Vol. 121, No. 4, p. 153–159, April 2015 doi: 10.5575/geosoc.2015.0004

Report New specimens of a primitive , Kenyapotamus coryndonae, from the Upper Nakali Formation, Kenya Takehisa Tsubamoto＊1, Yutaka Kunimatsu＊2, In particular, the origin of hippos is hotly debated re- Hideo Nakaya＊3, Tetsuya Sakai＊4, cently (e.g., Pickford, 2007, 2008, 2011; Boisserie et al., Mototaka Saneyoshi＊5, Emma Mbua＊6 and 2010; Orliac et al., 2010; Tsubamoto et al., 2011). How- Masato Nakatsukasa＊7 ever, the fossil collection size of the species is not enough to understand the morphology and variation of Received November 6, 2014 the species precisely, so that the early evolution of hip- Accepted January 23, 2015 ＊ pos is not well resolved. 1 Graduate School of Science and Engineering, Ehime Uni- versity, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan Since 2002, a joint research team from Kyoto Univer- ＊2 Faculty of Business Administration, Ryukoku University, sity and the National Museums of Kenya has been con- 67 Tsukamoto-cho, Fukakusa Fushimi-ku, Kyoto 612-8577, ducting field geological and paleontological research in Japan ＊3 Faculty of Science, Kagoshima University, Korimoto 1-21- the lower Upper Miocene Nakali Formation at the Na- 35, Kagoshima 890-0065, Japan kali locality, central Kenya (Nakatsukasa et al., 2006). ＊4 Interdisciplinary Faculty of Science and Engineering, Shi- mane University, 1060 Nishikawatsu-cho, Matsue 690- The team has discovered many vertebrate fossils includ- 8504, Japan ing a new great ape, nakayamai, which ＊5 Faculty of Biosphere-Geosphere Science, Okayama Uni- attracted a great deal of attention from anthropologists versity of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700- 0005, Japan and primatologists because the species is a candidate for ＊6 Department of Earth Sciences, National Museums of Ke- the latest common ancestor of the extant African great nya, P. O. Box 40658-00100, Nairobi, Kenya apes and humans (Kunimatsu et al., 2007). Handa et al. ＊7 Graduate School of Science, Kyoto University, Kitashi- rakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan (in press) has recently described one dental specimen from a fossil rhinoceros from the Nakali Formation dis- Corresponding author: T. Tsubamoto, covered by the joint team. [email protected] This material report describes and illustrates new den- Abstract: New dental and astragalar specimens of a tal and astragalar specimens of K. coryndonae from the primitive hippopotamus, Kenyapotamus coryndonae Nakali Formation discovered by the joint research team (Mammalia, Cetartiodactyla, , Kenya- from Kyoto University and the National Museums of potaminae) from the lower Upper Miocene Nakali For- Kenya. Only a few specimens of K. coryndonae from mation at the Nakali locality, central Kenya, are de- scribed and illustrated. The new specimens increase the Nakali Formation have been described by previous the known morphological and size variations of the researchers (Pickford, 1983; Boisserie et al., 2010). The dentition and astragalus in this primitive hippopota- specimens from the Nakali Formation will contribute to mid species, which is important to understand the ori- a more precise understanding of the morphology and gin and early evolution of the Hippopotamidae. variation of the species and to the analyses of the origin Keywords: Hippopotamidae, Kenya, Kenyapotaminae, of the hippos from the paleontological perspectives. Kenyapotamus, Miocene, Nakali Dental terminology.―Mostly follows Boisserie et al. (2010) and Orliac et al. (2010). Dental abbreviations.―M/m, upper/lower molars. P/ p, upper/lower premolars. Introduction Other abbreviations.―KNM, National Museums of Kenyapotamus coryndonae Pickford, 1983 (Mamma- Kenya, Nairobi, Kenya. NA, Nakali locality, Kenya. lia, Cetartiodactyla, Hippopotamidae, Kenyapotaminae) SH, Samburu Hills locality, Kenya. is a species of primitive fossil in the Miocene of Africa (Boisserie et al., 2010). It is an im- Systematic paleontology portant species in the African mammalian chronology Family Hippopotamidae Gray, 1821 and in the origin of the Hippopotamidae (e.g., Pickford, Subfamily Kenyapotaminae Pickford, 1983 1983, 1989, 1990, 2007, 2008; Boisserie et al., 2010). Genus Kenyapotamus Pickford, 1983 ©The Geological Society of Japan 2015 153 154 T. Tsubamoto et al. 2015―4

Kenyapotamus coryndonae Pickford, 1983 et al. (2007), the tuff bed in the upper most part of the “Lower” Member was dated at 9.90±0.09 and 9.82± Figs. 1–3 0.09 Ma and the tuff bed in the lower most part of the “Upper” Member was dated at 10.10±0.12 Ma. Based Kenyapotamus coryndoni Pickford, 1983 [sic]. on these 40Ar–39Ar ages and magnetostratigraphic analy- sis by Kunimatsu et al. (2007), the age of the Nakali Comments on the specific name.―The specific name Formation ranges from the Paleomagnetic Polarity was originally spelled as‘ coryndoni’ (Pickford, 1983), Chron C5n.2n to C5n.1n, with the Chron C5n.1r (9.92– but it was later corrected to a feminine form‘ coryndo- 9.88 Ma) corresponding to the upper part of the“ Lower” nae’ as it is named after a woman (Boisserie et al., Member to the lower most part of the“ Upper” Member. 2010; also see Articles 31.1, 32.4, and 32.5 of the Inter- Dental measurements.―Shown in Table 1. national Code of Zoological Nomenclature, Ride et al., Astragalar measurements (in mm).―NA11-12238’06: 1999). lateral proximo-distal length (a) = 61.5; medial proxi- New material.―KNM-NA 45748, a left M3; KNM- mo-distal length (b) = 51.8; medio-lateral width of tibial NA 52678, a distal part of the right upper molar; KNM- trochlea (c) = 33.2; medio-lateral width of distal troch- NA 47535, a right M3; NA12-12276’06, a right P2; lea (d) = 39.9; medial dorso-plantar height (e) = 30.2; KNM-NA 45747, a left maxillary fragment with P4; lateral length of tibial trochlea (f) = 37.3; medial length KNM-NA 45751, a broken left p3; KNM-NA 45754A, of tibial trochlea (g) = 34.7. KNM-NA 52680A: (b) = a broken left p3; KNM-NA 45746, a right mandibular 48.1; (d) = 36.7. The measurement positions (a–g) are fragment with p4; KNM-NA 45752, a right p4; KNM- the same as in Tsubamoto and Tsogtbaatar (2008). NA 45749, a left m3; KNM-NA 45756, a trigonid of a Comparative description.―P2 (NA12-12276’06; Fig. right lower molar; KNM-NA 45735A, a broken talonid 1A) is two-rooted and is as large as the previously re- of left m3; KNM-NA 47284, a broken talonid of right ported P2 specimen of K. coryndonae (Table 1). It dif- m3; NA11-12238’06, a left astragalus; KNM-NA fers from P3 of K. coryndonae in lacking a protocone 52680A, a right astragalus; KNM-NA 47290, a frag- and from P2 of the other kenyapotamine species Pal- mentary tusk. The specimen numbers starting with NA aeopotamus ternani in being more developed and more are formal field numbers in KNM. They will eventually P3-like (Pickford, 1983, 2007; Boisserie et al., 2010). be numbered with the prefix of KNM-NA. P4 (KNM-NA 45747; Fig. 1B) is as large as the previ- Repository.―Palaeontology Section, National Muse- ously reported P4 specimen of K. coryndonae (Table 1). ums of Kenya, Nairobi, Kenya. It has three roots. The first is below the mesial part of Locality.―Nakali locality (approx. 1°11′N and 36°22′E), the paracone. The second is below the distal part of the which is located on the eastern shoulder of the central paracone. The third is below the protocone. However, Kenya Rift, about 40 km west of Maralal, central Kenya the second and third roots are conjoined and are contin- (Kunimatsu et al., 2007). uous with each other, resulting in P4 superficially hav- Formation and horizon.―Nakali Formation. It is ing two roots (a smaller mesiobuccal one and a larger characterized by tuffaceous sandstones and mudstones distal one), as seen in the previously reported P4 speci- with conglomerate interbeds, a thick pyroclastic flow men of K. coryndonae (Pickford, 1983). deposit, and tuff beds (Kunimatsu et al., 2007; Sakai et The two M3 specimens (KNM-NA 45748 and 47535; al., 2013). It is stratigraphically subdivided into the Fig. 1C, 1E) are moderately worn and have no intersti- “Lower,”“ Middle,” and“ Upper” Members. The verte- tial wear facet on the distal face, indicating that they are brate fossils come from the upper part of the“ Lower” definitely M3s. KNM-NA 45748 is as large as the previ- Member and from the“ Upper” Member. KNM-NA ously reported M3 specimens of K. coryndonae, and 52678, 47535, 45735A, 47284, and 52680A are from KNM-NA 47535 is somewhat smaller than the previ- the“ Upper” Member. All other present specimens de- ously reported specimens (Table 1). In both specimens, scribed here are from the upper part of the“ Lower” the parastyle appears to be more mediomesially located Member. than that of P. ternani (Pickford, 1983, 2007; Boisserie Age.―The age of the vertebrate fossils from the Na- et al., 2010), also implying that these specimens should kali Formation is around 10.0–9.8 Ma (early Tortonian, be assigned to K. coryndonae. early Late Miocene). This age is correlated to the Val- KNM-NA 52678 (Fig. 1D) is a distal part of an upper lesian European Land Age (Steininger et al., molar and is only slightly worn. No interstitial wear fac- 1996). According to the 40Ar–39Ar dating by Kunimatsu et can be observed on the distal facet. Jour. Geol. Soc. Japan 121（ 4 ） Kenyapotamus from Nakali, Kenya 155

Fig. 1. Upper molars and premolars of Kenyapotamus coryndonae Pickford from Nakali, Kenya (Miocene). A, NA12- 12276’06, a right P2: A1, occlusal view (stereo pair); A2, buccal view; A3, lingual view. B, KNM-NA 45747, a left maxillary fragment with P4: B1, occlusal view (stereo pair); B2, buccal view; B3, lingual view; B4, mesial view; B5, distal view. C, KNM-NA 45748, a left M3: C1, occlusal view (stereo pair); C2, buccal view. D, KNM-NA 52678, a distal part of right up- per molar, occlusal view (stereo pair). E, KNM-NA 47535, a right M3: E1, occlusal view (stereo pair); E2, buccal view. 156 T. Tsubamoto et al. 2015―4

Fig. 2. Lower molars and premolars of Kenyapotamus coryndonae Pickford from Nakali, Kenya (Miocene). A, KNM-NA 45751, a broken left p3, occlusal view (stereo pair). B, KNM-NA 45754A, a broken left p3: B1, occlusal view (stereo pair); B2, buccal view; B3, lingual view. C, KNM-NA 45746, a right mandibular fragment with p4: C1, occlusal view (stereo pair); C2, lingual view; C3, buccal view. D, KNM-NA 45752, a right p4: D1, occlusal view (stereo pair); D2, lingual view; D3, buc- cal view. E, KNM-NA 45749, a left m3: E1, occlusal view (stereo pair); E2, lingual view; E3, buccal view. F, KNM-NA 45756, a trigonid of right lower molar: F1, occlusal view (stereo pair); F2, distal view. G, KNM-NA 45735A, a broken tal- onid of left m3, occlusal view. H, KNM-NA 47284, a broken talonid of right m3, occlusal view. Jour. Geol. Soc. Japan 121（ 4 ） Kenyapotamus from Nakali, Kenya 157

Fig. 3. Astragali and a tusk of Kenyapotamus coryndonae Pickford from Nakali, Kenya (Miocene). A, NA11-12238’06, a left astragalus: A1, dorsal (anterior) view; A2, plantar (ventral) view; A3, medial view; A4, lateral view; A5, proximal view; A6, distal view. B, KNM-NA 52680A, a right astragalus: B1, dorsal (anterior) view; B2, plantar (ventral) view. C, KNM-NA 47290, a fragmentary tusk: C1, buccal view; C2, lingual view; C3, lateral view; C4, cross-sectional view.

The lower premolar specimens, broken p3 (KNM-NA and 47284; Fig. 2E, 2G, 2H) are comparable in size and 45751 and 45754A; Fig. 2A–2B) and p4 (KNM-NA morphology to the previously reported m3 specimens of 45746 and 45752; Fig. 2C–2D), are comparable in size K. coryndonae (Pickford, 1983; Nakaya et al., 1987). and morphology to the previously reported p3 and p4, There is an ectohypocristulid. These specimens differ respectively, of K. coryndonae (Table 1; Pickford, 1983; from m3 of P. ternani in that the postectohypocristid is Tsujikawa, 2005). On the p4 specimen, there is a meta- very weak or cannot be observed (Boisserie at al., conid on the distolingual face of the protoconid, and a 2010). hypoconid, an entoconid, and a smaller distostylid are In the molar trigonid specimen (KNM-NA 45756; present on the talonid. The metaconid on the p3 speci- Fig. 2F), the basal part of the trigonid crown is some- men is smaller and lower than that on p4. The talonid on what broken. In distal view (Fig. 2F2), a well-developed p3 is buccolingually narrower than that on p4. The tal- M-structure on the distal trigonid wall is visible, con- onid cusps on p3 are less developed than those on p4. sisting of the postectoprotocristid, postprotocristid, The three m3 specimens (KNM-NA 45749, 45735A, postmetacristid, and postectometacristid. 158 T. Tsubamoto et al. 2015―4

Table 1. Dental measurements (in mm) of Kenyapotamus coryndonae Pickford from Nakali, Kenya (Miocene). The ranges of the previously reported specimens are from Pickford (1983, 1990), Nakaya et al. (1987), Tsujikawa (2005), and Boisserie et al. (2010).

The astragalar specimens (NA11-12238’06 and KNM- 2005), as in p4. NA 52680A; Fig. 3A–3B) are comparable in size and On a present m3 specimen (KNM-NA 45749; Fig. morphology to the previously reported astragali of K. 2E), there are short but prominent cingular shelves on coryndonae (Pickford, 1983; Nakaya et al., 1984; Bois- the distolingual and distobuccal aspects of the hypocon- serie et al., 2010). The cuboid facet on the distal troch- ulid and several prominent small tubercles between the lea is as wide as the navicular facet. The keel between hypoconulid and both of the entoconid and hypoconid. the two facets on the distal trochlea is centered medio- These characteristics are also seen in KNM-NA 250C laterally. (m3 of K. cf. coryndonae; Boisserie et al., 2010). In The tusk (canine) (KNM-NA 47290; Fig. 3C) is bro- contrast, these characteristics are not developed on the ken. Its cross-section is ovate triangular (or D-shaped), other m3 specimens of K. coryndonae (Fig. 2G–2H). In like the previously reported root of the lower canine of addition, KNM-NA 45749 (Fig. 2E) and the present K. coryndonae (Tsujikawa, 2005). This tusk differs from molar trigonid (KNM-NA 45756; Fig. 2F) differ from that of the suids in having thicker enamel. other lower molar specimens of K. coryndonae in hav- Remarks.―The present specimens have following ing a weaker mesial cingulum. features compared to the previously described speci- mens of K. coryndonae, increasing the known morpho- Acknowledgments logical variation of the species (Pickford, 1983, 1990; We are grateful to the Government of Kenya for re- Nakaya et al., 1987; Tsujikawa, 2005; Boisserie et al., search permission and to Martin Pickford (Muséum Na- 2010). tional d'Histoire Naturelle, Paris) for review. We also On the present M3 specimens, KNM-NA 45748 and thank Fredrick Kyalo Manthi, Mary Muungu, and Fran- 47535 (Fig. 1C, 1E), the lingual cingulum of the proto- cis Ndiritu (National Museums of Kenya, Nairobi) for cone is slightly discontinuous on the lingual aspect of graciously providing access to the specimens examined. the cusp, whereas it is continuous in the previously re- This research was supported by JSPS KAKENHI Grant ported M3s of K. coryndonae. Also, on KNM-NA Number 25257408 (to M. Nakatsukasa). 45748, the ectostyle, entostyle, and paraconule are de- veloped better than those of any other M3s of K. coryn- References donae. In buccal view, the crown height of KNM-NA Boisserie, J.-R., Lihoreau, F., Orliac, M., Fisher, R. E., Weston, 45748 is relatively higher than other M3s of K. coryn- E. M. and Ducrocq, S., 2010, Morphology and phylogenetic relationships of the earliest known hippopotamids (Cetar- donae and is slightly lower than M3 of K. aff. coryndo- tiodactyla, Hippopotamidae, Kenyapotaminae). Zool. Jour. nae (KNM-NA 251; Boisserie et al., 2010). Linnean Soc., 158, 325–366. Gray, J. E., 1821, On the natural arrangement of vertebrose ani- On p3, the expression of the postprotocristid is vari- mals. London Med. Reposit., 15, 296–310. able (Fig. 2A–2B). The postprotocristid is shorter and Handa, N., Nakatsukasa, M., Kunimatsu, Y., Tsubamoto, T. and disappears midway before it reaches the hypoconid in Nakaya, H., in press, New specimens of Chilotheridium (Perissodactyla, Rhinocerotidae) from the Upper Miocene KNM-NA 45754A (Fig. 2B), whereas it is longer and Namurungule and Nakali Formations, northern Kenya. Pa- extends down to the small hypoconid in KNM-NA leontol. Res., doi: 10.2517/2014PR035. 45751 (Fig. 2A) and KNM-SH 40142 (Tsujikawa, Kunimatsu, Y., Nakatsukasa, M., Sawada, Y., Sakai, T., Hyodo, M., Hyodo, H., Itaya, T., Nakaya, H., Saegusa, H., Mazuri- Jour. Geol. Soc. Japan 121（ 4 ） Kenyapotamus from Nakali, Kenya 159

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