Anthropol. Sci. 105(2), 117-141, 1997 New Species of Nyanzapithecus from Nachola, Northern Kenya YUTAKA KUNIMATSU Primate Research Institute, Kyoto University, Kanrin, Inuyama-shi, Aichi-ken, 484 Japan Received April 11, 1997 Abstract Nyanzapithecus is a genus of East African Miocene apes, which has been suggested to be an ancestral stock of oreopithecids. A new species of this genus, Nyanzapithecus harrisoni, is described in this article. The description is based on the material discovered from Nachola, a middle Miocene fossil locality (ca. 15 m.y.a.) in northern Kenya. N. harrisoni is slightly smaller than the previously known N. pickfordi and N. vancouveringorum, but shares with them such unique dental characters as elongated molar crowns, quite inflated cusps, and restricted occlusal basins and foveae. The presence of the new Nyanzapithecus species in Nachola has expanded the distribution and diversity of this genus, and suggests that oreopithecids may have been a relatively common element in the middle Miocene primate fauna of East Africa. Key Words: hominoids, oreopithecids, Nyanzapithecus, East Africa, Miocene INTRODUCTION The field research by the Japan-Kenya Joint Project has discovered hundreds of fossil primates from Nachola, a middle Miocene fossil locality in northern Kenya. The primate sample mainly consists of middle-sized hominoid fossils assigned to Kenyapithecus (Ishida et al., 1984), but it also contains about twenty specimens of a small Miocene hominoid (Kunimatsu, 1992a). Although the small hominoid sample from Nachola only contains isolated teeth and a mandibular fragment, the dental morphology indicates that it belongs to Nyanzapithecus (Kunimatsu, 1992a). The Nachola sample, however, can be distinguished in size and some morphological features from the previous species of Nyanzapithecus known from western Kenya, and deserves to be separated from them as a new species. The present study aims to describe the morphology of the new species in details, with comparisons to the other two species of Nyanzapithecus. Nyanzapithecus is a unique Miocene ape discovered from East Africa. The genus was proposed by Harrison (1986a), when he described the sample of a small catarrhine primate from the middle Miocene deposits in Maboko Island, western Kenya. Harrison named the Maboko sample as N. pickfordi, and he also transferred to the new genus a smaller species of Rangwapithecus, which had been known previously as Rangwapithecus vancouveringorum from the early Miocene localities in western Kenya. (Andrews, 1974, 1978). 118 Y. KUNIMATSU The presence of Nyanzapithecus in the early to middle Miocene of Kenya is quite interesting, because the dental morphology suggests that Nyanzapithecus is likely to be an ancestral stock of Oreopithecus, a unique and enigmatic fossil primate discovered from the late Miocene of Italy (Harrison, 1986a). Fragmentary evidence suggests that some other kinds of oreopithecids may have existed in the East African Miocene (Koenigswald, 1969; Harrison, 1986a, 1992). The recognition of a new species of Nyanzapithecus in the Nachola sample further extends the geographical distribution and the diversity of African oreopithecids. The origin of oreopithecids, therefore, may be traced back to the early/middle Miocene East Africa. It is important to study the African oreopithecids in understanding the origin of the modern hominoids, because the postcranial evidence of Oreopithecus suggests that it is in close relationship with the modern hominoids (Harrison, 1986b; Sarmiento, 1987). Unfortunately, the postcranium of Nyanzapithecus is only poorly known (McCrossin, 1992), and the oreopithecid status of Nyanzapithecus is a tentative hypothesis at present. Geological background Nachola is located on the eastern side of the Suguta Valley (=a part of the Eastern Rift Valley), ca. 350 km north of Nairobi. In Nachola, the Miocene series unconform- ably overlays the Precambrian basement. This series consists of volcanic and sedimen- tary rocks, and is divided into the Aka Aiteputh and Nachola Formations. The fossil primates were discovered from the Aka Aiteputh Formation along with abundant petrified wood (Sawada et al., in press). The fossil bearing sediments are sandwiched by volcanic rocks, and the age of the primate fossils was estimated by K-Ar method as 13 to 15 m.y.a. (Itaya and Sawada, in press). An analysis of the mammal- ian fauna suggests that the age might be slightly older (Pickford and Kuga, in press). Material All specimens of Nyanzapithecus in the present study were discovered from the site BG-X in Nachola during the field seasons of 1984 and 1986. Among the fossil sites in Nachola, the site BG-X is the most productive with plenty of primate fossils. From the site BG-X and some other sites in Nachola, a number of fossils of a middle-sized hominoid, which are tentatively assigned to Kenyapithecus sp., have been found (Ishida et al., 1984). However, Nyanzapithecus is a relatively rare element in the primate fauna of Nachola. SYSTEMATICS Order: Primates Linnaeus, 1758 Suborder: Anthropoidea Mivart, 1864 Infraorder: Catarrhini Geoffroy, 1812 Superfamily: Hominoidea Gray, 1825 New Species of Nyanzapithecus from Nachola, Northern Kenya 119 Family: Oreopithecidae Schwalbe, 1915 Genus: Nyanzapithecus Harrison, 1986a Species: Nyanzapithecus harrisoni sp. nov. Diagnosis A species of Nyanzapithecus which differs from N. vancouveringorum in the following features: smaller size (the average size of upper M2 being ca . 83% of that in N. vancouveringorum); upper molar higher-crowned; molar cusps more inflated and higher; occlusal basins and foveae more restricted; occlusal outline of the upper molars tending to taper distally; lingual cingulum of upper molars more distinct and wider and slightly approaching toward the cervix lingually; upper M3 crown much shorter; lower molar relatively short (especially M3); lower P4 more elongated; mandible more gracile . N. harrisoni differs from N. pickfordi in the following features: smaller size (the average size of the upper M2 being ca. 75% of that in N. pickfordi); upper and lower molar crowns much less elongated; lingual cingulum of the upper molars with less developed mesial part and better-developed lingual part, slightly approaching toward the cervix lingually; hypocone linked to the protocone instead of the crista obliqua; less waisted upper molar crown; hypoconulid on lower M3 tending to be located more medially; lower P3 having a weak but continuous buccal cingulum . Holotype KNM-BG15237: an isolated right upper M2 crown . The holotype and other specimens in the hypodigm are stored in the National Museums of Kenya (KNM) in Nairobi. The prefix BG indicates that the specimen comes from Nachola near Baragoi . Type Locality BG-X, Nachola, near Baragoi, northern Kenya Distribution Middle Miocene of Nachola (13-15 Ma) Hypodigm Twenty three isolated teeth and one mandibular fragment from Nachola are assigned to Nyanzapithecus harrisoni (Table 1). Detailed descriptions of the specimens except for a lower P4 (KNM-BG17862) were given previously in Kunimatsu (1992a) . The dental material includes upper and lower molars and lower third and fourth premolars . Etymology After Terry Harrison, who contributed much to the study of the East African Miocene primates. 120 Y. KUNIMATSU Table 1. Hypodigm of Nyanzapithecus harrisoni DESCRIPTION Mandible The mandibular fragment preserves part of the right corpus anterior to the P4/M1 boundary and the symphyseal region up to the level of the left canine. The crowns of the teeth are missing except for the right lower P3. The roots of the incisors, canines and right P4 remain in the alveoli. The distal half of the left canine root is exposed. A crack runs superoinferiorly in the mandibular body between the right canine and P3. The surface bone is missing from the buccal aspect at the level of the right canine. The labial surface of the symphysis is depressed by the postmortem deformation. The inferior border of the symphysis is slightly broken, but it appears that the inferior transverse torus is absent or very poorly developed. The superior transverse torus is developed about one third midway up from the inferior border, and extends posteriorly to the level of P3. The mandible is relatively gracile. New Species of Nyanzapithecus from Nachola, Northern Kenya 121 Upper Molars The upper Ml has a rounded occlusal outline. The preservation of the specimens is not good, as both specimens are weathered and/or lacking enamel. The crown is slightly longer than broad with the average length/breadth index of 106.0% (Table 1 and Fig. 1). The cusps are rounded cones of moderate height. The degree of cusp inflation seems weaker than in the upper M2, but this may possibly be a result of damage. The paracone, metacone and hypocone are similar in size, and the protocone is slightly larger. The distal cingulum adjacent to the hypocone is slightly swollen, contributing to the restriction of the talon basin. The occlusal crests are low, rounded, and poorly- developed. The mesial crest of the protocone runs mesiobuccally and ends at the slightly developed protoconule. The lingual cingulum is well-developed, and forms a broad and continuous C-shaped shelf surrounding the mesial and lingual aspects of the protocone. This shelf starts from the protoconule, and runs slightly up toward the cervix to end at the mesial aspect of the hypocone. The upper M2 is as long as broad or only slightly broarder than long (Table 2 and Fig. 2). The length/breadth index ranges between 97.1 and 100.0% with the average being 98.2%. The occlusal outline is more squared than in Ml, and the distal moiety is slightly narrower than the mesial moiety (Fig. 4). The enamel surface is smooth. The occlusal crests are low, rounded, and poorly-developed as in Ml. The cusps are inflated so that the trigon and talon basins are restricted. The mesial fovea is short and narrow, and is restricted by the voluminous paracone, well-developed protoconule and inflated crests. The hypocone is separated from the metacone by a distinct groove. There is no distal transverse crest between these two cusps.