Monte L. McCrossin

On the Relationships and Adaptations of Kenyapithecus, a Large-Bodied Hominoid from the Middle Miocene of Eastern Africa

The phylogenetic relationships and adaptations of Kenyapithecus have been of special interest since Leakey (1962, p. 696) first described the genus as possessing a number of characters exhibiting “a marked tendency in the direction of the Hominidae.” Expectations regarding the hominid affinities of Kenyapithecus influenced the reconstruction of many functionally and phylogenetically significant aspects of its anatomy in the absence of tangible fossil evidence. The type species Kenyapithecus wickeri is represented by four jaw fragments, 11 isolated teeth, and a distal humerus collected from a single site, Fort Ternan (Pickford, 1985). As of 1985, the hypodigm of the referred species Kenyapithecus africanus consisted of the type maxilla together with 46 isolated teeth, four incomplete postcranial pieces, and a poorly preserved mandible collected from the Maboko Formation and an isolated lower molar from Nyakach (Pickford, 1985). Features for which little or no fossil evidence existed, but which Kenyapithecus was said to share with hominids, include: small lower incisors relative to cheek tooth size, reduced incisor procumbency, arcuate dental arcade, short rostrum, and a humerus that is longer than the femur (Simons and Pilbeam, 1972). The supposed reduction in upper canine and lower incisor size, and facial abbreviation, in combination with thickenameled molars in Kenyapithecus was interpreted as being related to an australopithecine-like emphasis on molar grinding resulting from the consumption of hard objects (Andrews and Walker, 1976), with incisors being “relatively unimportant in food preparation” (Simons and Pilbeam, 1978, pp. 149, 152).

Fossil Evidence for the Origins of Terrestriality among Old World Higher Primates

Preference for terrestrial substrates is one of the most significant adaptive differences between some members of the radiation of Old World higher primates and the anthropoids of the Neotropics (Le Gros Clark, 1959; Napier and Napier, 1967, 1985; Fleagle, 1988; Martin, 1990). Adaptations for terrestriality are most conspicuous among savanna baboons (Papio — Rose, 1977), geladas (Theropithecus — Jolly, 1967; Dunbar and Dunbar, 1974), and humans (Napier, 1967). Varying degrees of semi-terrestriality and terrestriality are also present among the African great apes (Gorilla — Remis, 1995 and Pan — Hunt, 1992; Doran, 1993) and some of the Asian colobines (Presbytis entellus — Ripley, 1967 and Rhinopithecus roxellana — Davison, 1982), guenons (Cercopithecus aethiops and Erythrocebus patas — Hall, 1965), mandrills and drills (Mandrillus — Jouventin, 1975), mangabeys (Cercocebus — Waser, 1984), and macaques (e.g., Macaca nemestrina — Caldicott, 1986). In contrast, terrestrial adaptations are notably absent from the otherwise diverse adaptive array of New World anthropoids.

Earliest known Old World monkey skull

Similarities of the skull are commonly used to support hypotheses of ancestor–descendant relationships between fossil and living ape genera, especially between the late Miocene apes Sivapithecus and Dryopithecus from Eurasia and the living orang-utan (Pongo) from Borneo and Sumatra. Yet determining whether craniofacial traits shared by extant and Miocene apes are primitive or derived is severely hampered by the rarity of well-preserved fossil crania, particularly of early members of their closest outgroup, the Old World monkeys (Cercopithecoidea). The discovery of a complete and undistorted skull of Victoriapithecus at middle Miocene deposits from Maboko Island, Kenya, provides evidence of intact cranial-vault and basicranial morphology, brain size and craniofacial hafting for a primate from between 32 and 7 million years ago. Victoriapithecus represents a branch of Old World monkey that is intermediate between extant cercopithecids (Colobinae and Cercopithecinae) and the common ancestor they shared with apes (Hominoidea). The skull preserves traits widely thought to be derived for extant and fossil members of a proposed Sivapithecus/Pongo clade, but which now appear to be primitive features of ancestral Old World higher primates in general.

Evolution of locomotion in Anthropoidea: the semicircular canal evidence

Our understanding of locomotor evolution in anthropoid primates has been limited to those taxa for which good postcranial fossil material and appropriate modern analogues are available. We report the results of an analysis of semicircular canal size variation in 16 fossil anthropoid species dating from the Late Eocene to the Late Miocene, and use these data to reconstruct evolutionary changes in locomotor adaptations in anthropoid primates over the last 35 Ma. Phylogenetically informed regression analyses of semicircular canal size reveal three important aspects of anthropoid locomotor evolution: (i) the earliest anthropoid primates engaged in relatively slow locomotor behaviours, suggesting that this was the basal anthropoid pattern; (ii) platyrrhines from the Miocene of South America were relatively agile compared with earlier anthropoids; and (iii) while the last common ancestor of cercopithecoids and hominoids likely was relatively slow like earlier stem catarrhines, the results suggest that the basal crown catarrhine may have been a relatively agile animal. The latter scenario would indicate that hominoids of the later Miocene secondarily derived their relatively slow locomotor repertoires.

Systematics of early and middle Miocene Old World monkeys

New information about the early cercopithecoids Prohylobates tandyi (Wadi Moghra, Egypt) and Prohylobates sp. indet. (Buluk and Nabwal, Kenya) is presented. Comparisons are made among all major collections of Early and Middle Miocene catarrhine monkeys, and a systematic revision of the early Old World monkeys is provided. Previous work involving the systematics of early Old World monkeys (Victoriapithecidae; Cercopithecoidea) has been hampered by a number of factors, including the poor preservation of Prohylobates material from North Africa and lack of comparable anatomical parts across collections. However, it is now shown that basal cercopithecoid species from both northern and eastern Africa can be distinguished from one another on the basis of degree of lower molar bilophodonty, relative lower molar size, occlusal details, symphyseal construction, and mandibular shape. Results of particular interest include: 1) the first identification of features that unambiguously define Prohylobates relative to Victoriapithecus; 2) confirmation that P. tandyi is incompletely bilophodont; and 3) recognition of additional victoriapithecid species.

Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys

Analysis of the only complete early cercopithecoid (Old World monkey) endocast currently known, that of 15-million-year (Myr)-old Victoriapithecus, reveals an unexpectedly small endocranial volume (ECV) relative to body size and a large olfactory bulb volume relative to ECV, similar to extant lemurs and Oligocene anthropoids. However, the Victoriapithecus brain has principal and arcuate sulci of the frontal lobe not seen in the stem catarrhine Aegyptopithecus, as well as a distinctive cercopithecoid pattern of gyrification, indicating that cerebral complexity preceded encephalization in cercopithecoids. Since larger ECVs, expanded frontal lobes, and reduced olfactory bulbs are already present in the 17- to 18-Myr-old ape Proconsul these features evolved independently in hominoids (apes) and cercopithecoids and much earlier in the former. Moreover, the order of encephalization and brain reorganization was apparently different in hominoids and cercopithecoids, showing that brain size and cerebral organization evolve independently.

A New Rhinoceros, Victoriaceros kenyensis gen. et sp. nov., and Other Perissodactyla from the Middle Miocene of Maboko, Kenya

The middle Miocene site of Maboko (Lake Victoria, Kenya), dated to ca. 15xa0Ma, has yielded one of the best collection of rhinos in Africa. The most common taxon, Victoriaceros kenyensis n.gen., n.sp., is represented by an almost perfect skull (whose main features are the large nasal horn, an orbit located very anteriorly and with a prominent border, and very broad zygomatic arches) and numerous limb bones, probably belonging to only a few individuals. Characters of the teeth and skull support an assignment to the subfamily Elasmotheriinae, a group best known in the middle and upper Miocene, but whose monophyly is disputable, as some of their tooth characters could be adaptations to a grazing diet (in agreement with their distribution in the Maboko beds). In any case, Victoriaceros clearly differs from other East African middle Miocene rhinos, whose diversity is far greater than currently assumed. A few other specimens attest to the occurrence at Maboko of at least one other species, perhaps close to the brachypotheres; a single calcaneum is tentatively assigned to the Chalicotheriidae.

Terrestriality in a Middle Miocene Context: Victoriapithecus from Maboko, Kenya

As possibly the earliest higher primate to eschew the trees and descend to the ground, Victoriapithecus offers several important lessons that may have implications for understanding our own ancestry. First, while paleoenvironment obviously constrains paleoecology, it should not be equated with a species’ paleoecology. Although there is a definite correlate between substrate preference and locomotor mode, evidence from Victoriapithecus suggests there is no a priori relationship between locomotor mode and paleoenvironment; that is, wooded or forested environs do not preclude terrestrial behaviors.

A window into ape evolution

A fossil of an ape ancestor helps to explain gibbon evolution [Also see Report by Alba et al.] Humans, Old World monkeys, gibbons, and the great apes of Africa and Asia are the only survivors of a highly diverse evolutionary radiation that began at least 28 million years ago (1, 2). The fossil record of human evolution after we diverged from apes is rich, but much less is known about the evolutionary history of modern apes. Not only is the fossil record incomplete but also the morphology of primitive apes from the Miocene (25 to 5 million years ago) seldom conforms to expectations based on living species. The ancestors of gibbons are particularly elusive. On page 528 of this issue, Alba et al. describe a Miocene fossil from Catalonia, Spain, that may bridge the gap between earlier small-bodied African apelike primates and living gibbons (3).