Morphometric analysis of lumbar vertebra UMP 67-28: Implications for spinal function and phylogeny of the Miocene Moroto hominoid

Fossil hominoid remains from the Miocene site of Moroto II include a well preserved lumbar vertebra (UMP 67-28). This vertebra is associated with a palate that has been included in the hominoid tribe Afropithecini and referred by some to Afropithecus. UMP 67-28 originally was described as having close morphological affinities with lumbar vertebrae from African apes and humans (Walker & Rose, 1968). The present analysis, however, indicates that UMP 67-28 shares no exclusive proportional or structural similarities with lumbar vertebrae from any particular extant catarrhine. The Moroto vertebra is the size of lumbar elements from female chimpanzees and orang-utans or large male cercopithecoids. Regression analysis of vertebral body surface area suggests that the individual represented by UMP 67-28 had a body weight of approximately 38 kg. UMP 67-28 is similar to large-bodied hominoids in general, in position and orientation of its transverse process, absence of anapophyses, inclination of its neural spine, and pedicular shape, marking the earliest appearance in the catarrhine fossil record of lumbar morphology resembling that of modern hominoids. Conversely, vertebral body proportions in UMP 67-28 most closely resemble those in male baboons, and in other traits the specimen cannot be differentiated between hominoids and cercopithecoids. The overall morphology of UMP 67-28 indicates that lumbar vertebrae of the Moroto hominoid were mole derived toward the great ape condition than those of Proconsul heseloni and P. nyanzae. In contrast to Proconsul, UMP 67-28 shares features with other dorsostable-backed mammals, suggesting that the Moroto hominoid and Proconsul possessed very different locomotor capabilities. Dental traits linking the Moroto hominoid with other afropithecins are thought to correspond functionally to a fundamental shift in diet, relative to the primitive catarrhine condition. It is possible that evolution of the lumbar region of early hominoids toward the morphotype of extant large-bodied hominoids, evidenced in UMP 67-28, was adaptively driven by modifications in substrate use coincident with changes in resource acquisition.