Elbow anatomy of fossil cercopithecids from Nakali, Kenya: Functional anatomy and taxonomy

The locomotor behavior and substrate preferences of early colobines are a matter of debate. Some researchers advocate a partly terrestrial origin with the subsequent parallel evolution of arboreality in different clades during the Plio-Pleistocene, while others suggest that arboreality had already evolved earlier in the Miocene. Microcolobus, from the Late Miocene of Nakali (ca. 10 Ma, Kenya), is one of the earliest known and, for the most part, best-documented African Miocene fossil colobines. A large collection of postcranial remains was recovered at Nakali during the fieldwork of the Joint Japan-Kenya Paleontological Expedition, permitting us to test hypotheses regarding the locomotor behavior, substrate preferences, and taxonomy of the Nakali colobines. Here, we present novel interpretations of the functional adaptations of the elbow anatomy of previously published partial skeletons from Nakali (KNM-NA 47916 and KNM-NA 47915) and the description of 12 additional humeral, ulnar, and radial specimens that we attributed to Microcolobus. The elbow specimens from Nakali are morphologically homogenous except one humeral specimen that demonstrates equivocal evidence of functional and taxonomical diversity. Quantitative and qualitative analyses confirm arboreal adaptations of Microcolobus, corroborating hypotheses supporting early adaptations for arboreal locomotor substrate preferences in colobines. The elbow anatomy of Microcolobus can be qualitatively and quantitatively distinguished from that of the Late Miocene colobines Cercopithecoides bruneti and Paracolobus enkorikae. Microcolobus also present several anatomical features seen in the small African colobine Procolobus verus, reflecting a frequently flexed elbow with moderate pronosupination abilities and a stable humeroulnar joint consistent with the use of flexed-elbow climbing and arboreal quadrupedal running.