Hind limb of the Nacholapithecus kerioi holotype and implications for its positional behavior

Abstract

We present the definitive description of the hind limb elements of the Nacholapithecus kerioi holotype (KNM-BG 35250) from the middle Miocene of Kenya. Previously, it has been noted that the postcranial (i.e. the phalanges, spine, and shoulder girdle) anatomy of N. kerioi shows greater affinity to other early/middle Miocene African hominoids, collectively called ‘non-specialized pronograde arboreal quadrupeds, ’ than to extant hominoids. This was also the case for the hind limb. However, N. kerioi exhibits a unique combination of postcranial characters that distinguish this species from other early/middle Miocene African hominoids. The femoral neck has a high angle, but is relatively short, though the adaptive meaning of this form is not readily understood. In the distal femur, the shape of the patellar surface and symmetry of the femoral condylar widths suggest that the knee was not typically abducted but assumed more variable movements and/or postures. Whereas the morphologies of the talocrural and intertarsal joints are generally similar to those of the other fossil hominoids, the tibial malleolus is extremely thick and asymmetry of the talar trochlea groove is more emphasized due to a more prominent lateral trochlear rim than in Proconsul and other African fossil hominoids. The distal foot segment is more elongated. The (non-hallucal) metatarsals appear relatively gracile due to the elongation. The proximal joints of these metatarsals are, nonetheless, large. Ligamentous attachments of the tarsal/metatarsal bones are generally well developed. The distal tarsal row, which is represented only by the medial cuneiform, though, is extremely large for the presumed body mass. In terms of function, the femur, ankle, and tarsal joints are interpreted behaviorally to represent a slow-moving arboreal quadruped. However, the foot of N. kerioi appears to be more specialized for inverted grasping and subvertical support use. All of these foot features are suggestive of a greater role for antiprono-grade activities in N. kerioi relative to other Miocene ‘pronograde arboreal quadrupeds.