Postcranial bones of infant Nacholapithecus: ontogeny and positional behavioral adaptation

Abstract

We describe the postcranial bones of a partial skeleton of a male infant Nacholapithecus kerioi (KNM-BG 37800) from the Middle Miocene of Kenya. The skeletal elements are associated with dental remains and its age is presumed to be 6–15 months based on a comparison with an infant Proconsul heseloni with a similar dental growth stage. The postcranial elements include the left scapula, a lumbar vertebra, left distal humerus, right proximal ulna, right radius, right and left proximal femora, a proximal fragment of metatarsal, and proximal and intermediate phalanges. Its body mass was estimated as 3.0 kg from shaft dimensions of the humerus and femur. At this early postnatal developmental stage, N. kerioi can be distinguished from P. heseloni in several postcranial features: a caudally projecting lumbar spinous process, strong dorsal divergence of the scapular spine, a coronoid fossa which is more developed than the radial fossa, higher neck shaft angle of the femur, taller trochlea of phalanges, and a lower brachial index. These features, except for the last, agree with the previously proposed positional behavioral reconstruction of N. kerioi, in which vertical climbing, orthograde clambering, transferring, or bridging are relatively important compared with P. heseloni. The apparently low brachial index results from a comparatively short radius for the predicted body mass. This might imply a relatively slow pace of growth in the radius of N. kerioi. However, it might result from body mass overestimation, incorrect length reconstruction, or individual variation. Its interpretation must await further specimens.