The Carpus of Proconsul Africanus: Functional Analysis and Comparison with Selected Nonhuman Primates.

Abstract

Proconsul africanus is part of the adaptive radiation of early Miocene hominoids that is dentally similar to modern apes. This study examines reconstructed h and posture and forelimb function in this species, based on the carpus of a fairly complete subadult, KNM-RU 2036. Among extant anthropoids, grasping effectiveness is associated with antipronograde behavior. Effective grasping is dependent on the ability to position the h and so that the axis of the postaxial digits is oblique to the axis of the support. Increased postural flexion, ulnar deviation, and supination increase grasping effectivenss by positioning the h and more obliquely to the axis of the support. Analysis of h and posture and function in relation to different modes of locomotion indicates that quadrupedal walking places constraints on h and posture. H and posture must be relatively prone and dorsiflexed. The other modes also place constraints on h and posture and function. Based on the positional behavior of modern primates and the estimated body weight of KNM-RU 2036, one would expect to find evidence of a relative decrease in the importance of quadrupedal walking, as defined in this study, and evidence of at least a moderately effective grasp in P. africanus. Analysis of the components of h and posture in P. africanus indicates that h and posture was dorsiflexed and moderately supinated. There is evidence for increased active flexion and increased rotary mobility at the distal radioulnar joint compared to most modern monkeys; there is also evidence for a grasp in which the pollex opposed the other digits. Although P. africanus was quadrupedal, the reconstructed h and posture is inconsistent with quadrupedal walking. It is also inconsistent with the habitually suspended forelimb postures used during brachiation and suspensory climbing. Reconstructed h and posture suggests a moderately effective grasp; one would expect to find relatively more antipronograde behavior in this species than among similar-sized anthropoids with a less effective grasp. The pattern of postural components and active behavior are most similar to animals that could be described as "active climbers". Climbing, although not suspensory climbing, in addition to quadrupedal positional behavior, was a relatively important mode of locomotion in this species.