Kinematics of the trapezium-1st metacarpal joint in extant anthropoids and Miocene hominoids

Abstract

In his many papers on the functional morphology of the primate hand John Napier paid particular attention to the thumb and its kinematics. He suggested that in most New World monkeys the trapezium-1st metacarpal joint (t-lmcj) is almost cylindrical in form, allowing flexion-extension, limited abduction-adduction, and no rotation. He referred to this joint type as a modified hinge, allowing pseudo-opposition of the thumb. By contrast, the t-lmcj joint is sellar in catrrrhines, allowing all three of these movements to be combined in the movement of opposition. On the evidence of the trapezium, Napier & Davis (1959) considered Proconsul africanus to have a t-lmcj of the modified hinge type. These views were challenged by Lewis (1977), who pointed out that a sellariform t-lmcj is a primitive feature in mammals, although it is modified from this shape in some New World monkeys. Lewis (1977) also believed that the trapezium of P. africanus does in fact bear a sellar articular surface. Beard et al. (1986) observations on recently collected trapezia of P.africanus confirm Lewis' interpretation. Currently there is a considerably larger sample of fossil material than that available to Napier & Davis. A consideration of the functional morphology and kinematics of the t-lmcj in extant anthropoids, Proconsul spp. and Afropithecus turkanensis, suggests that the sellariform nature of the t-lmcj in these taxa is emphasized enough to result in abduction-adduction and rotatory movements as extensive as those typical of extant large hominoids. This contrasts with the more limited movements found at the t-lmcj of other extant anthropoids. This similarity of the t-lmcj and its kinematics in large hominoids, and the Miocene species is most likely to be a shared derived character.