Abstract

INTRODUCTION The fossil tibia StW 514a from the Sterkfontein Formation, Gauteng, South Africa, is argued to possess ape-like proximal tibial morphology (Berger & Tobias 1996). The specimen is regarded as belonging to an australopith from Member 4, which has been estimated to be 2.4–2.8 Ma (Vrba 1985; Delson 1988; Partridge & Watt 1991; McKee et al. 1995; Kuman & Clarke 2000; Pickering et al. 2004). Berger et al. (2002), in a revision of the Australopithecus-bearing deposits of Sterkfontein, interpreted Member 4 more likely to fall between 1.5–2.5 Ma. These revised age estimates have been refuted by Clarke (2002). StW 514a has a close association with large numbers of cranio-dental remains attributed to australopiths (MoggiCecchi et al. 2006), suggesting a high probability that the tibia should be assigned to this taxonomic group, and most probably to the species Australopithecus africanus (Berger & Tobias 1996). More recently we have analysed another proximal tibial fragment from Member 4 (StW 396) and we find that, in certain functionally important features, this specimen differs from StW 514a. Berger & Tobias (1996) described the convex anteroposterior curvature of the lateral tibial condyle of StW 514a as possibly being associated with a more chimpanzee-like locomotor function when compared to the flatter lateral tibial condyle of A. afarensis. Latimer et al. (1987) have observed that joints become flatter as size increases as a result of a response to increased transarticular loads accompanied by larger body mass. They suggest that variation observed among hominins, both within and between species, reflects size alone rather than function (Latimer et al. 1987). Recently, Organ & Ward (2006) in a detailed study, compared the three-dimensional surface areas of the lateral tibial condyle of humans and apes with those of A. afarensis (A.L. 129b, A.L. 288-1aq, A.L. 333–42, A.L. 333X-26) and A. africanus (StW 514a). Their results neither support the hypotheses that A. africanus and A. afarensis differ in condylar topology or that joint surfaces become flatter with increased transarticular load accompanying increased body size. Organ & Wards’ (2006) arguments are compelling and their methods would have been useful in solving the problem of the StW 396 proximal tibial fragment if a lateral condyle was present. However, the remainder of this specimen appears to vary enough from StW 514a to justify further consideration. Following is a description of StW 396 and a comparison of its morphology with that of StW 514a that highlights the morphological differences between these two specimens. The implications of the differences are discussed, with special reference to the claim of Clarke (1985, 1994, 2009) that two different hominin taxa may possibly be represented in Sterkfontein Member 4.