Revisiting the peroneal trochlea of the StW 352 calcaneus

Abstract

StW 352, from Sterkfontein Member 4 (South Africa), is a partial calcaneus attributed to Australopithecus africanus and is dated to ~2.0–2.6 Ma. The unusual robusticity of the peroneal trochlea (PT) of StW 352 has been commented on by several authors. The size of hominin PTs has been hypothesised to be positively correlated with the degree of recruitment of peroneus longus during bipedal locomotion and/ or climbing. Given the potential functional relevance of an enlarged PT for reconstructing hominin activity patterns, we present the following previously unrecognised structural details of the reconstructed StW 352 that affect current interpretations of its functional morphology: (1) we estimate that the PT has been reattached to the body of the calcaneus ~5 mm dorso-distally from its original anatomical position; and (2) the presence of intrusive matrix has artificially misshaped the PT by expanding it laterally and proximodistally. Future studies of this specimen that apply geometric morphometrics, or other shape analysis tools, should compensate for these inaccuracies before undertaking comparisons between it and other calcanei. Additionally, given that the PT is likely smaller than previously reported for StW 352, caution should be exercised when using it to infer muscle function and extrapolate activity patterns of this individual, and thus by extension, within Australopithecus africanus in general. Lastly, these findings highlight the importance of not only the production of accurate reconstructions, but also the critical evaluation of the accuracy of existing reconstructions when working with damaged fossil material.

The evolution of the human foot

“…africanus have only been discovered at Sterkfontein Member 4, dated to ~2.0 to 2.6 Ma . There is one calcaneus (StW 352) attributed to this species . The posterior tuberosity is missing, but what is preserved is relatively gracile, and the subtalar joint has an ape‐like degree of convexity, suggesting that this joint was fairly mobile .…”

“…94 There is one calcaneus (StW 352) attributed to this species. 92,95 The posterior tuberosity is missing, but what is preserved is relatively gracile, 96 and the subtalar joint has an ape-like degree of convexity, suggesting that this joint was fairly mobile. 97 The tali from Member 4 all display even trochlear rims with unwedged trochlear surfaces, indicating that the tibia was orthogonally positioned relative to the talus, as in humans today.…”

“…sediba provides evidence for yet another distinct locomotor strategy compared to that found in other Australopithecus species. The calcaneus (U.W [88][89][90][91][92][93][94][95][96][97][98][99]. is gracile compared to other hominin calcanei,21,96 with an ape-like dorsally positioned lateral plantar process.…”