The hominin talus has figured prominently in previous studies of the functional morphology of the talocrural joint, but the talocalcaneal and talonavicular joints have received comparatively less attention despite their functional importance as components of the subtalar joint complex. An associated complete talus and calcaneus attributed to the Malapa Hominin 2 (MH2) individual of Australopithecus sediba offers the opportunity to evaluate the subtalar joint complex in an early hominin. Furthermore, detailed morphological comparisons of A. sediba to other fossil hominins such as Australopithecus africanus have not yet been conducted. Here I quantify joint curvatures and angular measurements among extant hominoids and fossil hominins to evaluate the functional morphology of the subtalar joint complex of A. sediba. Australopithecus sediba uniquely combines talocalcaneal joint morphology indicative of mobility with specializations of the talonavicular joint that provide medial midtarsal stabilization. Multivariate analyses of talus and calcaneus variables show that A. sediba is most similar to extant gorillas in the morphology of the subtalar joint complex. In contrast, other hominins, such as OH 8, are more similar to modern humans. The morphological similarity between MH2 (U.W. 88-98/99) and specimens from Sterkfontein, Member 4 (StW 88, StW 102, StW 352) in morphologies of the talonavicular and talocalcaneal joints suggests that A. sediba may have possessed a foot that was functionally similar to that of A. africanus. This combination of morphologies in the A. sediba foot is probably derived among hominins and suggests that arboreality may have been adaptively significant for southern African Australopithecus.

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The longitudinal arch is one of the hallmarks of the human foot but its evolutionary history remains controversial due to the fragmentary nature of the fossil record. In modern humans, the presence of a longitudinal arch is reflected in the angular relationships among the major surfaces of the human talus and calcaneus complex, which is also known as the rearfoot. A complete talus and calcaneus of Australopithecus sediba provide the opportunity to evaluate rearfoot posture in an early hominin for the first time. Here I show that A. sediba is indistinguishable from extant African apes in the angular configuration of its rearfoot, which strongly suggests that it lacked a longitudinal arch. Inferences made from isolated fossils support the hypothesis that Australopithecus afarensis possessed an arched foot. However, tali attributed to temporally younger taxa like Australopithecus africanus and Homo floresiensis are more similar to those of A. sediba. The inferred absence of a longitudinal arch in A. sediba would be biomechanically consistent with prior suggestions of increased midtarsal mobility in this taxon. The morphological patterns in talus and calcaneus angular relationships among fossil hominins suggest that there was diversity in traits associated with the longitudinal arch in the Plio-Pleistocene.