The subtalar joint complex of Australopithecus sediba

Abstract

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.

From Australopithecus to Homo : the transition that wasn't

“…South African Australopithecus sediba (approx. 2.0 Ma) has also been claimed to have been a direct ancestor to Homo, possibly even to H. erectus [90], but is more plausibly considered a close relative of A. africanus [91][92][93].…”

Chimpanzee ankle and foot joint kinematics: Arboreal versus terrestrial locomotion

“…The subtalar joint, located between the talus proximally and the calcaneus distally, mainly allows inversion–eversion and abduction–adduction motions in humans and chimpanzees (Close et al, ; Inman, ; Lewis, ), and complements the plantarflexion–dorsiflexion at the talocrural joint in controlling the three‐dimensional position of the foot relative to the leg. Chimpanzees possess a more curved posterior subtalar facet than humans (Deloison, ; Latimer and Lovejoy, ; Prang, ), which is thought to be indicative of a greater range of subtalar joint motion. This mobility has been argued to represent an arboreal locomotor adaptation that enables chimpanzees to position their feet at highly inverted angles when climbing or traveling atop arboreal supports (Lewis, ; Zipfel et al, ), thereby enhancing pedal grasping by exposing a greater surface area of the foot to the substrate (Cartmill, ).…”

Stiffness of the human foot and evolution of the transverse arch

“…[3][4][5] By contrast, the feet of non-human primates are flat and softer. 6-8 Current understanding of foot stiffness is based on studies that focus solely on the longitudinal arch, [9][10][11][12][13][14] and little is known about the mechanical function of the transverse arch. However, common experience suggests that transverse curvature dominates the stiffness; a drooping dollar bill stiffens significantly upon curling it along the transverse direction, not the longitudinal.…”