The subtalar joint complex of Australopithecus sediba

Area of Science:

• Paleoanthropology and Functional Morphology
• Hominin Evolution
• Comparative Anatomy

Background:

• The talocrural joint of the talus has been extensively studied, but the talocalcaneal and talonavicular joints, crucial components of the subtalar joint complex, remain less understood in early hominins.
• The discovery of a complete talus and calcaneus from the Malapa Hominin 2 (MH2) individual of Australopithecus sediba provides a unique opportunity to examine the subtalar joint complex.
• Previous detailed morphological comparisons between Australopithecus sediba and other fossil hominins, like Australopithecus africanus, are limited.

Purpose of the Study:

• To evaluate the functional morphology of the subtalar joint complex in Australopithecus sediba.
• To conduct detailed morphological comparisons of Australopithecus sediba's subtalar joint complex with extant hominoids and other fossil hominins.
• To investigate the evolutionary significance of the subtalar joint complex morphology in early hominins.

Main Methods:

• Quantification of joint curvatures and angular measurements of the talus and calcaneus.
• Morphological comparisons were made among extant hominoids (including gorillas) and fossil hominins (including Australopithecus sediba and OH 8).
• Multivariate analyses were employed to assess similarities in talus and calcaneus variables.

Main Results:

• Australopithecus sediba exhibits a unique combination of talocalcaneal joint morphology indicating mobility and talonavicular joint specializations for medial midtarsal stabilization.
• Multivariate analyses reveal that Australopithecus sediba's subtalar joint complex morphology is most similar to extant gorillas.
• Specimens of Australopithecus sediba show morphological similarities to Australopithecus africanus in the talonavicular and talocalcaneal joints.

Conclusions:

• The subtalar joint complex of Australopithecus sediba presents a derived morphology among hominins, suggesting a unique functional adaptation.
• The foot morphology of Australopithecus sediba may have been functionally similar to that of Australopithecus africanus.
• The specialized foot structure of Australopithecus sediba suggests that arboreality might have been adaptively significant for this southern African early hominin.