The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


The primitive aspects of the foot and ankle of Australopithecus sediba

JEREMY M. DESILVA1,2, BERNHARD ZIPFEL2,3, ROBERT S. KIDD2,4, KRISTIAN J. CARLSON2,5, STEVEN E. CHURCHILL2,6 and LEE R. BERGER2,7.

1Department of Anthropology, Boston University, 2Institute for Human Evolution, University of the Witwatersrand, 3Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, 4School of Biomedical and Health Sciences, University of Western Sydney, 5Department of Anthropology, Indiana University, 6Department of Evolutionary Anthropology, Duke University, 7School of Geosciences, University of the Witwatersrand

Thursday 11:45-12:00, Grand Ballroom II Add to calendar

Foot fossils from the 1.977 myr Malapa site, South Africa display a unique mixture of primitive and derived morphologies. The foot and ankle fossils of Australopithecus sediba now include distal tibiae, a distal fibula, talus, calcanei, navicular, lateral cuneiform, several partial metatarsals, and a distal phalanx. Its calcaneal tuber is gracile, and lacks the weight-bearing lateral plantar process found in fossils attributed to Au. afarensis, making it surprisingly (given the bipedal advances in other parts of the Au. sediba skeletons) more ape-like than a hominin over one-million years its elder. The medial plantar process projects in a beaklike fashion, suggesting a strong role for flexor digitorum brevis, an important muscle for pedal grasping. The talar head is disproportionately large, implying some mobility at the talonavicular joint. The lateral cuneiform is proximodistally short, unlike the elongated tarsal region found in the foot of humans and other australopithecines. Unlike the condition in humans, or in any known fossil hominin, the base of the fourth metatarsal is slightly convex, indicating some mobility at the tarsometatarsal joint. These morphologies suggest that Au. sediba had a more mobile midfoot than that of any known Australopithecus. The small heel process and the mobile midfoot region both suggest that Au. sediba, while bipedal, practiced kinematically different upright walking than modern humans and other bipedal hominins. The thickness of the medial malleolus, beaklike projection of the medial plantar process, and midfoot laxity indicate that Au. sediba was probably arboreal, and perhaps the most arboreal of the australopithecines.

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