1Department of Biological Sciences, Idaho State University, 2Penn Museum, University of Pennsylvania, 3Human Evolution Foundation
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Based on non-statistically representative samples of human, Pan troglodytes, and Gorilla gorilla, Ward et al. 2011 concluded that a complete hominin fourth metatarsal (MT4) from Hadar, AL 333-160, belonged to a committed terrestrial biped with fixed transverse and longitudinal arches. According to Ward et al., the Hadar MT4 had 1) torsion value indicating a transverse arch, 2) angles between the diaphyseal long axis and the planes of the articular surfaces indicating a longitudinal arch, and 3) narrow mediolateral to dorsoplantar base ratio, an ectocuneiform facet, and tarsal articular surface contours, all indicating a rigid mid-foot. Comparisons of the Hadar MT4 characters to those of statistically representative samples of humans, five great ape species, baboons and proboscis monkeys show that none of their character interpretations are supported. The Hadar MT4 characters are common to catarrhines that have a midtarsal break and lack fixed transverse or longitudinal arches. Further comparison of length, as well as base, midshaft, and head circumferences to those of catarrhines shows that this bone is uniquely short with a large base suggesting poor leaping ability, with limited arboreal behaviors. The MT4 long axis was usually held perpendicular to gravity, i.e. the foot lacked a longitudinal arch. Overall, AL 333-160 is most similar to the MT4 of eastern gorillas, a slow moving quadruped that sacrifices arboreal behaviors for terrestrial ones. This study highlights the peril of using small and narrow samples for localized anatomy and relying on a single bony element to reconstruct overall locomotor behaviors.
This research was supported in part by a grant from the University of Pennsylvania Department of Anthropology and the Penn Museum.