Quantification of the peroneus longus groove in primate cuboids: Implications for Ardipithecus ramidus
Achilles, Erin Jean
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Significant debate surrounds the question of locomotor pattern in the last common ancestor (LCA) of chimpanzees and humans. Traditionally, most researchers have agreed that the morphology of the LCA was similar to that of extant African apes. However, recent analyses of the foot of the possible stem hominin Ardipithecus ramidus by Lovejoy et al. (2009) suggest the earliest hominins may have possessed a locomotor pattern more similar to that of a monkey. This argument is based in part on the morphology of the groove for the peroneus longus tendon located on the cuboid bone. The orientation of this groove is thought to relate to locomotor pattern, and a qualitative assessment of this angle in A.ramidus suggests that it compares favorably with the Old World monkey condition. However, no quantification of the groove angle is provided. In this study, I quantified the angle of the groove for peroneus longus relative to the facet for the fourth metatarsal using eight landmarks on the cuboid, analyzing both landmark data and linear and angular measurements derived from this data. A comparative sample including extant humans, apes, and monkeys, as well as fossil hominins and apes, shows that groove angle can be used to distinguish some taxonomic groups, but is highly variable within taxa. Additionally, there is only a weak link between particular locomotor patterns and groove angle. Therefore, this metric should not be used to make assumptions about locomotor behavior in A.ramidus or other fossil species and is not relevant to reconstructions of locomotor pattern in the human-chimpanzee LCA.