Fifth metatarsal morphology does not predict presence or absence of fibularis tertius muscle in hominids.

The leg muscle fibularis tertius (formerly peroneus tertius) is occasionally absent in humans, but it is rarely found in other primates. Phylogenetically and functionally it appears to be linked to efficient terrestrial bipedalism. An osseous indicator of the muscle would therefore be useful for interpreting the locomotor behavior of fossil hominids. To determine whether the presence of fibularis tertius can be detected osteologically, we isolated 58 human fifth metatarsals, noting which came from cadavers lacking the muscle. The bones were then ranked according to two characters that have been said to suggest presence of fibularis tertius in australopithecines: (1) sharpness of the dorsal shaft edge and (2) size and prominence of the dorsal tubercle. Presence of the muscle showed little association with the ranked characters, and the two criteria were uncorrelated. For example, one individual lacking a fibularis tertius exhibited nearly maximal expression of both features, whereas another possessing the muscle showed the weakest development of both. Only one of the 58 bones had a line comparable to that seen on SK 33380, a robust australopithecine fifth metatarsal from Member 3 of Swartkrans, South Africa. We conclude that fifth metatarsal morphology offers little reliable information about the presence of fibularis tertius or the timing of its appearance in the human career.