Morphology of the Hallucial Phalanges in Extant Anthropoids and Fossil Hominoids

Pedal phalanges of living anthropoids and several Miocene fossil hominoid taxa were studied to reveal functional adaptations of living anthropoid feet and to infer positional behavior of fossil hominoids. Among the examined living anthropoids, Pan has a very developed (long and robust) hallux. Proconsul and Nacholapithecus, a large hominoid from Nachola, northern Kenya, display a moderately long hallux like Alouatta and Cebus, suggesting the well-developed capability of a hallux-assisted power grip. Allometric analyses revealed that the Miocene hominoids examined (mainly from East Africa) as a whole displayed a different scaling pattern about the width of the proximal articular surface of the hallucial terminal phalanx from that of living anthropoids. Larger-sized hominoids display a wider articular surface than comparable-sized living anthropoids while smaller-sized fossil hominoids do the reverse. Such a difference was less marked for the height of the articular surface. These results may suggest that positional adaptations of Miocene hominoids are not merely resultants of a common body size function that is observed in living anthropods. The wide articular surface of fossil hominoid hallucial terminal phalanges suggests an adaptation for vertical climbing and clinging, in which the hallux is kept perpendicularly to the long axis of the vertical support.