Study of the origins of the terrestriality in hominoids viewed from functional morphology of the talar joint

点击翻译按钮获取中文摘要

This study aims to clarify the evolutionary processes concerning the origin(s) of the terrestriality in the human-African ape clade. For this purpose, the talar joint of the Miocene apes was examined. Movements of the subtalar joint during inversion-eversion were observed in living anthropoids by using the CT scanner, and osteological and kinesiological associations were investigated. The 3-D morphology of the posterior talar articular surface of the calcaneus was measured in Proconsul major, P.nyanzae (ca.19-18 Ma), and Nacholapithecus (ca.15Ma) as well as eight living anthropoids. The proportion of the posterior talar facet and the articular surface axis in the fossil calcanei exhibit the primitive anthropoid pattern. However, movements of the posterior subtalar joint in the fossil apes show a certain degree of rotations and ab-/adduction during in-and eversion of the foot as well as dorsi-and plantarflexion. This type of functional adaptation differs from those in living large platy … More rrhines in which only the dorsi-and plantarflexion are dominated. The posterior talar facet in these fossil apes is unique in that the posterior and anterior part of the facet are peculiarly flat and the intermediate part is well flexed or convexed. Probably, the posterior subtalar joint presumed either the fully plantarflexed or dorsiflexed position when the foot is variously inverted or everted.The posterior part of the articular surface exhibits strong inclination on the medial side. This causes a greater amount of the lateral rotations and/or adduction associated with the plantar flexion. Differing from this condition, cercopithecids has a shortened articular surface, resulting in a smaller range of movements and greater stability of the joint. This condition reflects the semi-terrestrial phase of the primitive cercopithecids. The articular surface of Pan has a less convex and smooth intermediate part. The long axis of the articular surface is strongly angled with the calcanean axis. These specializations enhance the stability of the joint through various positions and a greater component of the rotations in inversion-eversion movement. The fossil calcanei have a wide range of inversion-eversion and high stability in fully inverted of everted position though less than in living apes, thus, are unlikely adapted for ground locomotion. The morphology seen in Pan could have been developed from that of the fossil calcanei. However, the possibility that terrestrially adapted (like morphology of cercopithecids) stage existed in between them is low. Probably, The terrestrial adaptation in human-African ape clade does not go back up to the Middle Miocene. Less