Capitular morphology in primates and fossils: 3-D measurements of the capitulum and possible implications for reconstructing locomotor adaptations in fossil primates

Abstract

The changes in habitual posture associated locomotor behavior have well-documented effects on the skeleton. This paper focuses on the humeral capitulum, which is functionally significant in bipedal, quadrupedal, and knuckle-walking locomotion. Using a sample of 43 great apes, 59 cercopithecids, and 31 humans, 3D images were used to take measurements of the area, length, and angle of the capitulum to test if differences in joint area occurred with different locomotion patterns. The results were compared KNM-ER 1504, KNM-ER 6020, KNM-ER 739, A.L. 228-1, SKX 10924, TM 1517 to determine if they could clearly be associated with a particular locomotor group. The results of this analysis indicate that the capitulum was significantly more anterior in humans than in other groups, which may be related to use of the elbow in nonlocomotor activity. Distal capitular area and length measurements were larger for cercopithecids than for other groups, however proximal capitular length, proximal capitular area measurements, and approximations of shape were not significantly different. Fossil specimens were indistinguishable from human, great ape, and cercopithecid values using both angle and a discriminant function analysis of all measurements. While significant patterns were found within locomotor groups, these measurements do not appear to work to adequately distinguish locomotor patterns in fossil hominins.