Joint Surface Area Proportions and Articular Curvature In AL 288-1: A Functional Interpretation

Body size has been recognized by several authors as one of the most important parameters affecting the biology of an organism. It has been argued that body size plays roles in metabolic cost, mobility, thermoregulation, and foraging strategy. For extinct species body masses can only be estimated using fossil remains and extant reference samples. To accurately estimate body mass the reference sample must have the same relationship between body mass and skeletal elements. Establishing a reference sample with similar body proportions as the fossil species is imperative.

The purpose of this study is to investigate forelimb to hindlimb joint surface area proportions and articular surface curvature in the Australopithecus afarensis specimen AL 288-1 “Lucy”. This specimen is compared to reference samples of humans, African apes, and orangutans to determine which most accurately reflects the joint surface area proportions and joint curvature observed in “Lucy”. Joint surface area and articular surface curvature are known to be related to body mass and locomotor repertoire and hence provide clues about body proportions, locomotor and postural behaviors. Findings in this analysis indicate that “Lucy” is a mosaic of human and pongid postcranial joint features. Analyses of joint surface area reveal proportions intermediate between apes and humans but which suggest heavy reliance on the hindlimbs for locomotion. Analyses of joint curvature reveal highly curved joint surfaces consistent with high mobility and multidirectional stability indicative of an arboreal component to Lucy’s locomotor repertoire.