Looking beyond phalangeal length and curvature: Functional correlation between manual phalangeal articular and collateral ligamentous morpohology and anthropoid locomotor adaptations

Abstract

Primates exhibit more diversity in locomotor behavior than any other mammalian order. Discrete locomotor adaptations are characterized by distinct patterns of phalangeal orientation with dissimilar patterns of mechanical loading. Phalangeal shaft length and curvature are frequently used to infer locomotion in fossil anthropoids given the direct and repetitive contact between the hand and substrate; however, most fossil anthropoid phalanges are fragmentary or incomplete. Given that articular epiphyses and associated collateral ligaments are subject to the same locomotor behaviour‐specific mechanical loading that influences longitudinal shaft curvature and length, it is reasonable to predict that the proximal and distal articular surface morphology and collateral ligament fossae (CLF) are also influenced, at least in part, by locomotion.We test the hypothesis that anthropoid phalangeal articular and collateral ligament fossae (CLF) morphology (i.e. contour curvature, size) varies with respect to locomotion and that more terrestrial primates experiencing higher compressive loads will have larger CLF's and increased mediolateral articular contour curvatures to resist mediolateral joint displacement. Segmented 3D models of proximal and distal articular surfaces and CLF's were obtained for the proximal phalanges (digits 2–5) of 16 locomotor diverse anthropoid genera (n=234 individuals). Direct comparisons among individual taxa and locomotor groupings reveal that articular and CLF morphology is a composite function of size, phylogeny, and locomotion but that more terrestrial primates have greater mediolateral articular contour curvatures and larger, deeper CLF's relative to more arboreal primates who have less mediolaterally restrictive joints with increased dorsoventral curvatures and smaller CLF's.