Inferring aspects of the locomotion of extinct primate species via biomechanically informed, morphometric analysis of the ulna

Abstract

Notharctids are euprimates from the Eocene of North America. Yet the nature and variety of locomotor modes used by this group is not fully understood. In this study we aim to add to discussions of notharctid locomotion through comparative analysis of functional morphology of limb bones in extant primates. The ulna was chosen for its potential correlation with variation in forelimb posture, load-bearing, mobility, and muscular strength. We identified six locomotor categories (vertical clinging and leaping, suspensory/brachiation, slow climbing, arboreal quadrupedalism, terrestrial quadrupedalism, and knuckle-walking quadrupedalism) likely to impose different functional requirements on the ulna. We evaluate associations between locomotor category and bone structure, and report results from preliminary analysis of 9 ulnar features in 30 extant primate species (n= 71). We found ulna morphological traits measured in this study to distinguish different locomotor modes, allowing the metrics of notharctid ulnae (n= 5) to be plotted into the locomotor group their ulnar morphology most closely resembled. The features of Notharctus and Smilodectes ulnae were found to be consistent with those of living arboreal quadrupeds and vertical clingers and leapers. Additionally, N. pugnax and N. tenebrosus exhibit a range of morphologies similar to that of a typical extant species suggesting no locomotor behavior diversity in Notharctus over time.