Central Connecticut State University New Britain, Connecticut, United States
Abstract Body : In human evolutionary studies the role of the forelimb in locomotion prior to the adaptation of obligate bipedality and the origins of tool use are central and unresolved debates. When the hand is used in locomotion the palmar surface needs to make contact with the substrate. Across primate locomotor modes and ecologies, this ranges from a prone position to meet a horizontal substrate, semiprone to meet arboreal supports oriented vertically or of a diameter smaller than the width of the animal, or even supine in suspensory postures. Similarly, manipulative behaviors necessitate supination in addition to intrinsic manual proportions necessary for grip. The act of pronation-supination of the hand occurs at the articulation between the radius and ulna. Whereas the ulna defines the long axis of the forelimb in its rotationally-limited articulation with the humerus proximally, the position of the radial notch of the proximal ulna is variable among taxa. The radial head rotates within the radial notch in response to the action of pronator teres and pronator quadratus for pronation and the supinator and biceps brachii for supination. Among these, biceps brachii has the most identifiable and definable enthesis, the radial tuberosity. Using 3D geometric morphometrics, this study aims to evaluate the relationship between both articular and entheseal morphology of the radius and ulna to identify metrics related to pronation and supination. 3D landmarks were collected from 32 primate genera (N (radius) =388, N (ulna) =333) representing variation in body mass, locomotor mode, and clade. Series of 3D landmarks defined planes, and angles between planes were calculated to quantify the position of the radial notch of the ulna and radial tuberosity of the radius relative to the long axis of each bone. Angles were used in predictive models of degrees of pronation and supination collected in vivo for a subset of species, locomotor frequency when available, and locomotor and cladistic categories for all taxa. Results between taxa categorically (clade, locomotor mode) and in relationships with measured degrees of pronation and supination and observational frequencies of locomotion demonstrate more stability and a stronger relationship with the articular surface angle – the orientation of the radial notch - compared to the entheseal angle – the orientation of the radial tuberosity. This result contributes to the contentious debate regarding the utility of entheseal morphology in functional analyses and specifically calls into question of the use of landmarks of the radial tuberosity in functional studies of the forelimb. Fossil metrics can be compared to the ulnar results to discern between quadrupedal and non-quadrupedal locomotor functional reconstructions of preserved proximal ulnae.
