Inferred musculature and kinematics of the forelimb in a lambeosaurine hadrosaurid dinosaur

Abstract

Hadrosaurid dinosaurs are largely considered to be facultatively bipedal. However, a distinct lack of well-described hadrosaurid postcranial material in the literature makes comparative analysis of hadrosaurid biomechanics difficult. A newly recovered lambeosaurine (crested) hadrosaurid from the Oldman Formation of southern Alberta presents an ideal specimen for comparative biomechanical analysis due to the high level of preservation of its pectoral girdle, forelimbs, manus, and sternal plates. Here, the osteology of the forelimb and pectoral girdle of TMP2015.044.0036 is described in detail and its morphology is compared with other known ornithopods. The musculature of the pectoral girdle and forelimb of TMP2015.044.0036 is inferred based on phylogenetic bracketing. Finally, the range of motion is assessed for the shoulder, elbow, and wrist of TMP2015.044.0036, and this inferred range of motion is used to plot muscle trajectories of the shoulder musculature onto a three-dimensional computational model of the articulated pectoral girdle and forelimb of TMP2015.044.0036. This model is used to estimate moment arms for these muscles. Findings suggest that TMP2015.044.0036 habitually utilized its forelimbs in quadrupedal locomotion, as evidenced by a rigid pectoral region, a pillar-like manus, cartilaginous carpals, and hoof-like unguals for distal phalanges. Limited range of motion of the shoulder, elbow, and manus suggest that movement was limited to anterior-posterior swing which is corroborated by very well-developed muscles involved in humeral protraction and retraction, namely M. pectoralis, M. supracoracoideus and M. coracobrachialis, and a lack of well-developed humeral adduction-abduction musculature. This study lends support to the hypothesis that hadrosaurids represent principally quadrupedal animals.