Forelimb biomechanics of Mussaurus patagonicus (Dinosauria, Sauropodomorpha): insights from three dimensional computer modelling

ALEJANDRO OTERO; JOHN HUTCHINSON; DIEGO POL; VIVIAN ALLEN

Berlin

Congreso; 73° Annual Meeting of the Society of Vertebrate Paleontology; 2014

We present a three-dimensional (3D) reconstruction and biomechanical analysis of the forelimb of the basal sauropodomorph Mussaurus patagonicus, from the Late Triassic of southern Patagonia, Argentina. Mussaurus has been interpreted as more closely related to derived sauropodomorphs (i.e., Sauropodiformes) than to more basal forms (e.g., Plateosaurus). 3D data were acquired with a laser scanner, and 3ds Max software was used to estimate the positions and axes of the joints between bones and the ranges of motion (ROMs) of each joint. Finally, we integrated the whole forelimb into a musculoskeletal model in reasonably well-validated biomechanics software (SIMM), producing the first such complete model of an archosaur forelimb. The musculoskeletal model includes a full range of degrees of freedom in joint rotation: flexion/extension, ab/adduction, and long-axis rotation, using joint morphology to estimate more realistic axes that could be non-orthogonal and not aligned to the world coordinate system or main body axes. Also, we reconstructed 28 muscle groups crossing the glenoid, elbow, and wrist joints. We used this model to estimate all major muscle moment arms about the main limb joints, varying limb orientation to quantify how these changed. Two forelimb models were analyzed, considering the two current hypotheses about antebrachium architecture to evaluate biomechanical differences between these: with manus pronation and with no pronation. Our analysis of joint articulations indicates that the forelimb of Mussaurus was capable of similar ROMs for the gleno-humeral and phalangeal joints in both models, differing in the elbow and wrist joints, which produced different ROMs in each model. In this sense, the model with a pronated manus presented a larger elbow ROM due to a better fit of the radius to the cuboid fossa of the humerus. Wrist ROM, on the other hand was larger in the model with no pronation. The antebrachium was not capable of active pronation, with minimal radial rotation against the ulna. The muscle moment arm data are important for addressing how Mussaurus used the forelimbs, testing hypotheses of forelimb function, such as locomotion, defense, grasping, and digging. We also present a comparative analysis of these moment arms against data from extant Crocodylus (as a rough approximation of plesiomorphic, quadrupedal musculoskeletal function), which illuminates numerous potential changes of muscle actions from basal archosaurs to bipedal ? and then convergently quadrupedal ? dinosaurs.