Three-Dimensional Musculoskeletal Modeling of the Sauropodomorph Hind Limb: The Effect of Postural Change on Muscle Leverage.

The biomechanical constraints for life at massive size can be directly observed in the evolutionary history of sauropodomorph dinosaurs. Members of this lineage underwent a number of major postural transitions as they increased in size from relatively small bipedal dinosaurs to massive titanosaurs that include the largest terrestrial animals of all time. To better understand the impact of gigantic size on the biomechanics of sauropods, we used three-dimensional musculoskeletal modeling to investigate how hind limb musculature was affected, first by the development of a quadrupedal stance from a bipedal one, and later in the transition from a narrow-gauge to a wide-gauge stance. Muscle moment arms were measured in four sauropodomorph taxa: the bipedal basal sauropodomorph Plateosaurus engelhardti, the narrow-gauge diplodocid Diplodocus carnegii, the titanosauriform Giraffatitan brancai, and the wide-gauge titanosaur Diamantinasaurus matildae. In Plateosaurus, low moment arm leverage in the hip extensors and knee flexors and extensors was observed suggesting high-velocity movement for fast locomotion. A reduction in hip extensor leverage in Diamantinasaurus was found which suggests a reduced role for the hind limb in forward propulsion in titanosaurs. An increase in overall hip adductor leverage and leverage of adductors 1 and 2 in Diamantinasaurus, compared with other taxa studied, might relate to the development of a wide-gauge stance. High knee flexor-extensor leverage in Giraffatitan but not Diamantinasaurus partially refutes the idea that broader femoral condyles in titanosauriforms increased knee torque production capabilities. Sauropodomorph postural changes clearly had an impact on the function and leverage of hind limb muscles. Anat Rec, 301:2145-2163, 2018.