Sauropods were gigantic herbivorous dinosaurs with elongate necks, diminutive skulls, and long tails. Despite their extensive, global fossil record and the paleobiological implications of their large size, many aspects of sauropod paleobiology remain uninvestigated because of the combined size, mass, and fragility of their postcranial remains. The evolutionary history and phylogenetic relationships of sauropods have been evaluated and established predominantly on postcranial characters. Although sauropods are diagnose primarily on limb and foot characters related to locomotion, the evolution and functional morphology of these characters remains poorly understood.
The purpose of the present study is to evaluate the evolution and functional morphology of sauropod locomotion utilizing primary osteological and ichnological data in combination with statistical analyses of bone shape. This study establishes a generalized model of sauropod locomotion and uses statistical shape analysis to infer terrain preferences and paleoeconiche partitioning among three contemporaneous Late Jurassic sauropod genera.
The results of this study suggest that sauropods had a limited locomotor scope constrained by a rigid joint morphology. The cylindrical or hinge-like joint surfaces of sauropod limb and foot bones acted to constrain limb movements to a parasagittal plane. The manus was rigid, the pes was mobile, and foot claws engaged the substrate during each footfall. Statistically significant differences in limb and foot bones of Apatosaurus, Diplodocus, and Camarasaurus suggest these contemporaneous sauropods had differing terrain preferences. In particular, Apatosaurus and Diplodocus appear to have been better suited to traversing wetter sediments than Camarasaurus.
Sauropod gigantism may be correlated, in part, with a reversion to quadrupedal locomotion from a bipedal ancestor and the rigid joint morphology of their limbs. As sauropod forelimbs resumed a weight-bearing role, the radius shifted internally to pronate the manus, subsequently altering a primitively flat dinosaurian metacarpus into a digitigrade, semi-tubular, load-bearing arrangement. This shift allowed both the radius and ulna to be loaded as parallel columns in the sauropod forearm, providing additional compressive strength to the forelimb. Overall, the rigid joint morphology of sauropod limbs and feet served to increase stability and weight support at large size by restricting the range of limb movement and mobility.