Biomechanical simulations of hindlimb function in Alligator provide insights into postural shifts and body size evolution.

The shift from sprawling to erect limb postures in archosaurs during the Triassic represents a major evolutionary transformation in vertebrates. One unresolved question regarding the limb posture transition is its association with body size evolution. If adopting more erect limb postures reduces mass-specific muscle forces and bone stresses, it would enable the evolution of larger body sizes. We tested this prediction using computational modeling and simulation of hindlimb muscle activations and femoral stresses across limb postures and body sizes in juvenile to adult American alligators and a scaled model of the extinct giant alligatoroid Deinosuchus riograndensis. We showed that larger alligators and D. riograndensis encounter challenges in generating sufficient muscle forces to support their bodies and maintain bone stresses, whereas adopting more erect hindlimb postures helps mitigate bone stresses among individuals of similar sizes. These results show how the shift from sprawling to erect limb posture relaxed biomechanical constraints, potentially facilitating the evolution of larger body sizes in terrestrial tetrapods.