Late acquisition of erect hindlimb posture and function in the forerunners of therian mammals.

The evolutionary transition from early synapsids to therian mammals involved profound reorganization in locomotor anatomy and function, centered around a shift from "sprawled" to "erect" limb postures. When and how this functional shift was accomplished has remained difficult to decipher from the fossil record alone. Through biomechanical modeling of hindlimb force-generating performance in eight exemplar fossil synapsids, we demonstrate that the erect locomotor regime typifying modern therians did not evolve until just before crown Theria. Modeling also identifies a transient phase of increased performance in therapsids and early cynodonts, before crown mammals. Further, quantifying the global actions of major hip muscle groups indicates a protracted juxtaposition of functional redeployment and conservatism, highlighting the intricate interplay between anatomical reorganization and function across postural transitions. We infer a complex history of synapsid locomotor evolution and suggest that major evolutionary transitions between contrasting locomotor behaviors may follow highly nonlinear trajectories.