Stance and gait in the hindlimb of a therocephalian mammal‐like reptile

The ilium, femur, tibia, fibula, astragalus and calcaneum of the therocephalian therapsid Regisaurus jacobi Mendrez are described. Functional consideration of the bones and particularly the nature of the hip, knee and ankle joints indicate that this animal was capable of at least two distinct gaits, a primitive, sprawling type and an advanced upright type. The ankle joint had a moveable articulation between the astragalus and the calcaneum analogous to that of the crocodiles. The possibility of such a multi-gaited intermediate stage having occurred in the course of the evolution of the mammalian locomotory pattern is suggested.