Evolution of hindlimb posture in nonmammalian therapsids: biomechanical tests of paleontological hypotheses

Analyses of limb joint morphology in nonmammalian therapsid "mammal-like re ptiles" have suggested that among many lineages, individual animals were ca pable of shifting between sprawling and upright hindlimb postures, much lik e modern crocodilians. The ability to use multiple limb postures thus might have been ancestral to the generally more upright posture that evolved dur ing the transition from "mammal-like reptiles" to mammals. Here I derive a biomechanical model to test this hypothesis through calculations of expecte d posture-related changes in femoral stress for therapsid taxa using differ ent limb postures. The model incorporates morphological data from fossil sp ecimens and experimental data from force platform experiments on iguanas an d alligators. Experimental data suggest that the evolutionary transition from sprawling t o nonsprawling posture was accompanied by a change in the predominant loadi ng regime of the limb bones, from torsion to bending. Changes in the cross- sectional morphology of the hindlimb bones between sphenacodontid "pelycosa urs" and gorgonopsid therapsids are consistent with the hypothesis that ben ding loads increased in importance early in therapsid evolution; thus, bend ing stresses are an appropriate model for the maximal loads experienced by the limb bones of therioaont therapsids. Results from the model used to est imate stresses in these taxa do not refute the use of both sprawling and mo re upright stance among basal theriodont therapsids. Thus, the hypothesis t hat the use of multiple postures was ancestral to the more upright posture typical of most mammals is biomechanically plausible. Model calculations al so indicate that the axial rotation of the femur typical in sprawling locom otion can reduce peak bending stresses. Therefore, as experimental data fro m alligators and iguanas suggest, the evolution of nonsprawling limb postur e and kinematics in therapsids might have been accompanied by increased lim b bone bending stress.