9520719 Modern mammals differ from reptiles in having an upright, rather than a sprawling stance. This research will test the hypothesis that therapsid dinosaurs, which were ancestral to mammals, were capable of switching between a sprawling and an upright stance. The biomechanically plausible range of fossil therapsid limb postures will be calculated from measured limb bone dimensions, estimated body weights, and model values of limb bone safety factors. The safety factor of a bone is the ratio between its strength and the maximum stress to which it is normally exposed. In mammals, safety factors of limb bones range from two to four. Weights of fossil therapsids will be estimated from measurements of extant reptiles and non-upright mammals. Three-dimensional analyses of locomotor forces and limb motion in reptiles will integrate high speed video, recordings from force platforms during foot contact, and recordings of limb bone strains. If safety factors of reptile bones match those of mammals, limb postures for therapsids that yield similar safety factors are biologically feasible. Postures that yield safety factors outside the acceptable range were probably not part of therapsid biology. The research will contribute to knowledge of locomotor biomechanics in living reptiles and locomotor evolution in reptiles and mammals. The biomechanical approach can be extended to other extinct tetrapods, and will place objective mechanical constraints on the reconstruction of fossil animals.
