Evolution of hindlimb posture in archosaurs: Limb stresses in extinct vertebrates

During the Triassic, some 250-200 million years ago, the basal archosaurs showed a transition from sprawling to erect posture. Past studies focused on changes in bone morphology, especially on the joints, as they reorientated from a sprawling to an erect posture. Here we introduce a biomechanical model to estimate the magnitude of femur stress in different postures, in order to determine the most reasonable postures for five basal archosaurs along the line to crocodiliforms (the rhynchosaur Stenaulorhynchus, the basal archosaur Erythrosuchus, the 'rauisuchian' Batrachotomus, the aetosaurs Desmatosuchus and Typothorax). The results confirm a sprawling posture in basal taxa and an erect posture in derived taxa. Erect posture may have evolved as a strategy to reduce large bending stresses on the limb bone caused by heavy body weights in larger forms.