Locomotion in rauisuchid thecodonts

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Abstract
The systematic approach to the Rauisuchidae suggests that the lowest level of locomotor organization of the Thecodontia is represented by the Proterosuchidae, which can be regarded as forming the suborder Proterosuchia. The morphological and ecological relationships of the families Erythrosuchidae and Rauisuchidae suggest that they should be referred to a new suborder, the Erythrosuchia, whose locomotor apparatus shows several important characters derived from the primitive condition present in the Proterosuchia (Proterosuchidae). In the advanced Rauisuchidae the ilia moved from vertical to almost horizontal position, with the acetabulum projecting nearly ventrally. It required significant modifications in the sacral ribs and proximal areas of the pubes and ischia, and resulted in a type of graviportal relation with the subvertical femur. Because of this condition, the femur did not require the anteromedially projecting head and the prominent fourth and lesser trochanters as in the two orders of dinosaurs. The Rauisuchidae with vertical hind limbs remained plantigrade, with a crurotarsal tarsus and the tibia shorter than the femur. It is suggested that the evolutionary history of the vertical hind limbs in archosaurs followed at least two very different models of improvements as shown by the rauisuchids and the dinosaurs.