Phylogeny and classification of thecodontian reptiles

Thecodontians have long been recognized as the basal stock from which all other archosaurs were derived, but their phylogeny and classification are very confused, and the origin and evolution of archosaurs are consequently still poorly understood. The skull and skeletal morphology of thecodontians is confusingly uniform, and many palaeontologists feel that the ankle joint may provide a more useful clue to the phylogeny and classification of these groups^1,2. I attempt here to trace some of the thecodontian lineages on the basis of tarsal anatomy. The major structural changes from thecodontian to dinosaurs are most clearly visible in the pelvis and hind-limbs, associated with improvement of gait from the 'sprawler' through the 'semi-improved' to the 'fully-improved' condition^3,4. As a result, the function of the ankle joint was modified from a simple flexible base for the leg to the more advanced role of a lever. For this reason, an understanding of thecodontian tarsi is necessary if we are to understand the evolution of the later archosaurs.