Theoretical morphology of the Archosaur (Reptilia : Diapsida) pelvic girdle

Theoretical models of skeletal structures provide suitable frameworks to as sess macroevolutionary patterns of form change. We discuss three theoretica l approaches to account for morphological patterns of the pelvic girdle in archosaurs. Every approach targets a different level of organization within the concept of morphospace. First, we build a morphocline by applying a ma thematical transformation to the outline of the hip of the theropod dinosau r Deinonychus antirrhopus, in order to look at theoretical paths of evoluti onary change based on changes of proportion. Second, we analyze the variabi lity of a sample of 86 hips within a theoretical construction that incorpor ates information about the spatial orientation of the three paired bones th at build this skeletal compound. Finally, we look at boundary patterns with in these hips as a basis for generating a formalism based on graph theory. Insights about the evolution and development of the archosaur triradiate pe lvis and its morphological trends are suggested in the light of each theore tical approach, with a special focus on the convergent evolution of a retro verted pubis in ornithischians and birds.