Synapsid ("mammal-like") reptiles and fossil mammals of Mesozoic age are the fossils that record the evolutionary transition from reptiles to mammals. Dr. James Hopson has been a leading investigator into the anatomy and functional capabilities of these intensely studied groups. He now proposes to incorporate important new fossil representatives into an analysis of evolutionary relationships within and among these critical groups. These new anatomical data come from museum collections in Europe and South Africa, and from new collections in Mexico and Argentina. Despite decades of focused study, the evolution from reptiles to mammals is not understood in detail. The proposed research will illuminate important anatomical details from relevant groups, thereby establishing the evolutionary affinities among early mammals and their most likely point of divergence from their reptilian progenitors. The results of this research will find a wide audience among zoologists and evolutionary biologists in general.