 In 1988, Jacques Gauthier proposed a cladistic definition of Reptilia as a monophyletic node-based crown group containing turtles, lizards and snakes, crocodilians, and birds, their common ancestor and all its descendants.While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.