The prosauropods from the Lower Jurassic Portland Formation of the Connecticut Valley are referred to two monospecific genera: the slender-footed Anchisaurus polyzelus (junior synomyns A. colurus, Yaleosaurus colurus) and the broad-footed Ammosaurus major (junior synonyms Ammosaurus solus, Anchisaurus solus); the material from Arizona is referred to Ammosaurus cf. major. The family Anchisauridae is restricted to three slenderfooted genera (Anchisaurus, Efraasia, Thecodontosaurus); the remaining anchisaurids are broad-footed forms which are transferred to the family Plateosauridae along with Ammosaurus, a genus long regarded as an extremely primitive coelurosaur. The replacement of prosauropods by ornithischians as the dominant "smallto medium-sized" (up to 10 m) terrestrial herbivores is attributed to the development in ornithischians of cheeks and self-sharpening teeth that dealt much more efficiently with resistant plant material than could the prosauropod dentition. The sauropodomorphs remained essentially quadrupedal, because they were herbivorous with the pubes anteroventrally directed as in most other reptiles.