New specimens of Eucoelophysis baldwini from the Upper Triassic Chinle Formation, northern New Mexico, U.S.A., and their implications for the evolution of Silesauridae (Dinosauriformes)

Eucoelophysis baldwini is a silesaurid dinosauriform known only from a single fragmentary postcranial skeleton from the Petrified Forest Member of the Upper Triassic Chinle Formation near Ghost Ranch in northern New Mexico, U.S.A. The relative incompleteness of that specimen has hampered the ability to diagnose Eucoelophysis baldwini and determine its phylogenetic relationships. However, abundant dinosauriform material has been recovered at the nearby Hayden Quarry from a similar stratigraphic level to the holotype. Our examination of all available specimens indicates that the holotype and the Hayden Quarry pelvic and hindlimb materials share a unique combination of character states which distinguish them from all other avemetatarsalians, including a reduction in the size of the fourth trochanter of the femur and a flat surface along the lateral margin of the tibia. Although the isolated Hayden Quarry cranial material cannot be directly compared with the holotype of Eucoelophysis baldwini, it does possess silesaurid synapomorphies (e.g., leaf-shaped teeth with delayed ankylosis, or fusion to the jaw) and there is no evidence to suggest more than one silesaurid taxon in the Hayden Quarry. Taken together, these data support the referral of the Hayden Quarry material to Eucoelophysis baldwini. The results of our study indicate that the diversification of most Late Triassic silesaurid lineages occurred by the Carnian Stage, approximately coincident with the initial radiation of saurischian dinosaurs.