BIOSTRATIGRAPHY AND EVOLUTION OF TRILOPHOSAURUS, AN UNUSUAL ARCHOSAUROMORPH FROM THE UPPER TRIASSIC CHINLE GROUP, SOUTHWESTERN USA

Two species of the unusual archosauromorph Trilophosaurus, T. buettneri Case and T. jacobsi Murry, are known from diverse localities in the Upper Triassic Chinle Group of Texas, New Mexico and Arizona. The tricuspid teeth from which Trilophosaurus derives its name are identifiable to the species level, so fragmentary tooth-bearing fossils, and even isolated teeth, have biostratigraphic utility. T. buettneri is the better-known taxon, but is restricted to a handful of localities stratigraphically low in the Chinle Group of Texas and Arizona. Until recently, T. jacobsiwas poorly known, leading some workers to speculate that it was actually a procolophonid ("Chinleogomphius"). However, we have recovered fossils from an extremely rich bonebed that demonstrate that T. jacobsi is congeneric with T. buettneri. Teeth of T. jacobsi are readily distinguished from those of T. buettneri by their tall, faceted central cusp, low, transversely elongate lingual cusp, and the presence of cingulae along their anterior and posterior margins. T. jacobsi occurs in lower Chinle Group strata in Texas, New Mexico, and Arizona. The two taxa are only known to co-occur at the upper Kalgary locality (NMMNH L-1430) in the lower Tecovas Formation in West TexasÑotherwise T. buettneri always occurs in stratigraphically lower strata than T. jacobsi. The first occurrence datum (FAD) of T. buettneri in Texas corresponds to that of Otischalkian index taxa such as the phytosaur Paleorhinus and the aetosaur Longosuchus, and implies an Otischalkian age for most T. buettneri occurrences. The FAD of T. jacobsi is often at or near the FAD of Adamanian index taxa such as the phytosaur Rutiodon and the aetosaur Stagonolepis, indicating that T. jacobsi is a possible index fossil of the Adamanian. T. jacobsi is probably an anagenetic descendant of T. buettneri, as it comes from generally younger strata and appears to be more derived. Trilophosaurus is endemic to southwestern North America, and may represent a relatively rare occurrence of an arboreal herbivore.