THE LATE TRIASSIC ARCHOSAUROMORPH TRILOPHOSAURUS AS AN ARBOREAL CLIMBER

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 in the southwestern USA. 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. jacobsi was poorly known, leading some workers to speculate that it was actually a procolophonid ("Chinleogomphius"). However, fossils from an extremely rich bonebed in West Texas demonstrate that T. jacobsi is congeneric with T. buettneri. T. jacobsi is probably an anagenetic descendant of T. buettneri, as it comes from generally younger strata and appears to be more derived. Both species likely occupied similar ecological niches, based on similarities in the postcrania. Trilophosaurus occurences in the Chinle Group are relatively rare, but individual sites are exceptionally rich, suggesting that Trilophosaurus lived in a different paleoenvironment than more typical Chinle vertebrates, which typically lived in or near streams (phytosaurs, metoposaurs) or floodplains (aetosaurs, rauisuchians, and dinosaurs). Two potential interpretations are that Trilophosaurus was either an arboreal climber or a fossorial digger. However, the gross skeletal features of Trilophosaurus are not compatible with a fossorial mode of life: the limbs are too long and gracile, proximal limb elements are longer than distal ones, and the claws are laterally compressed, not transversely broadened. The intermittent study of Trilophosaurus has allowed the idea of it being an arboreal climber, originally proposed by Gregory, to receive little mention in subsequent studies. We restudied the functional morphology of Trilophosaurus using qualitative functional morphological analysis of the skeleton of and a quantitative examination of claw curvature. The claw curvature analysis is based on a similar anaylsis by Fedduccia, used to argue for an arboreal lifestyle in the Late Jurassic bird Archaeopteryx. Claw morphology of Trilophosaurus shows similarities to the drepanosaurs Drepanosaurus and Megalancosaurus. Our analysis indicates ample evidence to suggest that Trilophosaurus was arboreal.