POSTCRANIAL ANATOMY OF A NEW, WELL-PRESERVED SPECIMEN OF THE DINOSAUR DRYOSAURUS ALTUS (ORNITHISCHIA, DRYOSAURIDAE) FROM ALBANY COUNTY, WYOMING

The ornithopod dinosaur Dryosaurus altus is a small herbivore known from the well-studied Morrison Formation of the North American Late Jurassic, which was deposited in an environment dominated by giant sauropods where smaller herbivores like Dryosaurus seem to have been comparatively rare. Although Dryosaurus is the best-known member of the ornithopod clade Dryosauridae, most specimens are quite fragmentary, and the Dryosauridae as a whole is not well-known. The “Middleton Dryosaurus,” donated to Amherst College from a private collection in 2013, is a very well-preserved individual collected from East Camarasaurus Quarry in southeastern Wyoming, in the Kimmeridgian-aged Brushy Basin Member of the Morrison Formation. This specimen is minimally distorted and preserves a considerable amount of the skull and postcranial skeleton. Three cervical vertebrae, five dorsal vertebrae, the sacrum and pelvic girdle, 23 caudal vertebrae, several ribs and chevrons, both forelimbs (except the manus), and both hindlimbs are completely or partially preserved; the right hindlimb is especially well-preserved, as all elements except digit I are present and most are complete. Anatomical description of the Middleton Dryosaurus provides additional information that is useful to understanding this genus and other members of Dryosauridae, which appears to have been a widespread group in the Late Jurassic and Early Cretaceous. For example, the several largely-complete caudal vertebrae in the Middleton Dryosaurus confirm differences in characters between this genus and other dryosaurids. Information on dryosaurid anatomy, especially from well-preserved specimens like the Middleton Dryosaurus, provides insight into the evolution of widespread and successful iguanodontian ornithopods from a body plan similar to that of more basal ornithischians. It also gives a basis for interpreting dryosaurid paleoecology, which could aid in understanding the structure of dinosaur faunas and the ecological role of and potential constraints on small herbivorous dinosaurs.