New Mexico Geological Society Annual Spring Meeting — Abstracts

The Redonda Formation (Upper Triassic: Rhaetian) is the uppermost preserved unit of the Chinle Group in eastern New Mexico Contemporaneous strata in Colorado and Utah preserve abundant fish and footprints, but the only significant tetrapod bodyfossils of Rhaetian age in western North America are from the Redonda Formation in Quay County, NM and the Rock Point Formation in Rio Arriba County, NM. The Rock Point assemblage is from the Ghost Ranch quarry and yields a low diversity fauna dominated by the ceratosaurian dinosaur Coelophysis. The Redonda produces a much more varied vertebrate fauna including phytosaurs, aetosaurs, rauisuchians and sphenosuchians. The Redonda Formation also yields numerous ichnotaxa that include tridactyl theropod dinosaur tracks (Grallator) and tetradactyl prints of prosauropod dinosaurs ("Tetrasauropus", "Pseudofefrasauropus''). However, until recently there were no osteological specimens of dinosaurs. This is somewhat surprising given the abundance of dinosaur specimens at the Ghost Ranch quarry, but the northern New Mexico fauna is very terrestrial in aspect whereas the Redonda fauna includes many sem-aquatic organisms. However, there are now two localities in the Redonda Formation that have yielded theropod bones.

The first specimen consists of a partial ischium from the uppermost portion of the Redonda Formation -above the massive yellow sandstone bench that is present in the fluvial interval of the Redonda Formation in eastern Quay County. The second specimen is only partially excavated and occurs stratigraphically just below the Redonda "ledge" -the uppermost lacustrine shoreline calcarenite in eastern Quay County. This specimen includes at least one partial femur, a podial and teeth. Both specimens represent theropods more derived than Herrerasauridae with hip heights of about 1 m. Neither specimen is generically determinate and both may represent the same taxon. These are the first dinosaur body-fossils recorded from the Redonda Formation and represent the only Rhaetian dinosaur specimens from western North America outside the Ghost Ranch quarry.