New Mexico Geological Society Annual Spring Meeting — Abstracts

The Norian Bull Canyon Formation of east-central New Mexico yields the most diverse Late Triassic dinosaurian fauna in the world: (1) 2 new large herrerasauridsi (2) a new large ceratosaur and a small indeterminate form; (3) a new incertae sedis theropod; (4) the ornithischians Revueltosaurus and Lucianosaurusi and (5) an indeterminate prosauropod. In West Texas the Bull Canyon Formation also includes the ?theropod Protoavis, the ornithischian Technosaurus and an indeterminate prosauropod.

Three vertebrate taphofacies are present in the Bull Canyon Formation: (1) channel sandstone; (2) floodplain mudstone; and (3) paleosol. Channel-sandstone assemblages are characterized by isolated and abraded fragments, principally of phytosaurs. Floodplain-mudstone assemblages represent the majority of fossils and include skulls and rare, articulated skeletons of large tetrapods (femoral length > 30 cm), principally phytosaurs and aetosaurs. Paleosol assemblages are depauperate in aquatic/semiaquatic taxa (e. g., phytosaurs) and include articulated skeletons of small tetrapods (femoral length < 30 cm).

All dinosaurian specimens derive from paleosol-hosted assemblages. Thus, it appears that dinosaurs were an element of a community living in wellI drained environments and were absent from floodplain communities. Taphonomic studies in the Petrified Forest Formation at Petrified Forest National Park show similar distributional patterns for dinosaurian specimens -restricted to the paleosol taphofacies. The rarity of Late Triassic dinosaurs in western North America is mainly a function of the predominance of fossil localities that only sample the floodplain community. The few localities that sample the paleosol taphofacies contain abundant dinosaurian specimens.