EXAMINING LATE TRIASSIC (NORIAN-RHAETIAN) TERRESTRIAL FAUNAL ASSEMBLAGE COMPOSITIONS IN THE CHINLE FORMATION OF NORTHEASTERN ARIZONA; IMPLICATIONS FOR TRIASSIC EXTINCTION EVENTS

The Triassic Period is the only geological period in Earth history bracketed by two major extinction events, the end-Permian and end-Triassic events. The Triassic is also punctuated by other smaller-scale extinction events that mainly pertain to epoch and/or stage boundaries. The Chinle Formation of western North America potentially preserves a mid-Norian extinction event represented by the boundary between two terrestrial biozones, the Adamanian and Revueltian. This turnover affects primarily aetosaurian and phytosaurian archosauriform reptiles, but also local palynology. This event has been preliminarily dated to between 214-216 Ma and roughly coincides with the Manicouagan impact event in Quebec. Ongoing work at Petrified Forest National Park in Arizona, where the Chinle Formation is exposed extensively, seeks to enhance our understanding of the Adamanian and Revueltian vertebrate assemblages to document other taxa potentially affected by this turnover. Recent new finds include a variety of vertebrate taxa, punctuated by an emphasis on the microvertebrate record. These include new records of actinopterygians, temnospondyl amphibians, diapsid reptiles, and a variety of archosauromorph reptiles. These records show a potential step-wise reduction in species richness as certain clades go extinct in the last 25 million years prior to the end-Triassic mass extinction. The Late Triassic dinosaur record of western North America shows an extinction of non-dinosaurian dinosauromorphs, leading to the introduction of sauropodomorphs and ornithischians, radiation of coelophysoid theropods, and dramatic increases in body size in the Early Jurassic.