EVALUATING LATE TRIASSIC FAUNAL CHANGE AND THE RISE OF DINOSAURS: EXAMPLES FROM THE CHINLE FORMATION OF NORTH-CENTRAL NEW MEXICO

Testing hypotheses of biotic turnover in the fossil record requires well-sampled and stratigraphically precise records from multiple locations through time and space. Although many hypotheses have been put forth regarding Late Triassic faunal change and the early diversification of dinosaurs, few of the necessary detailed records exist to appropriately test these ideas. The Upper Triassic Chinle Formation in the Chama Basin of north-central New Mexico is one area that is ideally suited for this task because it contains abundant vertebrate fossils, including early dinosaurs, from a variety of well known localities with large sample sizes. I placed known localities in a precise stratigraphic framework and created a specimen-level database to evaluate the biotic record preserved in this basin. These data indicate that overall vertebrate assemblage composition is stable through time; there is only minor species-level turnover. Furthermore, comparison with available paleoenvironmental proxy data indicates that there is no correlation between vertebrate faunal change and climate change. Dinosaurs comprise a minor component of assemblages throughout the preserved interval, with the exception of the taphonomically unusual Coelophysis Quarry, suggesting they did not become an abundant component of the fauna until the Early Jurassic Period. The vertebrate assemblages from New Mexico are more diverse than penecontemporaneous records from northern Arizona, possibly reflecting faunal differences between the edge and center of the Chinle depositional basin.