REVISED CHRONOSTRATIGRAPHY AND CORRELATION OF THE EARLIEST DINOSAUR-BEARING STRATA IN THE AMERICAN SOUTHWEST: U-PB GEOCHRONOLOGIC CONSTRAINTS ON THE LATE TRIASSIC TETRAPOD EVOLUTION

The distribution in space and time of the earliest dinosaurs and their closest non-dinosaurian relatives – together, the clade Dinosauromorpha – is key to unraveling the early evolution and diversification of dinosaurs. Faunal assemblages involving basal dinosauromorphs are recorded in a few Upper Triassic terrestrial sedimentary successions, widely scattered throughout the Pangean Supercontinent. Nevertheless, correlation of the relevant rocks and fossils and their integration into the global Triassic remain problematic, largely due to the lack of reliable radioisotopic age data, and reliance on uncalibrated bio- and magneto-stratigraphy.

Central to our understanding of the patterns and tempo of early dinosaur evolution is the correlation of dinosauromorph-bearing faunal assemblages between the Late Triassic Chinle Formation of the Colorado Plateau and those of the Ischigualasto-Villa Union Basin (Argentina) and the Paraná Basin (Brazil) in South America. Here we present new data to supplement the comprehensive geochronologic framework of the Chinle Formation established based on high-precision U-Pb ID-TIMS analyses of zircon from closely-spaced tuffaceous beds in multiple stratigraphic sections. These provide new robust correlations among geographically distant Chinle strata that were deposited between ~228.1 Ma and ~207.2 Ma during Norian. Our results indicate that the first documented appearance of dinosauromorphs in the Chinle Formation followed a ~9 m.y. gap in the North American tetrapod record, which itself was preceded by a ~10 m.y. depositional hiatus associated with the sub-Chinle regional Tr-3 unconformity. This ~19 m.y. interval, in which no fossils are preserved, coincides in age with the dinosauromorph-bearing Chañares, Los Rastros and the basal Ischigualasto Formations from the Ischigualasto-Villa Union Basin of Northwest Argentina. Consequently, the prominent absence of the pre-Norian dinosauromorphs from North America appears to be an artifact of the missing rock and/or fossil record, rather than manifestation of a diachronous evolutionary process.