TRIASSIC CHRONOSTRATIGRAPHY OF THE COLORADO PLATEAU (SOUTHWEST UNITED STATES) AND THE PROBLEM OF FIRST APPEARANCE OF DINOSAURS IN NORTH AMERICA

The Colorado Plateau of the Southwest USA preserves an early Mesozoic terrestrial sedimentary succession with important ramifications for the early evolution of Dinosauria. Revisions to the historically litho- and bio-stratigraphy-based correlations made possible by high-precision U-Pb geochronology now provide a robust temporal context for the earliest North American dinosauromorph assemblages.

The oldest Mesozoic deposits on the Colorado Plateau are the disconformity-bounded Moenkopi Formation (Fm.), a Middle Triassic (Anisian) unit containing a temnospondyl-dominated fauna with no ornithodirans. The earliest fossil record of dinosauromrphs in North America is preserved in the fluvial rocks of ca. 229 to <208 Ma Chinle Fm. that overlie the Moenkopi Fm. with a ca. 10 Myr hiatus. Eleven CA-ID-TIMS U-Pb dates from the Petrified Forest National Park (PEFO) in Arizona establish a detailed chronostratigraphy for the rich vertebrate faunal record of the Chinle Fm. Strata representing the first ca. 6 Myr of Chinle deposition appear barren of any fossils. The known range of the basal (non-dinosaur) dinosauromorphs is ca. 220 – 210 Ma and that of true dinosaurs is ca. 223 – 211 Ma. The PEFO tetrapod record highlights the coexistence of a mixed assemblage of hererrasaurid theropods (Chindesaurus), coelophysoid neotheropods (Coelophysis; Camposaurus), and basal dinosauromorphs (Dromomeron; silesaurids) for a minimum duration of 12 Myr in the Late Triassic. Although previous correlations placed other fossiliferous localities lower in the Chinle Fm., U-Pb age calibration reveals their ages to be uniformly younger. These localities include the Placerias Quarry, AZ (coelophysoids; Dromomeron); Ward Terrace, AZ; and the Blue Water Creek, NM.

Stratigraphic consideration indicate that the Chinle record does not capture the full temporal ranges of basal dinosauromorphs or primitive dinosaurs in North America. The sudden appearance of a mixed assemblage including evolutionarily advanced components following a ca. 16 Myr depositional and/or preservational gap underscores an incomplete record with an equivocal evolutionary signal. Contrary to their South American counterparts, the dinosaurs of North America did not supplant their close basal dinosauromorph relatives, but lived with them for at least 12 Myr.