THE LATE TRIASSIC EARLY EVOLUTION OF DINOSAURS: A GEOCHRONOLOGIC PERSPECTIVE

During the Late Triassic several tetrapod groups that are considered important elements of the ‘modern’ fauna, including the Dinosauria, emerged in terrestrial ecosystems. The rise of early dinosaurs is known to have been closely associated with, and preceded by, a prominent radiation of dinosaur precursors known as basal dinosauromorphs. However, the mode and tempo of this crucial faunal transition remains poorly understood mainly due to a fragmentary fossil record, taxonomic uncertainties and speculative correlations among widely dispersed fossil occurrences. The apparent delayed arrival of basal dinosauromorphs and earliest dinosaurs in North America as compared to those found in South America has led workers to postulate a “diachronous evolution” model for the global emergence of dinosaurs.

We present a comprehensive chronostratigraphy for the Upper Triassic Chinle Formation of the Colorado Plateau in Southwest United States that records the first appearance of dinosauromorphs in North America, based on high-precision U-Pb (ID-TIMS) zircon geochronology of multiple, closely spaced, volcanogenic sedimentary beds. Our results significantly reduce the hypothesized time gap between the North and South American early dinosaur occurrences and further indicate that the transcontinental correlations are often obscured by extended hiatuses in both the rock and fossil records. The high-resolution Chinle chronostratigraphy documents a unique coexistence of basal dinosauromorphs and true dinosaurs in North America that persisted for a minimum of 12 m.y., following their first known appearance at ca. 223 Ma. The remarkable presence of advanced theropodan dinosaurs in this early North American assemblage reveals a faunal record that is neither complete, nor primitive, and thus cannot form the basis for construction of any simple global radiation and diversification model.