NEW U-PB ZIRCON AGES FROM THE CHINLE FORMATION (WESTERN US) REVISE UNDERSTANDING OF LATE TRIASSIC TERRESTRIAL VERTEBRATE EVOLUTION

The Triassic is a critical interval for understanding the origin and evolution of modern terrestrial ecosystems, sandwiched between two major mass extinction events. Unfortunately, study of the tempo of Late Triassic biotic change is hampered by a lack of reliable radioisotopic constraints for both the timescale itself and important paleontological records. One of the best sampled and well-studied biotic records from the Late Triassic comes from the Chinle Formation on the Colorado Plateau of western North America. We present new ID-TIMS U-Pb single zircon ages from this unit that for the first time precisely constrain the age of this unit. Our median ²⁰⁶Pb/²³⁸U age of 218.7 +0.2/-0.4 Ma from a sandstone at the base of the Blue Mesa Member in western New Mexico indicates that most if not all of the Chinle Formation is Norian in age, in contrast to previous assignment of the lower part of the formation to the Carnian Stage. These data demonstrate that “Adamanian” vertebrate assemblages from the Chinle Formation are much younger than the South American “Ischigualastian” assemblages they had been correlated with. Our median age of 213.3 +1.1/-0.8 Ma from the Hayden Quarry in the Petrified Forest Member of northern New Mexico provides the first radioisotopic age constraints for the rich fossil record of this area, which also includes the Canjilon, Snyder, and Coelophysis quarries. This new maximum age constraint demonstrates that the rarity of early dinosaurs persisted much longer in western North America than in South America, where they were both diverse and abundant by 220 Ma. Finally, ongoing analyses of samples from Petrified Forest National Park in northern Arizona will help constrain the age of an abrupt mid-Norian biotic turnover. These examples, along with new U-Pb ages from marine strata, demonstrate the necessity of precise geochronological data for accurately reconstructing both regional and global patterns of biotic change in geologic deep time.