High-resolution U–Pb ages from the Upper Triassic Chinle Formation (New Mexico, USA) support a diachronous rise of dinosaurs Randall B. Irmis a, ⁎, Roland Mundil b , Jeffrey W. Martz c , William G. Parker c, d a Utah Museum of Natural History and Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112-0050, USA b Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA c Division of Resource Management, Petrified Forest National Park, P.O. Box 2217, Petrified Forest, AZ 86028, USA d Department of Geological Sciences, 1 University Station-C1100, The University of Texas at Austin, Austin, TX 78712, USA abstract article info Article history: Received 4 April 2011 Received in revised form 12 July 2011 Accepted 14 July 2011 Available online 12 August 2011 Editor: G. Henderson Keywords: Triassic timescale geochronology biostratigraphy Dinosauria Colorado Plateau Ischigualasto Formation Though the Late Triassic preserves major paleoenvironmental fluctuations and is key for understanding the evolution of Mesozoic and modern terrestrial ecosystems, comparisons of Late Triassic non-marine sedimentary and fossil records are difficult because global correlations lack precise radioisotopic ages, and have instead been based upon unconstrained biostratigraphic ranges of palynomorph and vertebrate fossils. The Chinle Formation in southwestern North America preserves a major Late Triassic record of paleoenvironmental and biotic change, including significant early dinosaur fossils. Previous high-resolution radioisotopic age constraints for the formation are limited to a single U–Pb zircon age from the upper third of the formation. The extraction of a geologically meaningful age is challenging from these redeposited units and preference is given to considering the youngest age of a deposit as a maximum age and closest approximation of the depositional age. Because calculating a weighted mean age (or median age) from a group of ages from such deposits is often not adequate, the precision of our two new CA-TIMS single crystal zircon U–Pb ages from the Chinle Formation of New Mexico is limited to ca 0.3% (or ± 0.7 Ma) of the youngest crystal age. Our 206 Pb/ 238 U age of ~218 Ma from the Blue Mesa Member in Six Mile Canyon, western New Mexico, demonstrates that strata, palynomorphs, and vertebrate fossils previously considered to be late Carnian in age are actually middle Norian in age. Our new age of ~ 212 Ma from the Hayden Quarry within the Petrified Forest Member at Ghost Ranch, northern New Mexico, provides the first maximum age for important vertebrate assemblages from this area that record the rise of dinosaurs, and demonstrates that basal dinosauromorphs (‘dinosaur precursors’) co-existed with dinosaurs for at least 18 Ma. These new radioisotopic data allow a new correlation of the Chinle Formation to the Late Triassic timescale, suggesting that most if not all of the lower Chinle is Norian in age. This new correlation has global implications as it allows us to make more precise comparisons with early dinosaur assemblages from the Ischigualasto Formation of Argentina, indicating that Chinle dinosaur assemblages are significantly younger than those from South America. The revised age of the Chinle Formation also demonstrates that dinosaurs were much rarer in North America at a time when they were abundant in South America, supporting hypotheses of paleolatitudinal variation during the rise of dinosaurs. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The Late Triassic Epoch was a critical time in earth history that encompassed major changes in global biotic and climate systems (e.g., Irmis and Whiteside, 2010). These events included final recovery from the largest Phanerozoic mass extinction (e.g., Payne et al., 2004), the onset of another mass extinction (e.g., Hesselbo et al., 2002; Schoene et al., 2010; Whiteside et al., 2010), major changes in the concentrations of greenhouse gases (e.g., Berner, 2006; Whiteside et al., 2010), and the origin and rise of dinosaurs (e.g., Brusatte et al., 2010; Langer et al., 2010; Rogers et al., 1993). Unfortunately, the lack of radioisotopic ages for the Late Triassic (Mundil, 2007; Mundil et al., 2010) limits the ability to resolve the relative timing and absolute tempo of these events across the globe (Irmis et al., 2010). For the ~35 Ma of the Late Triassic preceding the Triassic–Jurassic boundary interval, there are only three published U–Pb and 40 Ar/ 39 Ar ages (Furin et al., 2006; Mundil, 2007; Mundil et al., 2010; Riggs et al., 2003; Rogers et al., 1993). This lack of data means that the duration of major Late Triassic earth-history events, and the correlation of the individual sections containing the records of these events, is poorly known. Earth and Planetary Science Letters 309 (2011) 258–267 ⁎ Corresponding author at: Utah Museum of Natural History and Department of Geology & Geophysics, University of Utah, 1390 E. Presidents Circle, Salt Lake City, UT 84112-0050, USA. Tel.: + 1 8015850561; fax: + 1 8015853684. E-mail addresses: irmis@umnh.utah.edu (R.B. Irmis), rmundil@bgc.org (R. Mundil). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.07.015 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl