title: The precise temporal calibration of dinosaur origins.
abstract: Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic-Jurassic boundary.
authors.affiliation: Departamento de Ciencias Geológicas, Instituto de Estudios Andinos-Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad de Buenos Aires, Ciudad Autónoma de Buenos Aires C1428EHA, Argentina;
authors.forename: Claudia A
authors.initials: CA
authors.lastname: Marsicano
authors.affiliation: Natural History Museum of Utah, University of Utah, Salt Lake City, UT 84108-1214; Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112-0102; irmis@umnh.utah.edu.
authors.forename: Randall B
authors.initials: RB
authors.lastname: Irmis
authors.affiliation: Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Centro Científico y Tecnológico-Consejo Nacional de Investigaciones Científicas y Técnicas-Mendoza, Mendoza CC330, Argentina;
authors.forename: Adriana C
authors.initials: AC
authors.lastname: Mancuso
authors.affiliation: Berkeley Geochronology Center, Berkeley, CA 94709;
authors.forename: Roland
authors.initials: R
authors.lastname: Mundil
authors.affiliation: Instituto de Geociências, Universidade de Brasília, Brasília-DF 70864-050, Brazil.
authors.forename: Farid
authors.initials: F
authors.lastname: Chemale
mesh_terms: D000818:Animals; D001118:Argentina; D005075:Biological Evolution; D002138:Calibration; D025061:Dinosaurs / Q000033:anatomy & histology*; D005843:Geography; D013997:Time Factors
keywords: Chañares Formation; Triassic; biostratigraphy; dinosaur origins; geochronology