The precise temporal calibration of dinosaur origins
Claudia A. Marsicano a, Randall B. Irmis b,c,1, Adriana C. Mancuso d, Roland Mundil e, and Farid Chemale f
aDepartamento 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;
bNatural History Museum of Utah, University of Utah, Salt Lake City, UT 84108-1214;
cDepartment of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112-0102;
dInstituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Centro Científico y Tecnológico-Consejo Nacional de Investigaciones Científicas y Técnicas-Mendoza, Mendoza CC330, Argentina;
eBerkeley Geochronology Center, Berkeley, CA 94709;
fInstituto de Geociências, Universidade de Brasília, Brasília-DF 70864-050, Brazil
Edited by Paul E. Olsen, Columbia University, Palisades, NY, and approved November 6, 2015 (received for review June 25, 2015)
Source/Fonte: Victor Leshyk - Nature
Significance
Many hypotheses have been put forth to explain the origin and early radiation of dinosaurs, but poor age constraints for constituent fossil assemblages make these scenarios difficult to test. Using precise radioisotopic ages, we demonstrate that the temporal gap between assemblages containing only dinosaur precursors and those with the first dinosaurs was 5–10 million years shorter than previously thought. Thus, these data suggest that the origin of dinosaurs was a relatively rapid evolutionary event. Combined with our synthesis of paleoecological data, we demonstrate there was little compositional difference between the dinosaur precursor assemblages and the earliest dinosaur assemblages, and thus, the initial appearance of dinosaurs was not associated with a fundamental shift in ecosystem composition, as classically stated.
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.
dinosaur origins Chañares Formation geochronology Triassic biostratigraphy
Footnotes
1To whom correspondence should be addressed. Email: irmis{at}umnh.utah.edu.
Author contributions: C.A.M., R.B.I., and A.C.M. designed research; C.A.M., R.B.I., and A.C.M. performed research; R.M. and F.C. contributed new reagents/analytic tools; C.A.M., R.B.I., A.C.M., R.M., and F.C. analyzed data; and C.A.M., R.B.I., A.C.M., R.M., and F.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1512541112/-/DCSupplemental.
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