A Late Triassic Dinosauromorph Assemblage from New Mexico and the Rise of Dinosaurs
Randall B. Irmis,1*
Sterling J. Nesbitt,2,3*
Kevin Padian,1
Nathan D. Smith,4,5
Alan H. Turner,3
Daniel Woody,6
Alex Downs7
It has generally been thought that the first dinosaurs quickly replaced more archaic Late Triassic faunas, either by outcompeting them or when the more archaic faunas suddenly became extinct. Fossils from the Hayden Quarry, in the Upper Triassic Chinle Formation of New Mexico, and an analysis of other regional Upper Triassic assemblages instead imply that the transition was gradual. Some dinosaur relatives preserved in this Chinle assemblage belong to groups previously known only from the Middle and lowermost Upper Triassic outside North America. Thus, the transition may have extended for 15 to 20 million years and was probably diachronous at different paleolatitudes.
1 Museum of Paleontology and Department of Integrative Biology, University of California, Berkeley, CA 94720-4780, USA.
2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
3 Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA.
4 Committee on Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA.
5 Geology Department, Field Museum of Natural History, Chicago, IL 60605, USA.
6 Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USA.
7 Ruth Hall Museum of Paleontology, Ghost Ranch Conference Center, Abiquiu, NM 87510-9601, USA.
* To whom correspondence should be addressed. E-mail: irmis{at}berkeley.edu (R.B.I.); snesbitt{at}ldeo.columbia.edu (S.J.N.)
