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Early dinosaur evolution and revised chronostratigraphy of the Lower Chinle Formation

Ben Creisler

News release:

Rise of the dinosaurs


A recent paper not yet mentioned:

Jahandar Ramezani, David E. Fastovsky, and Samuel A. Bowring (2014)
Revised chronostratigraphy of the Lower Chinle Formation strata in
Arizona and New Mexico (USA): High-precision U-Pb geochronological
constraints on the Late Triassic evolution of dinosaurs.
American Journal of Science 314(6):  981-1008
doi: 10.2475/06.2014.01

The early history of dinosaurs in North America is obscured by an
incomplete fossil record, taxonomic uncertainties and speculative
correlations of tetrapod-bearing rocks, as well as poor calibration of
the Late Triassic time scale. High-precision U-Pb geochronology
provides a reliable means of correlating terrestrial rock formations
independent of equivocal lithostratigraphy or vertebrate
biostratigraphy, and hence the possibility of properly evaluating
models for the early radiation and diversification of Dinosauria. Here
we present new, high-precision, U-Pb ID-TIMS zircon geochronology from
the presumed lowermost strata of the Upper Triassic Chinle Formation
of the Colorado Plateau in Southwest United States, including a mean
206Pb/238U date of 219.39 ± 0.16 Ma from the renowned Placerias Quarry
Bone Bed in eastern Arizona. The new results prompt revisions to the
chronostratigraphy of the lower Chinle and provide a new temporal
context for its rich tetrapod fauna.

The oldest documented dinosaurs of North America coexisted with their
non-dinosaurian near-relatives for a minimum of 12 m.y., from ca. 223
Ma to ca. 211 Ma, in the Norian. This early dinosauromorph record
follows a ca. 6 m.y. period from which no tetrapod fossils have been
documented and which was itself preceded by a ca. 10 m.y. depositional
hiatus spanning nearly the entire Ladinian and Carnian stages of the
terrestrial North America. The supposed late appearance of
dinosauromorphs in North America compared to those in South America
thus appears to be an artifact of incomplete preservation, as well as
unsubstantiated age interpretations. This, together with the
conspicuous biogeographic distinctions among the Triassic
dinosauromorph assemblages, invalidates a simple diachronous model for
the transcontinental radiation of early dinosaurs.