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Age constraints on the dispersal of dinosaurs in the Late Triassic from magnetochronology of the Los Colorados Formation (Argentina)
Edited* by Neil D. Opdyke, University of Florida, Gainesville, FL, and approved April 17, 2014 (received for review February 7, 2014)

Significance
Uncertainties in reported 40Ar/39Ar dates from the Ischigualasto Formation of Argentina allow its dinosaur-bearing fauna to be Norian in age and possibly contemporaneous with some of the older U-Pb dated dinosaur-bearing units in the Chinle Formation of the American Southwest. Our magnetochronology of the previously undated Los Colorados Formation, which also contains a diverse dinosaur assemblage, constrains its age to the interval from 227 to 213 Ma (Norian) and thereby largely restricts the underlying Ischigualasto Formation to the Carnian. Rise of early dinosaurs was thus diachronous across the Americas with their dispersal from the austral temperate belt blocked until later in the Norian. The breakout may have resulted from critically lowered climatic barriers associated with decreasing atmospheric pCO2 levels.
Abstract
A measured magnetozone sequence defined by 24 sampling sites with normal polarity and 28 sites with reverse polarity characteristic magnetizations was established for the heretofore poorly age-constrained Los Colorados Formation and its dinosaur-bearing vertebrate fauna in the Ischigualasto–Villa Union continental rift basin of Argentina. The polarity pattern in this ∼600-m-thick red-bed section can be correlated to Chrons E7r to E15n of the Newark astrochronological polarity time scale. This represents a time interval from 227 to 213 Ma, indicating that the Los Colorados Formation is predominantly Norian in age, ending more than 11 My before the onset of the Jurassic. The magnetochronology confirms that the underlying Ischigualasto Formation and its vertebrate assemblages including some of the earliest known dinosaurs are of Carnian age. The oldest dated occurrences of vertebrate assemblages with dinosaurs in North America (Chinle Formation) are younger (Norian), and thus the rise of dinosaurs was diachronous across the Americas. Paleogeography of the Ischigualasto and Los Colorados Formations indicates prolonged residence in the austral temperate humid belt where a provincial vertebrate fauna with early dinosaurs may have incubated. Faunal dispersal across the Pangean supercontinent in the development of more cosmopolitan vertebrate assemblages later in the Norian may have been in response to reduced contrasts between climate zones and lowered barriers resulting from decreasing atmospheric pCO2 levels.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: dvk{at}rutgers.edu.
Author contributions: D.V.K. and O.A.A. designed research; D.V.K., P.S.M., C.E.C., O.A.A., and R.N.M. performed research; D.V.K. and P.S.M. analyzed data; D.V.K., P.S.M., C.E.C., and O.A.A. participated in fieldwork; O.A.A. initiated and organized field sampling expeditions; R.N.M. verified vertebrate paleontology associations; and D.V.K. wrote the paper.
The authors declare no conflict of interest.
↵*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1402369111/-/DCSupplemental.
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