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Age Constraints on the Dispersal of Dinosaurs in the Late Triassic from Magnetochronology of the Los Colorados Formation (Argentina) Dennis V

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Age Constraints on the Dispersal of Dinosaurs in the Late Triassic from Magnetochronology of the Los Colorados Formation (Argentina) Dennis V Age constraints on the dispersal of dinosaurs in the Late Triassic from magnetochronology of the Los Colorados Formation (Argentina) Dennis V. Kenta,b,1, Paula Santi Malnisc,d, Carina E. Colombic,d, Oscar A. Alcoberd, and Ricardo N. Martínezd aDepartment of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854; bLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964; cConsejo Nacional de Investigaciones Científicas y Técnicas, 1033 Buenos Aires, Argentina; and dInstituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan 5400, Argentina Edited* by Neil D. Opdyke, University of Florida, Gainesville, FL, and approved April 17, 2014 (received for review February 7, 2014) A measured magnetozone sequence defined by 24 sampling sites Placerias assemblage much younger than the Ischigualastian with normal polarity and 28 sites with reverse polarity character- fauna if the Herr Toba date is taken at face value. istic magnetizations was established for the heretofore poorly In contrast, Ramezani et al. (8) suggested that the rise of di- age-constrained Los Colorados Formation and its dinosaur-bearing nosaurs may have occurred at about the same time across the vertebrate fauna in the Ischigualasto–Villa Union continental rift Americas. Their new U-Pb zircon dates for seven tuffaceous basin of Argentina. The polarity pattern in this ∼600-m-thick red- horizons in the Chinle Formation at Petrified Forest National bed section can be correlated to Chrons E7r to E15n of the Newark Park, Arizona, indicated that the entire succession spans from astrochronological polarity time scale. This represents a time in- ∼225.0 to 207.8 Ma (or younger), with the Adamanian–Revueltian terval from 227 to 213 Ma, indicating that the Los Colorados For- faunal transition (9) between 219 and 213 Ma. More pertinently, mation is predominantly Norian in age, ending more than 11 My their assessment of the full dating error envelope for the Ischi- before the onset of the Jurassic. The magnetochronology confirms gualasto Formation, including 40Ar/39Ar data in a thesis (10), sug- that the underlying Ischigualasto Formation and its vertebrate gested that an age of ∼218 Ma (or younger) cannot be excluded for assemblages including some of the earliest known dinosaurs its contact with the overlying Los Colorados Formation. Such a are of Carnian age. The oldest dated occurrences of vertebrate EARTH, ATMOSPHERIC, younger age would allow a closer temporal correspondence AND PLANETARY SCIENCES assemblages with dinosaurs in North America (Chinle Formation) are younger (Norian), and thus the rise of dinosaurs was diachro- between the geographically separated assemblages, signifying nous across the Americas. Paleogeography of the Ischigualasto and that the Adamanian was effectively the age equivalent of the Los Colorados Formations indicates prolonged residence in the aus- Ischigualastian (11) and thus that there was virtually parallel tral temperate humid belt where a provincial vertebrate fauna development of early dinosaurs across the Americas. Olsen et al. with early dinosaurs may have incubated. Faunal dispersal across (12) also expressed doubts about the reliability of the dating of the Pangean supercontinent in the development of more cosmo- the Ischigualastian vertebrate assemblages that would necessarily politan vertebrate assemblages later in the Norian may have been make them of Carnian age. in response to reduced contrasts between climate zones and low- There are only two dated levels to formally constrain the nu- ered barriers resulting from decreasing atmospheric pCO2 levels. merical age of the Late Triassic epoch: a 230.1 ± 0.06 Ma U-Pb zircon date on a volcanic ash in late Carnian marine strata from magnetostratigraphy | paleolatitude | La Esquina | Adamanian southern Italy (13) and an age of 201.3 ± 0.18 Ma calculated for he leading candidate for the oldest known occurrence of Significance Tdinosaurs is the tetrapod assemblage of the Ischigualasto Formation of Argentina (1–4) where a preferred recalculated Uncertainties in reported 40Ar/39Ar dates from the Ischigua- 40Ar/39Ar date of 231.4 ± 0.3 Ma (1σ analytical uncertainty as lasto Formation of Argentina allow its dinosaur-bearing fauna reported) (5) on the Herr Toba tuff from near the base of the to be Norian in age and possibly contemporaneous with some formation points to a Carnian age for the dinosaur-bearing fauna. of the older U-Pb dated dinosaur-bearing units in the Chinle However, two recent studies of high-precision U-Pb zircon dates Formation of the American Southwest. Our magnetochronology from the Chinle Formation in the American Southwest, practically of the previously undated Los Colorados Formation, which also the only other Late Triassic strata with radioisotopic age con- contains a diverse dinosaur assemblage, constrains its age to the straints on vertebrate assemblages, arrive at very different interval from 227 to 213 Ma (Norian) and thereby largely restricts interpretations of the timing of the dispersal of dinosaurs the underlying Ischigualasto Formation to the Carnian. Rise of depending on the accepted degree of total uncertainty for the early dinosaurs was thus diachronous across the Americas with Ischigualasto 40Ar/39Ar data. their dispersal from the austral temperate belt blocked until later The study by Irmis et al. (6) advocated a diachronous rise of in the Norian. The breakout may have resulted from critically dinosaurs, starting in the Ischigualasto Formation of Argentina lowered climatic barriers associated with decreasing atmospheric in the Southern Hemisphere and appearing only later in the pCO2 levels. Chinle Formation. They pointed to a new U-Pb zircon date of 211.9 ± 0.7 Ma (2σ uncertainty as conventionally reported for 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., U-Pb dates) for a dinosaur-bearing vertebrate assemblage in Hay- and O.A.A. participated in fieldwork; O.A.A. initiated and organized field sampling ex- den Quarry at Ghost Ranch, New Mexico, that was considerably peditions; R.N.M. verified vertebrate paleontology associations; and D.V.K. wrote younger than the nominal dates from the Ischigualasto Forma- the paper. tion. The Placerias Quarry in northeastern Arizona apparently The authors declare no conflict of interest. contains even older dinosaurs from the Chinle Formation (7), *This Direct Submission article had a prearranged editor. and although it was not dated directly, Irmis et al. (6) suggested 1To whom correspondence should be addressed. E-mail: [email protected]. that a new U-Pb zircon date of 218.1 ± 0.7 Ma from presumably This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. age-correlative strata in New Mexico would make even the 1073/pnas.1402369111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1402369111 PNAS Early Edition | 1of6 – the Triassic Jurassic boundary from U-Pb zircon dates on volcanic Ischigualasto-Villa Union ashes bracketing the boundary in ammonite-bearing sediments K? Cerro from Peru (14). In the current absence of other reliable radio- Rajado isotopic age controls on fossiliferous marine strata that are the La Esquina basis for a global chronostratigraphy, correlations to the Newark Los astrochronological polarity time scale [APTS (15)] have provided Colorados 3000 important constraints on ages for standard subdivisions of the Talampaya National – Late Late Triassic (16 18), which have largely been adopted in re- Park cently published geologic time scales (19, 20). High-precision Ischigualasto 2500 U-Pb geochronology on earliest Jurassic volcanics of the Central Atlantic Magmatic Province (CAMP) interbedded with sediments 2000 3500 30°00’ S 30°00’ in the upper part of the Newark continental rift sequence Los Valle Fértil Megafault Ischigualasto strongly affirms the astrochronological methodology (21). Rastros Provincial Triassic Park 1500 Over the entire 35-My-long Late Triassic epoch, there are Bermejo ∼ Middle La Sal only four land vertebrate biozones recognized in North America Ischichuca and just two in South America (11). The low temporal resolution Chanares 1000 Gallinita combined with endemism of faunas for the Late Triassic make it Tarjados difficult to disentangle temporal and spatial components gov- 500 Morado Early Agua de la Peña erning the distribution of dispersed vertebrate assemblages and Hill Talampaya El Alto Fault for progress requires age control aside from biostratigraphy. In this 010km 0 regard, dating of the Los Colorados Formation would help de- P-C Paganzo Group m termine if the underlying Ischigualasto Formation extends into the 68°00’ W Norian or is confined to the Carnian and if the temporal range of Fig. 1. Columnar section of rock formations (Left)andlocationandgeo- the dinosaur-bearing Coloradian fauna actually extends to the end logic sketch map (Right) of the Ischigualasto–Villa Union basin where map of the Triassic as sometimes supposed (e.g., 11). The apparent patterns for formations in the basin are keyed to the columnar section (with absence of volcanic ash layers suitable for radioisotopic dating in the basalts near Morado Hill shown as gray fill). The Valle Fertil megafault Los Colorados Formation motivated this magnetostratigraphic represents Neogene tectonic inversion activity that exhumed the Mesozoic study of the unit and enabled us to address these objectives.
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