Samuel Bowring, MIT, EAR-1024196 David Fastovsky, Univ. Rhode Island, EAR-1023788
ABSTRACT PIs propose a program of high-precision U-Pb zircon geochronology on tuffaceous rocks preserved in the Late Triassic Chinle Formation of the southwestern United States. The Chinle is a fluvial sedimentary sequence that preserves a rich fossil record that reveals the ascendancy of dinosaur faunas and the shaping of the modern biota. Reconstructing the tempo and mode of paleoenvironmental and biotic evolution recorded in the Chinle, however, has been problematic. This is largely because although a detailed lithostratigraphy and biostratigraphy have been constructed, they are not well calibrated geochronologically. High-resolution isotopic dating of key fossiliferous Chinle outcrops, such as PIs propose, would allow the wealth of information that it contains to be integrated into the global Triassic timescale. The Chinle preserves a unique record of terrestrial biotic evolution and paleoclimate in a critical time interval of Earth history. Its sequential biostratigraphic assemblages, codified as Land Vertebrate Faunachrons, are core to the understanding of the early dinosaur evolution during a period of remarkable environmental change. Proposed high-precision temporal calibration of the Chinle litho- and bio-stratigraphy will greatly enhance their utility and will allow their integration into the global late Triassic time scale. EARTHTIME standards and tracer solutions will be used facilitating comparison between labs. The work proposed is associated with a national park with the potential of exposing an estimated 500,000 visitors per year from a wide spectrum of people to the results of this study in the form of interpretive exhibits and literature. PIs are also committed to the education of Native American communities who inhabit the land which is in part the subject of the research.
The earliest dinosaurs from North America are known from the Chinle Formation, southwest USA. Our goal was to date the rich Chinle biotas. Such dates would give us insights about the early evolution of dinosaurs. The work began in Petrified Forest National Park (PEFO), Arizona, USA. We: 1. established a bed-by-bed, measured stratigraphic framework through the thickness of PEFO; and 2. applied CA-TIMS (chemically abraded thermal ionization mass spectrometry). The former maximized the precision that could be applied to the locations of our samples; the latter maximized dating precision. Ultimately we obtained a suite of 11 high-precision dates across the thickness of the Chinle were able to date almost all of the fossils found in PEFO with heretofore unknown accuracy (Ramezani et al., 2011). These dates were published in Ramezani et al. 2011, and Atchley et al., 2013). The youngest date was 207.79+0.17 Ma; the oldest date was 227.604+0.082 Ma. Dating the Chinle accomplished several things. First, the Chinle became arguably the best-dated pre-Quaternary fluvial section in the world. This allowed for a precise study of the fluvial sequence stratigraphy in the unit (Atchley et al., 2013). Second, the age of the Chinle appeared to overlap that of the Ishigualasto Formation of Argentina. The oldest dinosaurs in the world are known from the Ischigualasto Formation. Having successfully constructed a robust, high-precision chronostratigraphy for PEFO, we moved outward, to other fossil-rich exposures of the Chinle. Using the same approach, we sampled for dating three major areas: the St. Johns, Arizona, region (south and east of PEFO), the Ward Terrace region (north and west of PEFO), and the Ghost Ranch, New Mexico region (east of PEFO). The St. Johns region contains a famous fossil-rich quarry, the Placerias quarry; Ward Terrace is a rich locality with a variety of vertebrate fossils said to be upper Chinle in age; and Ghost Ranch contains a famous quarry with abundant Coelophysis, a derived dinosaur typical of Chinle. These localities had been previously correlated with rock units identified in PEFO, on the basis of lithological similarity. In Ramezani et al. (2014), which came out of this second phase of study, we successfully dated the Placerias Quarry at 219.39+0.16 Ma. This showed that it was considerably younger than had been supposed previously using lithostratigraphy. Moreover, we made significant progress towards understanding the global rise of dinosaurs. Until then, many workers thought that a single evolutionary sweep of dinosaurs originating from South America, beginning with the Ischigualasto Formation, occurred in the pre-Jurassic time, as evidenced by a supposed delayed arrival of dinosaurs in North America and elsewhere. This was termed the "diachronous" hypothesis of dinosaur evolution. Because we determined that the Chinle significantly overlaps in time with the Ischigualasto, the diachronous hypothesis is an artifact of an incomplete fossil record, as well as miscorrelation of rocks within and between North and South America. Whereas the dinosaur precursors (basal dinosauromorphs), the most primitive dinosaurs, and the more derived theropod dinosaurs appeared in South America successively beginning in the mid-Triassic (about 240 million years ago), the three groups are now known to have coexisted in North America for a minimum of 12 million years from approximately 223 to 211 million years ago. More importantly, we now know that the critical 16 million-year fossil record of land vertebrates before 223 Ma is essentially missing from the North American continent, such that the oldest documented dinosaurs here are represented by relatively derived theropods. Given the unique nature of the North American dinosaur mixed assemblages in terms of age, completeness of record and phylogeny, the idea of diachronous dinosaur evolution is not well supported. Overall, we affirmed that high-precision geochronology is a powerful way – perhaps the only way – to resolve complex questions ranging from biotic and ecosystemic evolution, to paleoenvironmental and climate change. Atchley, S. C., Nordt, L. C., Dworkin, S. I., Ramezani, J., Parker, W. G., Ash, S. R., and Bowring, S. A., 2013, A linkage among Pangean tectonism, cyclic alluviation, climate change, and biologic turnover in the Late Triassic: The Record From The Chinle Formation, Southwestern United States:Journal of Sedimentary Research, v. 83, p. 1147-1161. Ramezani, J., Hoke, G. D., Fastovsky, D. E., Bowring, S. A., Therrien, F., Dworkin, S. I., Atchley, S. C., and Nordt, L. C., 2011, High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs: Geological Society of America Bulletin, v. 123, p. 2142-2159. Ramezani, J., Fastovsky, D.E., and Bowring, S.A., 2014, Revised chronostatigraphy 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, v. 314, p. 981 – 1008.
