Dinosaur Relatives and First Dinosaurs More Closely Connected Than Previously Thought: Precise Rock Dating Calls Ages of Other Fossil Sites across the Globe into Question
Animals escaping from an erupting volcano 235 million years ago in northwestern Argentina. These species, found as fossils in the Chanares Formation, include early mammal relatives (the dicynodont Dinodontosaurus in the left background, and the cynodont Massetognathus in the left foreground) and early dinosaur precursors (Lewisuchus in the right background, and Lagerpeton in the right foreground). By measuring radioactive isotopes in zircons crystals from the volcanic ash, scientists were able to determine the precise age of this fossil assemblage.
Credit: Victor Leshyk
The team found that the formation, and therefore the fossils found in it, is 234 to 236 million years old, from the Late Triassic Period; this is 5-10 million years younger than previous estimate of a Middle Triassic age.
“To discover that these early dinosaur relatives were geologically much younger than previously thought was totally unexpected,” said Irmis.
The results were recently published in the Proceedings of the National Academy of Sciences. The lead author is Claudia A. Marsicano from the University of Buenos Aires, and the co-authors are Irmis, Adriana C. Mancuso, Roland Mundil and Farid Chemale.
The Chañares Formation: The quintessential dinosauromorph cemetery
The Chañares Formation is an approximately 75-meter-thick-(250-foot) geologic formation comprised of sediments deposited by rivers, streams and lakes during the Triassic Period in present-day La Rioja Province, northwestern Argentina.
“Among Triassic geologic formations containing fossils, the Chañares Formation is a classic. It contains a variety of complete fossil specimens of early dinosauromorphs, which are essentially dinosaur aunts, uncles and cousins,” said Irmis.
Co-author Adriana Mancuso (far left) investigates badlands of the Chañares Formation, deposited 236 to 234 million years ago in what is now Talampaya National Park, Argentina.
Credit: Randall Irmis
Today the Chañares Formation’s landscape looks reminiscent of southern Utah — high desert badlands. Back when dinosauromorphs roamed, however, the Chañares Formation was more tropical and lush. At that point in time the Earth was in a hothouse state — extremely warm without any polar ice caps and high levels of carbon dioxide in the atmosphere.
The team collected two rock samples from the Chañares Formation: One from the base of the level where fossils were preserved and one from near the formation’s top.
“The Chañares Formation contains tuffs, or volcanic ashes, that can be dated with great accuracy. So in this basin, not only are there many high-quality, complete fossils that allow us to easily identify taxa, but there are also tuffs in between that allow us to date the entire column,” said Marsicano.
The rock samples include volcanic ash that erupted from volcanoes at the same time the sediments were deposited. The samples were then crushed to extract zircon crystals from this ash.
When these crystals formed during the Triassic Period, they only incorporated uranium; over time, some of this uranium decayed into lead at an exact known rate. By measuring the uranium/lead ratio in each zircon crystal with a mass spectrometer, an instrument that separates elements and isotopes by mass and concentration, the researchers were able to determine the precise date age the crystals formed.
“In a coarse way, you can place fossils in certain time periods, but if you wish to constrain a major evolutionary event, like the beginning of a group or the diversification of a group, the methodology has to have a margin of error that is much smaller. For something that happens over the course of, say, 600 million years, an error of 3-4 million years is not statistically significant. But if something happens over the course 10-12 million years, like the diversification of a group, an error of 3-4 million years is a major problem,” said Marsicano.
“By working with an international group that has the technological capability to date these rocks with great accuracy, we’ve dated the rocks for the first time.”
The radioisotopic dating demonstrated that the base formation sample is no older than 236 million years old and the top layer is no older than 235 million years old, with a possibility of being as young as 234 million years old. This demonstrates that the time elapsed between the appearance of early dinosauromorphs and the origin of dinosaurs is much shorter than previously thought.
Younger dinosauromorphs across the globe
The team’s results also call the timelines of a wide variety of other fossil-bearing formations across the world, all believed to be from the Middle Triassic Period, into question.
The new Late Triassic age for the Chañares Formation may hold true for the Santa Maria Formation in southern Brazil, which shares the same fossil species as the Chañares Formation, and the Karoo sequence in South Africa, to name a few. None of these other formations have been radioisotopically dated.
“We always thought these ‘Middle Triassic’ fossils showed the ecological recovery from the worst mass extinction of all time, the end-Permian extinction, but if these fossils are actually Late Triassic in age, they really have nothing to do with that recovery,” said Irmis.
Major funding for this study was provided by the Ann and Gordon Getty Foundation. Other funding was provided by the Universidad de Buenos Aires (University of Buenos Aires), University of Utah, Consejo Nacional de Investigaciones Científicas y Técnicas (National Scientific and Technical Research Council of Argentina), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (National Council for Scientific and Technological Development of Brazil). Fieldwork was conducted in Talampaya National Park under scientific permits issued by the Administración de Parques Nacionales (National Park Administration of Argentina) and the Province of La Rioja.
Contacts and sources:
University of Utah
Source: http://www.ineffableisland.com/2015/12/dinosaur-relatives-and-first-dinosaurs.html
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