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Dating the first dinosaurs

Geologists from Argentina and the U.S. today announced a new dinosaur that roamed what is now South America 230 million years ago at the beginning of the age of the dinosaurs. The discovery, backed up by careful dating of dinosaur fossils and the volcanic ash around them by researchers from UC Berkeley and UC Davis, casts doubt on the idea that dinosaurs appeared and opportunistically replaced other animals. Instead — at least in this valley — they seem to have existed side by side and gone through similar periods of extinction.

Eodramaeus head

See a picture gallery of the new dinosaur here

Eodramaeus, or “dawn runner” was a predatory dinosaur that walked (or ran) on two legs and weighed ten to 15 pounds. The new fossil is described in a paper by Ricardo Martinez and colleagues published in the journal Science today, Jan. 13. (See here for a news article from Science).

The fossils come from a valley in the foothills of the Andes in north-western Argentina. Over 200 million years ago, it was a rift valley on the western edge of the supercontinent Pangaea, surrounded by volcanoes. It’s one of the few places in the world where a piece of tectonically active continental margin has been preserved to the present day, said Isabel Montañez, a UC Davis geology professor and a coauthor on the Science paper.

Montañez, with Brian Currie from Miami University, Ohio and Paul Renne at UC Berkeley’s Geochronology Center have previously studied the ancient soils from the valley, dating layers of ash and studying how the climate changed. Those climate studies have been published previously.

There are two major boundaries in the geology of the era, Montañez said: The Carnian-Norian boundary at 228 million years ago and the Triassic-Jurassic transition at about 210 million years ago. Geologists have long thought that dinosaurs jumped in number and variety at both points, opportunistically replacing other reptiles.

But the carefully aged fossils from the South America show no such increase at the Carnian-Norian boundary, Montañez said. Rather dinosaurs were present and as diverse and abundant before the transition as later in the Jurassic, although several species of both dinosaurs and other animals went extinct at the boundary.

At that time, the climate in the valley changed from semi-arid, like the Mojave desert, to more humid. It’s not clear whether that was a global phenomenon or local to that area, Montanez said.

“Those dinosaurs were perfectly happy before the Carnian-Norian transition,” Montanez said. There’s no indication that the dinosaurs appeared and wiped out other animals, the prevailing hypothesis for the origin of dinosaurs based on fossils from North America and elsewhere in the world.

It may be that there are missing pieces from the fossil records elsewhere, Montanez said, noting that, “nowhere else is this well dated.”

More news stories about Eodramaeus: National Geographic (picture gallery), MSNBC, BBC (with video), University of Chicago press release (via Eurekalert)

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