Rise of dinosaurs in Late Triassic more gradual than once thought

Berkeley -- Fossils discovered in the oft-painted arroyos of northern New Mexico show for the first time that dinosaurs and their non-dinosaur ancestors lived side by side for tens of millions of years, disproving the notion that dinosaurs rapidly replaced their supposedly outmoded predecessors.

The fossils were excavated from the Hayden Quarry at Ghost Ranch, an area made famous through the paintings of Georgia O'Keefe, by a team of paleontologists from the University of California, Berkeley, the American Museum of Natural History and The Field Museum. The finds, including fossil bones of a new dinosaur predecessor the researchers have named Dromomeron romeri, are described in a cover story in the July 20 issue of Science.

"Up to now, paleontologists have thought that dinosaur precursors disappeared long before the dinosaurs appeared, that their ancestors probably were out-competed and replaced by dinosaurs and didn't survive," said co-author Kevin Padian, professor of integrative biology at UC Berkeley and a curator in the campus's Museum of Paleontology. "Now, the evidence shows that they may have coexisted for 15 or 20 million years or more."

According to primary authors Randall Irmis and Sterling Nesbitt, graduate students, respectively, at UC Berkeley and at New York's American Museum, the new bones provide anatomical information that tells paleontologists about the evolution of dinosaur precursors, their transition into true dinosaurs and how dinosaurs diversified.

"Finding dinosaur precursors, or basal dinosauromorphs, together with dinosaurs tells us something about the pace of changeover," Irmis said. "If there was any competition between the precursors and dinosaurs, then it was a very prolonged competition."

An alternative hypothesis held that the sudden extinction of many animals in the Late Triassic period allowed dinosaurs to diversify and eventually populate the globe. Based on the new findings, ho

Contact: Robert Sanders
University of California - Berkeley

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