'Dinosaurs rose gradually'
2007-07-23 09:48
Washington - Fossils found in the United States show dinosaurs lived along side their ancestors for tens of millions of years, disproving long-held theories assuming they quickly replaced their predecessors, according to a new study.
Dinosaurs appeared at the end of the Triassic period about 235 to 200 million years ago and came to dominate the planet during the Jurassic period about 200 to 120 million years ago.
"Up to now, palaeontologists 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 Kevin Padian, professor of integrative biology at the University of California at Berkeley and one of authors of the study.
"Now, the evidence shows that they may have coexisted for 15 or 20 million years or more," said Padian, whose findings were published in Friday's edition of the journal Science.
235 million years old
The study was based on an analysis of fossils discovered in the desert of New Mexico at the Hayden Quarry by a team of palaeontologists from the UC Berkeley, the American Museum of Natural History and The Field Museum in Chicago.
The bones reveal anatomical information that sheds light on the evolution of dinosaur ancestors, the study said.
The newly discovered fossil, about 235 million years old, is a dinosaur predecessor called Dromomeron romeri.
Lead authors Randall Irmis and Sterling Nesbitt of the University of California and New York's American Museum respectively, said the new fossils shed light on anatomical evolution in dinosaurs' ancestors, their transition into dinosaurs and the ways in which dinosaurs diversified.
"Finding dinosaur precursors ... together with dinosaurs tells us something about the pace of changeover," said Irmis. "If there was any competition between the precursors and the dinosaurs, then it was a very prolonged competition."
Jurassic period
An alternative hypothesis was that the sudden extinction of many animal species at the end of the Triassic period helped dinosaurs diversify and dominate around the world.
But the new findings of New Mexico fossils show "quite a few of the groups proposed to go extinct survived well into the Late Triassic," Irmis added.
Dinosaurs and many other animals including mammals, lizards, crocodiles, turtles and frogs appeared toward the end of the Triassic, 235-200 million years ago.
But it was only during the Jurassic period, 200-120 million years back, that dinosaurs dominated the Earth and their predecessors died out.
Dinosaur fossils from the late Triassic period were rare until 2003 when a creature dubbed the Silesaurus was found in Poland.