Instead of an abrupt takeover, the dinosaurs' rise to dominance on land may have been gradual as they co-existed with a closely related group of reptiles for millions of years, fossils found in New Mexico reveal.
The finding, published in the July 20 issue of the journal Science, challenges the theory that dinosaurs quickly replaced or out-competed their close relatives, the "dinosauromorphs," or that dinosauromorphs were extinct before dinosaurs arrived on the scene.
"Up to now, paleontologists have thought that dinosaur precursors disappeared long before the dinosaurs appeared," said study team member Kevin Padian of the University of California, Berkeley. "Now, the evidence shows that they may have coexisted for 15 or 20 million years or more."
Dinosauromorphs resembled dinosaurs on the exterior, but they lacked some of the anatomical features of true dinosaurs, such as a hole in the pelvis for the femur. The two groups were very closely related and likely descended from a common ancestor.
Because fossils of the two groups had never been found together, some scientists had proposed that dinosaurs replaced dinosauromorphs relatively quickly by either out-competing them or filling in their ecological niches following some major extinction event.
"Dinosauromorphs that were known before our study were from a time when true dinosaurs never existed," said study team member Sterling Nesbitt of the American Museum of Natural History in New York City.
The new cache of fossils, discovered in the Hayden Quarry of New Mexico, challenges this "lucky break" scenario in which dinosaurs rapidly replace dinosauromorphs. The fossils include remains of various dinosaurs — including therapods, the group that gave rise to birds and to which T. rex belongs — and early dinosauromorphs, including some from groups previously thought to have been extinct by the late stages of the Triassic Period. (The Triassic spanned from about 235 to 200 million years ago.)
"If there was any competition between the precursors and the dinosaurs, then it was a very prolonged competition," said study team member Randall Irmis, also of UC Berkeley.
Source: Xinhua/agencies
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