Fossils uncovered in New Mexico show dinosaurs co-existed with a closely related group of reptiles for millions of years before overtaking them to become the dominant land animals on Earth.
The finding, detailed in the July 20 issue of the journal Science, suggests the dinosaurs' rise to dominance was a gradual ascent rather than a sudden takeover.
It challenges the notions that dinosaurs quickly replaced or out-competed their close relatives, the "dinosauromorphs," or that dinosauromorphs were long gone before the dinosaurs' appearance.
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"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."
Outwardly, dinosauromorphs resembled dinosaurs, 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, much as chimpanzees and humans later evolved from one species.
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.
One of the unearthed dinosauromorphs represented a completely new species, which the team dubbed Dromomeron romeri, after Alfred Sherwood Romer, the paleontologist who first described dinosauromorphs in the early 1970s.
The researchers think D. romeri was between three and five feet long (0.9 to 1.5 meters), and possibly bipedal.
A 'first order' discovery
Paul Olsen of Columbia University, who is an expert on Late Triassic and Jurassic ecosystems, called the new findings a "first-order major discovery."
"It shows that the Late Triassic was a time of unusually high diversity, something that we didn't realize at all before," Olsen said in a telephone interview. "These guys are finding wonderful and fantastic new things in what was formerly thought to be a pretty well trodden area."
Jack Horner, a paleontologist at Montana State University, said the new findings will force him to revise how he teaches his classes.
"I'm going to have to revise my lectures concerning the turnover rates of these animals," Horner said in an e-mail interview.
The finding "clearly illustrates the meager amount of information we have had concerning the Triassic Period and indicates that there is much more to be found if only people go out looking," Horner added.
Paul Sereno, a paleontologist at the University of Chicago, said that "all in all, it is heartening to see new bones and evidence from this earliest part of the dinosaur era."
However, Sereno found the idea of a gradual dinosaur takeover more speculative.
"The most important fact rising from these ... intriguing, but isolated bones is that some of the small proto-dinosaurs lived onward in time to overlap with true dinosaurs," Sereno said.
It could be that dinosaurs replaced dinosauromorphs at different rates in different parts of Pangaea, the super-continent that existed at the time.
Sereno notes that in South America, where the only other dinosauromorphs have been found, dinosaurs appeared to take over fairly quickly.
In any case, by the start of the Jurassic Period about 200 million years ago, dinosaurs were on their way to becoming the reigning land animals on Earth, and dinosauromorphs were gone.
Why one group survived and the other didn't is still a mystery.
"We don't know," study team member Nesbitt said. "Unfortunately, we can't answer that question yet."
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