Dinosaurs' Rise Due to "Blind Luck," Study Says

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The researchers hoped to gauge the rate of evolution and the range of body shapes in the two groups.

To their surprise, the team found no difference in dinosaur and crurotarsan evolution rates.

"If dinosaurs were superior to crurotarsans, you might suspect they would be evolving faster, or that over time, crurotarsans would start evolving slower," Brusatte said.

Even more striking, the scientists said, was that crurotarsans had more diverse body types and occupied more niches than Triassic dinosaurs did.

Many Triassic crurotarsans looked nothing like crocodiles, and some had body shapes reminiscent of those which evolved much later in dinosaurs.

There were bulky four-legged predators; swift, graceful animals that ran on two legs; and tank-like herbivores covered in armor plates.

"If we were standing in the Late Triassic, 210 million years ago or so, and had to bet on which group would eventually dominate ecosystems, all reasonable gamblers would go with the crurotarsans," Brusatte said.

(Related: "Dino-Era Fossil Reveals Two-Footed Croc Relative" [January 25, 2006].)

"A Whole Different Horse"

But other scientists say attributing the dinosaurs' success to luck isn't a very satisfactory explanation.

"Organisms don't become extinct at random, and they don't succeed at random," said Kevin Padian, a paleontologist at the University of California, Berkley, who was not involved in the study.

If no differences existed between crurotarsans and dinosaurs, then it might be appropriate to talk about the chance elimination of one group and not the other, Padian said. But dinosaurs were different, he noted.

"[Dinosaurs] grew faster, they had higher metabolic rates, they were bipedal, and they were presumably more alert, agile, and lightly built," Padian said.

That dinosaurs and crurotarsans coexisted for tens of millions of years suggest the competitive edge dinosaurs had was not very great—at least not initially.

Their differences may have proved crucial only at the end of the Triassic, when life on Earth was threatened on a mass scale.

"Maybe dinosaurs didn't take over the world right away, but something in the end made them more successful," Padian said.

Brusatte, the study co-author, said that even if the dinosaurs were competing with the crurotarsans, it wasn't competition in the classical sense, and it was much briefer than previously thought.

"What our research has done has demolished this idea that dinosaurs and crurotarsans were competing, red in tooth and claw, for 30 million years across the Late Triassic," Brusatte said.

"Call it competition if you will, but it's a whole different horse. What we're essentially saying is that without the extinction [event], dinosaurs would have never made it."

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