Dinosaurs are often seen as unlucky, having been wiped out by an asteroid.

Yet they dominated Earth for more than 160 million years, evolving into a wild array of body types and sizes suited for many different ecological niches.

Scientists previously thought that it was this evolutionary diversity that enabled the dinosaurs' reign, allowing them to out-compete similar groups of reptiles, but a new study, detailed in the Sept. 11 issue of the journal Science, shows that it was really just a matter of luck.

"For a long time it was thought that there was something special about the dinosaurs that helped them become more successful during the Triassic, the first 30 million years of their history, but this isn't true," said lead author of the study, Steve Brusatte, a Ph.D. student at Columbia University and affiliate of the American Museum of Natural History in New York.

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The closest competitors to the dinosaurs during the Triassic period (about 251 to 199 million years ago) were the crurotarsans, the ancestors of today's crocodiles.

Both dinosaurs and crurotarsans evolved and filled some of the same ecological niches after a massive extinction event at the end of the Permian period some 250 million years ago.

Both groups also survived a less massive extinction event in the middle of the Triassic about 228 million years ago.

But only the dinosaurs, pterosaurs (and crocodiles) made it through a period of rapid global warming at the end of the Triassic 200 million years ago.

And avian dinosaurs are still with us today in the form of modern birds, which evolved from theropod dinosaurs and survived a separate and later mass extinction event at the end of the Mesozoic era and Cretaceous period.

Special or lucky?

To see if there was indeed something "special" about the non-avian dinosaurs that allowed them to out-compete the crurotarsans, Brusatte and his colleagues compared the reptiles' rates of evolution and morphological disparities — or range of different body plans.

The team used a database of 437 features from 64 skeletons and relied on a new family tree of the group Archosauria.

Archosaurs include dinosaurs and crurotarsans, plus their descendants modern birds and crocodilians, as well as pterosaurs, the large flying reptiles which co-existed with the dinosaurs.

The research was funded by the Marshall Scholarship and the Paleontological Society.

The researchers found no difference in the rates of evolution of the two groups.

Yet if dinosaurs were truly out-competing the crurotarsans, they should have been evolving more rapidly.

Crurotarsans also had a much higher disparity. In other words, they were exploring a wider range of body types, diets and lifestyles.

Again, this should have given them a leg up on the dinosaurs.

"If any of us were standing by during the Triassic and asked which group would rule the world for the next 130 million years, we would have identified the crurotarsans, not dinosaurs," Brusatte said.

The dinosaurs only surpassed the crurotarsans when most of the latter were killed off at the end of the Triassic and the dinos no longer had to compete with them.

So why did the dinosaurs survive the end-Triassic mass extinction event, while the crurotarsans (except for a few lineages of crocodiles) disappeared?

"We don't know the answer to that," Brusatte said, "but we suspect that it was nothing more than luck, plain and simple."

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