When most people imagine the age of dinosaurs, we think of giant beasts rising above competitors and evolving to a position of dominance some 200 million years ago, a time when they ruled the Earth.

But British and U.S. researchers suggest in a study published Thursday in the journal Science that the dominance of dinosaurs owed more to good fortune than any inherent evolutionary advantage.

In short, they got lucky.

The researchers from the University of Bristol and Columbia University looked at how dinosaurs spread throughout the land in the late Triassic period, while a rival group of archosaurs, or "ruling reptiles," called crurotarsans mostly died out.

Near the beginning of the Triassic period, 250 million years ago, dinosaurs and crurotarsans split off into two distinct groups. A few species of crurotarsans live on today — crocodiles and alligators, for example — but at their height in the middle Triassic, roughly 230 million years ago, they were the dominant terrestrial carnivores and herbivores.

The prevailing theory is that the dinosaurs "outcompeted" the crurotarsans, paving the way for their future expansion and the rise of later dinosaurs like the carnivorous Tyrannosaurus Rex, horned Triceratops and giant Supersaurus.

But for that theory to hold water, the researchers said they would have expected to find both greater diversity and faster evolutionary rates among dinosaurs during the period when the two rival groups lived side-by-side.

In other words, if dinosaurs had really outcompeted their crocodile-like cousins, it should have shown up in faster and more diverse changes in skull and skeleton characteristics during that time frame.

To test the theory, they looked at skeletal data from 64 different taxa, or major groups of the dinosaurs and crurotarsans.

What they found instead was little difference in evolutionary rates and greater diversity in crurotarsans.

That counterintuitive find leads the scientists to suggest a cataclysmic event at or near the boundary of the Triassic and subsequent Jurassic period — some 200 million years ago — was more likely to be responsible for the decline of the crurotarsans, though they could not elaborate on what that event was.

"It is difficult to explain why crurotarsans and not dinosaurs went extinct at [that time], which may have been a catastrophic event or an ecologically drawn-out affair triggered by eruption and elevated CO2 levels," the authors wrote.

"Either way, as in most mass extinction events, the death of species is often more random than ecologically selective."

It was the second such timely extinction for dinosaurs, who along with crurotarsans at around 230 million years ago were the beneficiaries of the extinction of a number of competing land animals from the early Triassic period.

"It is likely that dinosaurs were the beneficiaries of two mass extinction events — and some good luck," the authors wrote.

But like a gambler who spends too much time in the casino, the dinosaurs' luck eventually ran out. They were wiped out by their own cataclysmic event some 65 million years ago.