Dinosaurs' long reign on Earth may have had more to do with lady luck than with superiority, according to a study published today in Science. The study challenges the old notion that dinosaurs out-competed their reptilian contemporaries.

It is a longstanding mystery why dinosaurs became and remained so plentiful for more than 180 million years. The traditional theory: dinosaurs suddenly replaced other land animals because of special traits that gave them an evolutionary advantage, such as being warm-blooded, nimble and able to occupy varied habitats. This new research presents a fresh mathematical analysis of previous fossil data that indicates that ancestors of modern-day crocodiles had as diverse body types as early dinos, with whom they co-existed for some 30 million years.

Although the data doesn't directly contradict the idea of dinosaur superiority, the authors say it is likely that these crocodilians were even more successful than dinosaurs, the latter of which may have survived major extinctions due to sheer luck.

"If you dissect the past, you can see that luck is a big part of everything in the grand scheme of evolution," says lead author Stephen Brusatte, a researcher at the American Museum of Natural History.

The idea that dinosaurs lived at the same time as similar reptile species is nothing new. And data from the past few years has many paleontologists rethinking whether dinosaurs were really so special after all. The fossil record shows that dinos lived alongside comparable groups of reptiles for millions of years without overtaking them.

For example, the early dinosaurs were contemporaries of  crurotarsans, croc ancestors, during the late Triassic period about 230 to 200 million years ago. This reptilian group ranged from quick predators to two-legged vegetarians to leisurely grazers. Then, as the Triassic turned into the Jurassic, the creatures roaming the planet changed drastically. Most crurotarsans disappeared from the fossil record. But many dinosaurs survived—and flourished, diversifying into meat-eating giants, armored warriors and winged aviators.

Brusatte and researchers from the University of Bristol in England expanded this research by analyzing the existing fossil record to show crurotarsans may have even been more successful than dinos. First, the team constructed a new family tree to separate the dinosaurs from the croc ancestors. They then assembled a database of 65 dinosaur and crurotarsan species that included over 400 skeletal features, such as whether they had beaks or shorter arms than legs.

If dinosaurs were more fit for the environment, they should have had a higher rate of evolution and more diverse body types. Instead the researchers found that the two groups evolved at similar rates and that the crurotarsans had a wider range of body types, suggesting that they had actually adapted to more lifestyles and ecological niches.

The authors argue that because dinosaurs and crurotarsans were living parallel lives together for so long, it is unlikely the dinosaurs necessarily ruled.  If you could travel back to the Triassic, Brusatte says, you would have guessed that the crocodilians would have won out. "There's no way you could argue that dinosaurs were superior to them," he says. Instead, he thinks an extinction event at the beginning of the Jurassic some 205 million years ago—like runaway global warming or an asteroid crash—may have just been bad luck for the crurotarsans.

Many paleontologists consider these findings a major step in dinosaur science. "It's really refreshing," says Kristi Curry-Rogers, a dinosaur paleontologist at Macalester College in Saint Paul, Minn. "It definitely challenges the standard story of dinosaur evolution…. In the world of dinosaurs, we see a lot that portrays them in ways that science doesn't really follow."

But not all experts agree. "I think that the conclusions of the authors aren't warranted," says Kevin Padian, a dinosaur paleontologist at the University of California, Berkeley. "Good luck isn't an evolutionary force….  Extinctions aren't random." He believes that dinosaurs are different enough from crurotarsans that they may have had a competitive edge.

Whether dinosaurs rose to fame from fitness or a roll of the dice should become clearer as paleontologists discover more fossils to fill in the sparse record of dinosaurs' early history and elucidate what caused the extinctions at the end of the Triassic. Rogers says that Brusatte's analysis will probably challenge people to support their claims of dinosaur superiority with stronger evidence. "It gives people something to shoot at that is based on data," she says, "and not just assumption."