What allowed dinosaurs to be the dominant form of life on earth for all those millions of years? Would you believe they just got lucky?

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 later extinction event about 228 million years ago.

The researchers found no difference in the rates of evolution of the two groups. If dinosaurs were out-competing the crurotarsans, they should have been evolving faster.

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.

So why did the dinosaurs survive that second 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."

Personally, I find the "luck" explanation less than satisfying. There must have been a real difference between the two groups that allowed one to survive the extinction event. You can say that the dinosaurs were "lucky" to have that trait -- or set of traits -- or you can describe those traits as an evolutionary advantage against at least that particular kind of extinction event.

I guess unless one argues that the dinosaurs deliberately developed a resistance to that kind of event -- and I'm certainly not saying that -- the two arguments mean the same thing. Looks like some pretty important things really do come down to luck.