WASHINGTON Thanks to a big stroke of luck 200 million years ago, dinosaurs beat out a fearsome group of creatures competing for the right to rule the Earth, scientists said on Thursday.
Dinosaurs appeared about 230 million years ago, during the Triassic Period, and competed for 30 million years with a group of reptiles called crurotarsans, cousins of today's crocodiles that grew to huge sizes and looked a lot like dinosaurs.
Many scientists believed dinosaurs were simply superior to crurotarsans and fared better because the earliest dinosaurs walked on two legs, not four, and because they may have been warm-blooded.
But scientists led by Steve Brusatte of Columbia University and American Museum of Natural History in New York conducted an extensive review of fossils and found that the two groups were evolving at roughly the same pace and the crurotarsans actually had a larger range of body types, diets and lifestyles.
The dinosaurs won out, Brusatte concluded, because some type of planetary calamity 200 million years ago -- dramatic climate change or maybe a large meteorite impact -- nearly wiped out the crurotarsans while sparing the dinosaurs.
"The fundamental question is why were the dinosaurs able to become so dominant," Brusatte, whose study is in the journal Science, said in a telephone interview. "Evolution on a big scale oftentimes is a matter of luck."
The event that toppled the crurotarsans 200 million years ago enabled dinosaurs to become the reigning form of land animals for a long time, until their luck ran out too. An asteroid struck Earth 65 million years ago and doomed the dinosaurs.
The crurotarsans were a fabulous bunch of monsters.
Some called phytosaurs looked and lived a lot like today's crocodiles, staying submerged in rivers or lakes until attacking a victim. The North American phytosaur Smilosuchus grew to 39 feet long.
Others called rauisuchians were land predators with four powerful legs, massive skulls and flesh-tearing teeth. South America's Fasolasuchus grew to 33 feet long.
Both were far more impressive than the typical dinosaur predator of the time like Coelophysis, a relatively lightly built, two-legged hunter about 10 feet long.
The dinosaur plant eaters of the time like Plateosaurus were getting big, but were less varied than the plant-eating crurotarsans such as the heavily armored aetosaurs.
"If we were standing around in the late Triassic period 210 million years ago and were asked what group is going to go on and take over the world, I think a reasonable gambler would say the crurotarsans. It's not that the dinosaurs weren't doing well. The crurotarsans were doing more," Brusatte said.