Apparently a lucky break allowed dinosaurs to rule Earth — by wiping out their rivals:
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.