Around 200 million years ago, during the Triassic Period, dinosaurs were competing with a group of reptiles called crurotarsans, huge crocodile-like creatures, for dominion over the earth. Thanks to a lucky break, the dinos we know and love won the struggle. What we consider to be dinosaurs began to really walk the earth during the Triassic, around 230 million years ago.
After extensive review of fossils by scientists at the New York Museum of Natural history, researchers 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 crurotarsans were a horrifying bunch as well. Some grew up to 40 feet in length, and few were smaller than 30. Some were crocodile-like creatures that waited underwater until they had a chance to strike. Others were land-based predators with four powerful legs, massive skulls and flesh-tearing teeth. This diversity should have given them an evolutionary edge, but luck won over shear percentages.
The dinosaurs won out, Brusatte concluded, because some type of planetary disaster 200 million years ago, a planetary disaster like dramatic climate change or maybe a large meteorite, for all intensive purposes, wiped out the crurotarsans while sparing the dinosaurs. But what went around came around, and the dinos went the way of the dodo in the same way that their counterparts did 65 million years ago.