A huge desert created by global warming likely prevented early dinosaurs from migrating out of South America for millions of years, suggests an analysis of ancient rocks. (Related: "Oldest Dinosaur Found?")

At that time, during the Triassic period, the world's continents were bound together in a supercontinent called Pangaea, home to the first dinosaurs. But as dinosaurs spread, they didn't become evenly distributed across this landmass, suggests a team led by Columbia University geophysicist Dennis Kent. (Related "Supercontinent Pangaea Pushed Into Place.")

In a Proceedings of the National Academy of Sciences report Kent and colleagues note that there's an extraordinarily long time between when dinosaur fossils begin showing up in rocks in what is now South America, dated to 231 million years ago, and when they begin showing up in rocks in North America, dated to 218 million to 212 million years ago.

"Dinosaurs seemed to have been corralled for a long time down there, even with no obvious mountains or seaways to stop them from dispersing north over the Americas," Kent says.

Most likely, he says, there was "a vast, hyperarid Sahara-like desert across Pangea" that blocked the dinosaurs from heading north.

High levels of carbon dioxide, a greenhouse gas, had melted the Poles and led to global warming during the Triassic, the study notes. The resulting desert eventually shrank as the climate cooled heading into the Jurassic period, allowing dinosaurs to spread across the world.