The rise of dinosaurs on Earth has not always been well understood. For the past ten years, it was generally thought that dinosaurs first appeared in South America, before spreading to North America millions of years later. However, according to a recent study by a team of geoscientists from the University of Rhode Island and the Massachusetts Institute of Technology, that hypothesis is incorrect.

As published in the American Journal of Science this past June, work done by URI geosciences professor David Fastovsky and MIT’s Jahan Ramezani and Samuel Bowring revealed that dinosaurs in North America date as far back as 219 million years ago, rather than only 211 million as previously thought. By comparison, the oldest dinosaurs from South America are thought to be 229 million years old.

According to the “diachronous hypothesis” of dinosaur evolution, the dinosaurs that had evolved in South America migrated north over an extended period of time, setting the “cradle of Dinosauria” in South America.

“That was the first time that anybody really tried to put together a comprehensive idea about the early appearance of dinosaurs and how they spread all over the globe,” Fastovsky said. The problem, Fastovsky explained, was that this theory was not based on any numerical dates.

“In short, they didn’t really know what age these things were, how long they took to evolve and what the age of the South American deposits were relative to the age of the North American deposits,” he said. “The hypothesis was based on a lack of information about those dates.”

Fastovsky’s research was focused on dating rocks in Arizona’s Petrified Forest National Park. While he was not specifically looking for evidence for or against the diachronous hypothesis, he was very interested in getting the exact ages of the earliest dinosaurs preserved there and in similar rocks nearby.

“We were interested in the dates for lots of reasons,” he said. “We wanted to understand the evolution of the earliest dinosaurs, and to do that, we need to know something about when they first appeared.”

The dates also revealed another flaw in the south-to-north radiation theory: there are no rocks or fossils from the time interval during with the migration is supposed to have occurred.

“It turns out this key interval where you would see this evolution from South America to North America is completely missing,” Fastovsky said. “There’s a big 16 million year gap. So to make this claim that there’s this big evolutionary event is not correct.”

Also brought to light by Fastovsky and his colleagues is the continued existence of proto-dinosaurs alongside early North American dinosaurs. While these more primitive forms

disappeared earlier in South America, their northern counterparts appear to have coexisted with dinosaurs for at least 11 million years.

Fastovsky plans to continue researching the ages of ancient rocks and is looking at a site in Texas with rocks that are thought to be the same age as those in Arizona. He would also like to see better dates on the South American specimens.

“This is some of the highest precision dating that’s ever been done on ancient rocks,” Fastovsky said. “So for people who are interested in the age of the history of the Earth, this is pretty exciting stuff.”