Scientists had previously concluded that dinosaurs appeared in the United States about 212 million years ago, significantly later than in South America, where they first appeared about 230 million years ago. But URI Professor of Geosciences David Fastovsky and colleagues at the Massachusetts Institute of Technology have precisely dated rocks in which the earliest dinosaur fossils were discovered in the southwestern U.S. and found that the dinosaurs appeared there as early as 223 million years ago.
“This work essentially upends the well-entrenched hypothesis that dinosaurs first got their start in South America, and radiated extremely slowly into North America,” said Fastovsky. “It demonstrates that the whole idea of the South America to North America radiation – slow or fast – is just not correct. We still don’t know where the cradle of Dinosauria was, but the evidence suggests that once dinosaurs appeared, they spread all over the globe very rapidly.”
The researchers also demonstrated that these earliest dinosaurs coexisted with close non-dinosaur relatives for more than 12 million years. And they identified a 16-million-year gap in the rock record in which there is no trace of any vertebrates, including dinosaurs.
“Right below that horizon where we find the earliest dinosaurs [in North America], there is a long gap in the fossil and rock records across the sedimentary basin,” said MIT research scientist Jahan Ramezani. “If the record is not there, it doesn’t mean the dinosaurs didn’t exist. It means that either no fossils were preserved, or we haven’t found them. That tells us the theory that dinosaurs simply started in South America and spread all over the world has no firm basis.”
According to Fastovsky, the earliest record of early dinosaur evolution is found in Argentina, where layers of sedimentary rock preserve a distinct suite of early, primitive dinosaurs. The first dinosaurs in North America, by contrast, are more advanced forms, and thus were once thought to be significantly younger than those in South America.
“Our best evidence now suggests that they are the same age or very slightly younger than those in Argentina,” said Fastovsky.
Moreover, in North America, more primitive relatives of dinosaurs apparently co-existed with more evolved dinosaurs for more than 12 million years, according to the researchers’ analysis. “In South America, there is very little overlap,” Ramezani said, “But in North America, we see this unique interval when these groups were coexisting. You could think of it as Neanderthals coexisting with modern humans.”
The 16-million year gap in preservation obscures understanding when the very first dinosaurs appeared in North America. “The fact that our record starts with advanced forms tells us there was a prior history,” Ramezani said. “It’s not just that advanced dinosaurs suddenly appeared 223 million years ago. There must have been prior evolution in North America — we just haven’t identified any earlier dinosaurs yet.” Ramezani notes that fossil preservation is “an exceptional process, requiring exceptional circumstances.” Dinosaurs may well have first appeared during this time interval. If they left any fossil evidence, it may have since been erased.
Fastovsky added, “We still don’t know where or even when dinosaurs first appeared. But regardless, we all agree that ever since dinosaurs showed up, for the next 223 million years at least – to this very day, in fact – they remain the most diverse terrestrial vertebrates on Earth.”
Results of this study were published this week in the American Journal of Science.
(Note: This story is adapted from materials provided by MIT.)
Pictured: The Chinle Formation where URI paleontologist David Fastovsky learned about the early evolution of dinosaurs in North America. (Photo courtesy of David Fastovsky)