Plant-eating dinosaurs likely arrived in the Northern Hemisphere millions of years after their carnivorous cousins, a slowdown likely caused by climate change, a new study found.
A new way of calculating the data from dinosaur fossils found in Greenland shows that the herbivores, called sauropodomorphs, were about 215 million years old, according to a study. in Monday’s Proceedings of the National Academy of Sciences. The fossils were once thought to be as old as 228 million years.
That changes the way scientists think about dinosaur migration.
The earliest dinosaurs all seemed to first evolve in what is now South America, about 230 million years ago or more. They then roamed north and around the world. The new study suggests that not all dinosaurs can migrate at the same time.
So far, scientists have not found a single example of the earliest herbivorous dinosaur family in the Northern Hemisphere that is more than 215 million years old. One of the best examples of this is the Plateosaurus, a seven-foot two-legged vegetarian who weighed 8,800 pounds (4,000 kilograms).
Still, scientists are discovering that meat eaters were virtually global at least 220 million years ago, said Randy Irmis, a paleontologist at the University of Utah who was not part of the study.
The herbivores “were latecomers in the Northern Hemisphere,” said lead author Dennis Kent of Columbia University. “Why did it take so long?”
Kent found out what likely had happened by looking at the atmosphere and climate at the time. During the Triassic era, 230 million years ago, the carbon dioxide content was 10 times higher than today. It was a warmer world with no polar ice caps and two bands of extreme deserts north and south of the equator, he said.
It was so dry in those regions that there weren’t enough plants for the sauropodomorphs to survive the journey, but there were plenty of insects that could feed carnivores, Kent said.
But about 215 million years ago, the carbon dioxide levels dropped by half briefly, and that allowed the deserts to have some more plants and the sauropodomorphs to make the journey.
Kent and other scientists said changes in the Triassic in carbon dioxide levels came from volcanoes and other natural forces – unlike now, when the combustion of coal, oil and natural gas are the main drivers.
Kent used changes in the Earth’s magnetism in the soil to determine the more exact date of the Greenland fossils. That underscored the time gap of the migration, several outside experts said, both in dinosaurs and in the ancient climate.
Kent’s theory that climate change is the difference in dinosaur migration “is super cool because it goes back to contemporary issues,” Irmis said.
It also suits some animals today who have migration problems that keep them out of certain climates, said Hans-Otto Portner, a climate scientist and biologist at the Alfred Wegener Institute in Germany who was not part of the study.
While the study makes sense, there is one potential flaw, said dinosaur expert Paul Sereno of the University of Chicago: Just because no fossils of herbivores older than 215 million years have been found in the Northern Hemisphere doesn’t mean there are no sauropodomorphs . The fossils may not have survived, he said.
Follow Seth Borenstein on Twitter at @borenbears
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