Herbivorous dinosaurs probably arrived in the northern hemisphere millions of years after their carnivorous cousins, which may be delayed by climate change, according to a new study.
A new method for calculating the age of dinosaur fossils found in Greenland shows that herbivores called sauropodomorphs were about 215 million years old, according to a study published in the National Academy of Sciences on Monday. Fossils were previously believed to be about 228 million years old.
This knowledge will change the way scientists think about the migration of dinosaurs.
The first dinosaurs first appeared to evolve in present-day South America about 230 million years ago or more. Later, they headed north and spread throughout the planet. New research suggests that not all dinosaurs were able to change at the same time.
So far, researchers have not found a single example of the oldest family of vegetarian dinosaurs in the northern hemisphere, more than 215 million years old. One of the best examples is the plateosaurus, a 7-meter (23-foot) tall vegetarian who weighed about 4 tons (8,800 pounds).
However, according to researchers, there were dinosaurs containing carnivores around the world at least 220 million years ago, according to Randy Irmis, a paleontologist at the University of Utah, who was not part of the study.
Vegetarians “were late for the northern hemisphere,” said editor Dennis Kent of Columbia University. “Why did it take so long?”
Kent found out what might have happened by examining the atmosphere and climate that prevailed at the time. During the Triassic, 230 million years ago, carbon dioxide levels were ten times higher than they are today. It was a warmer world with no ice caps on the poles, and two long deserts north and south of the equator, he said.
These areas were so dry that there were not enough vegetation for the sauropodomorphs to survive the trip, but there were enough insects to feed the carnivores, Kent said.
But then, about 215 million years ago, carbon dioxide levels halved for a moment and it gave the deserts a little more vegetation, so the sauropodomorphs were finally able to change.
While the study makes sense, according to Paul Sereno, a dinosaur expert at the University of Chicago, there is a potential mistake: Just because no vegetarian fossils over 215 million years old can be found in the northern hemisphere does not mean there were no sauropodomorphs. Fossils may simply not have survived, he said.
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