Herbivorous dinosaurs likely arrived in the Northern Hemisphere millions of years after their carnivorous cousins, a delay possibly brought on by climate change, according to a new study.
A new method for calculating the ages of dinosaur fossils found in Greenland shows that herbivores, called sauropodomorphs, were about 215 million years old, according to the study published Monday in the Proceedings of the National Academy of Sciences. The fossils were previously believed to be about 228 million years old.
This information changes the way scientists think about the migration of dinosaurs.
The first dinosaurs all seemed to first develop in what is now South America some 230 million years ago or more. Later they headed north and spread across the planet. The new study hints that not all dinosaurs were able to migrate at the same time.
So far, scientists haven’t found a single example of the oldest family of herbivorous dinosaurs in the Northern Hemisphere that’s more than 215 million years old. One of the best examples is the plateosaurus, a 7-meter (23-foot) tall vegetarian biped that weighed about 4 tons (8,800 pounds).
However, scientists say there were carnivorous dinosaurs 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.
Herbivores “were late to the Northern Hemisphere,” said lead author Dennis Kent of Columbia University. “Why did it take so long?”
Kent discovered what possibly happened by researching the atmosphere and climate at that time. During the Triassic period, 230 million years ago, carbon dioxide levels were 10 times higher than today. It was a warmer world with no ice caps at the poles, and with two long stretches of desert north and south of the equator, he said.
Those regions were so arid that there wasn’t enough vegetation for the sauropodomorphs to survive the journey, but there were enough insects to feed the carnivores, Kent said.
But then, about 215 million years ago, carbon dioxide levels briefly dropped by half and that allowed deserts to have a bit more vegetation, so sauropodomorphs were able to finally migrate.
Although the study makes sense, there is a possible flaw, according to University of Chicago dinosaur expert Paul Sereno: Just because no herbivore fossils older than 215 million years have been found in the Northern Hemisphere doesn’t mean that there were no sauropodomorphs. The fossils may simply not have survived, he said.
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Follow Seth Borenstein on Twitter as @borenbears
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