Herbivorous dinosaurs likely arrived in the Northern Hemisphere millions of years after their carnivorous cousins, a delay likely caused by climate change, a new study found.
A new way of calculating the dates of dinosaur fossils found in Greenland shows that plant eaters, called sauropodomorphs, were about 215 million years old, according to a study. in the Proceedings of Monday of the National Academy of Sciences. The fossils were previously thought to be 228 million years old.
That changes the way scientists think about dinosaur migration.
The first dinosaurs appeared to first develop in what is now South America some 230 million years ago or more. Then they wandered north and across the world. The new study suggests that not all dinosaurs could migrate at the same time.
So far, scientists have not found any examples of the first herbivorous dinosaur family in the Northern Hemisphere that is more than 215 million years old. One of the best examples of these is the Plateosaurus., a 23-foot (7-meter) two-legged vegetarian who weighed 8,800 pounds (4,000 kilograms).
However, scientists find that carnivores existed virtually all over the world at least 220 million years ago, said Randy Irmis, a paleontologist at the University of Utah, who was not part of the research.
Plant eaters “were late in the Northern Hemisphere,” said lead study author Dennis Kent of Columbia University. “Why did it take you so long?”
Kent found out what likely happened by looking at the atmosphere and weather at that time. During the Triassic era, 230 million years ago, carbon dioxide levels were 10 times higher than now. It was a warmer world with no ice caps at the poles 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 sauropodomorphs to survive the journey, but there were enough insects that carnivores could, Kent said.
But then, about 215 million years ago, carbon dioxide levels were briefly cut in half and that allowed deserts to have a little more plant life and sauropodomorphs were able to make the journey.
Kent and other scientists said that the Triassic changes in carbon dioxide levels came from volcanoes and other natural forces, unlike now, when the burning of coal, oil and natural gas are the main drivers.
Kent used changes in the Earth’s magnetism in the ground to determine the most accurate date of the Greenland fossils. That highlighted the migration time gap, several outside experts on both dinosaurs and ancient climate said.
Kent’s theory that climate change is the difference in dinosaur migration “is great because it brings it back to contemporary issues,” Irmis said.
It also fits some animals today that have migratory 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 a potential flaw, said University of Chicago dinosaur expert Paul Sereno: Just because no plant-eating fossils older than 215 million years have been found in the Northern Hemisphere, that doesn’t means there are no sauropodomorphs. The fossils may simply not have survived, he said.
Follow Seth Borenstein on Twitter at @borenbears.
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