A plant-eating dinosaur probably arrived in the northern hemisphere millions of years after its meat-consuming cousins, a delay likely caused by climate change, according to a new study.
A new way to calculate the dates of dinosaur fossils found in Greenland shows that plant eaters, called sauropodomorphs, were about 215 million years old, according to a study at the Monday Proceedings of the National Academy of Sciences. Fossils were previously thought to be 228 million years old.
This changes the way scientists think about dinosaur migration.
The first dinosaurs first appeared in present-day South America about 230 million years ago or more. Then they wandered north and around the world. The new study suggests that not all dinosaurs could migrate at the same time.
So far, scientists have found no example of the first plant-eating family of dinosaurs 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 4,800 kilograms (8,800 pounds).
However, scientists found that meat eaters were virtually worldwide 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 “arrived late in the northern hemisphere,” said study lead author Dennis Kent of Columbia University. “What took them so long?”
Kent discovered what probably happened by looking at the atmosphere and climate of the time. During the Triassic era, 230 million years ago, carbon dioxide levels were 10 times higher than today. It was a warmer world with no layers of ice 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 trip, but there were enough insects that meat consumers could make, Kent said.
But about 215 million years ago, carbon dioxide levels fell briefly by half and this allowed deserts to have a little more plant life and sauropodomorphs were able to make the trip.
Kent and other scientists said the Triassic changes carbon dioxide levels came from volcanoes and other natural forces, unlike now, when burning coal, oil and natural gas are the main engines.
Kent used changes in the Earth’s magnetism on the ground to determine the most accurate date for the Greenland fossils. This highlighted the migration time gap, said several external experts, both in dinosaurs and in ancient climate.
Kent’s theory of climate change as the difference in dinosaur migration “is fantastic because it brings it back to contemporary issues,” Irmis said.
It also fits with some animals today that have migratory problems that keep them away from 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 Paul Sereno, a dinosaur expert at the University of Chicago: “Just because no fossils of plant-eating plants have been found more than 215 million years old. in the northern hemisphere, this does not mean that there were no sauropodomorphs. Fossils may not have survived, he said.
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Follow Seth Borenstein on Twitter at @borenbears.
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