Plant-eating dinosaurs probably arrived in the Northern Hemisphere millions of years after their carnivorous cousins, a delay most likely caused by climate change, a new study has found.
A new way to calculate the dates of dinosaur fossils 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. It was previously thought that the fossils were as old as 228 million years.
It changes how scientists think about dinosaur migration.
It seems that the earliest dinosaurs only evolved 230 million years ago or longer in present-day South America. They then wander north and around the world. According to the new study, not all dinosaurs can migrate simultaneously.
To date, scientists have found no example of the earliest plant-eating 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 biped 23-foot (7-meter) vegetarian who weighed 4,000 kilograms.
Yet scientists find that at least 220 million years ago, carnivores were almost worldwide, said Randy Irmis, a paleontologist at the University of Utah who was not part of the research.
The herbivores “were late in the Northern Hemisphere,” said Columbia University lead author Dennis Kent. “What took them so long?”
Kent found out what probably happened by looking at the atmosphere and climate at the time. During the Triassic era, 230 million years ago, carbon dioxide levels were ten times higher than now. It was a warmer world without ice sheets 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 were not enough plants for the sauropodomorphs to survive the journey, but there were enough insects that carnivores could do, Kent said.
But about 215 million years ago, the levels of carbon dioxide dropped briefly in half, which could give the deserts a little more plant life, and the sauropodomorphs could make the ride.
Kent and other scientists say Trias change in carbon dioxide levels it was 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 more exact date of the Greenland fossils. This highlighted the migration time gap, said several outside experts in dinosaurs and ancient climates.
‘Kent’s theory of climate change as the difference in the migration of dinosaurs, is super cool because it brings it back to contemporary issues,’ Irmis said.
It also applies to some animals today that have migration 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.
Although the study makes sense, there is one possible flaw, the University of Chicago dinosaur expert Paul Sereno said: just because no fossils of herbivores older than 215 million years have been found in the Northern Hemisphere does not mean that there are no sauropodomorphs. The fossils may just not have survived, he said.
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Follow Seth Borenstein on Twitter @borenbears.
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