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 of calculating the dates of dinosaur fossils in Greenland has shown that the herbivores, called sauropodomorphs, were about 215m years old, instead of as much as 228m years old as previously thought, according to a study in Monday’s Proceedings of the National Academy of Sciences.
It changes how scientists think about dinosaur migration.
It seems that the earliest dinosaurs first evolved in present-day South America about 230 million years ago. They then wander north and around the world. According to the new study, not all dinosaurs can migrate simultaneously.
So far, scientists have not found an 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 bipedal 7 meter vegetarian that weighed 4,000 kg.
Yet scientists find that at least 220 million years ago, carnivores worldwide 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, carbon dioxide levels dropped in half from time to time, which could give the desert a little more plant life and make the sauropodomorphs travel.
Kent and other scientists have said that Triassic changes in carbon dioxide levels come 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 gap in migration time, said several experts on dinosaurs and the ancient climate.
Kent’s theory on climate change as the difference in dinosaur migration “is super cool, because it brings it back to contemporary issues,” Irmis said.
It also suits some animals today with migration problems that keep them away from certain climates, said Hans-Otto Pörtner, 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 one possible flaw, said dinosaur expert Paul Sereno of the University of Chicago: just because no fossils of herbivores older than 215 million years have been found in the northern hemisphere does not mean that no sauropodomorphic. The fossils may just not have survived, he said.