Plant-eating dinosaurs probably arrived in the northern hemisphere millions of years after their meat-eating cousins, a delay likely caused by climate change, a new study found.
A new way to calculate dinosaur fossil data found in Greenland shows that plant consumers, called sauropodomorphs, were about 215 million years old, according to a study in the Monday papers of the National Academy of Sciences. The fossils were thought to be 228 million years old.
This changes the way scientists think about dinosaur migration.
The oldest dinosaurs first appeared to develop in what is now South America, about 230 million years ago or more. They then wandered north and across the globe. The new study suggests that not all dinosaurs could migrate at the same time.
To date, scientists have found no example of the oldest plant-eating dinosaur family in the northern hemisphere, 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.
However, scientists are discovering that meat consumers were almost 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 consumers “came late to the northern hemisphere,” said study lead author Dennis Kent of Columbia University. “What took them so long?”
Kent realized what was probably happening by looking at the atmosphere and climate at the 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 layers of ice at the poles and two bands of extreme desert north and south of the equator, he said.
It was so dry in those areas that there weren’t enough plants for sauropodomorphs to survive the trip, but there were enough insects that meat eaters could, Kent said.
But about 215 million years ago, carbon dioxide levels fell short by half and this allowed deserts to have a little more plant life, and sauropodomorphs managed to make the journey.
Kent and other scientists have said Triassic changes Carbon dioxide levels come from volcanoes and other natural forces – unlike now, when burning coal, oil and natural gas are the main drivers.
Kent used changes in the Earth’s magnetism in the soil to identify the exact date of the Greenland fossils. This highlighted the time lag of migration, said several external experts in both dinosaurs and the ancient climate.
Kent’s theory of climate change being the difference in dinosaur migration “is super cool because it brings it back to contemporary issues,” Irmis said.
It also suits 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 study.
Although the study makes sense, there is a potential flaw, said University of Chicago dinosaur expert Paul Sereno: Just because fossils of plant consumers older than 215 million years old have not been found in the northern hemisphere, this is not the case. means that there were no sauropodomorphs. The fossils may not have survived, he said.
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Follow Seth Borenstein on Twitter at @borenbears.
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