A drop in carbon dioxide levels may help sauropodomorphs, the first relatives of the largest animal that ever walked the earth, to migrate thousands of miles north past the deserts they once banned about 214 million years ago.
Scientists have determined the timing of dinosaurs from South America to Greenland by correlating rock layers with sauropodomorphic fossils with changes in the Earth's magnetic field. Using that timeline, the team found that the creatures ’northward push coincides with a dramatic decrease in CO2, which may have removed climate-related barriers, the team reported Feb. 15 in Proceedings of the National Academy of Sciences .
Sauropodomorphs were a group of long-necked, plant-eating dinosaurs that included massive sauropods such as Seismosaurus and their smaller ancestors (SN: 17/11/20). About 230 million years ago, sauropodomorphs lived mainly in what is now northern Argentina and southern Brazil. But at some point, these first dinosaurs caught on and moved north to Greenland.
However, exactly when they were able to make that trip was an enigma. “In principle, it could walk from where they were to the other hemisphere, which was 10,000 miles away,” says Dennis Kent, a geologist at Columbia University. At that time, Greenland and the Americas were together in the supercontinent Pangea. There were no oceans blocking the way and the mountains were easy to get around, he says. If dinosaurs had walked slowly one to two miles a day, it would have taken about 20 years to reach Greenland.
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But for much of the late Triassic era, which spans 233 to 215 million years ago, the Earth’s carbon dioxide levels were incredibly high, up to 4,000 parts per million. (By comparison, CO2 levels are currently about 415 parts per million.) Climate simulations suggested that the CO2 level would have created hyperarid deserts and strong climate fluctuations, which could act as a barrier for giant beasts. With vast deserts stretching north and south of the equator, according to Kent, there would be few plants available for herbivores to survive the journey north for much of that time period.
Previous estimates have suggested that these dinosaurs migrated to Greenland about 225 to 205 million years ago. To get a more accurate date, Kent and colleagues measured magnetic patterns on ancient rocks in South America, Arizona, New Jersey, Europe, and Greenland, all places where sauropodomorphic fossils were discovered. These patterns record the orientation of the Earth's magnetic field at the time of rock formation. By comparing these patterns with previously excavated rocks whose ages are known, the team found that sauropodomorphs appeared in Greenland about 214 million years ago.
Dennis Kent and Lars Clemmensen
That more accurate date for the migration of sauropodomorphs may explain why it took so long to begin the trek north and how they survived the journey: the Earth’s climate was changing rapidly at that time.
Around the time sauropodomorphs appeared in Greenland, carbon dioxide levels dropped in a few million years to 2,000 parts per million, making the climate more pleasant for herbivores, the team reports. The reason for this drop in carbon dioxide, which appears in the climate records of South America and Greenland, but allowed a possible migration to the north, is unknown.
“We have evidence of all these events, but the confluence in time is remarkable here,” says Morgan Schaller, a geochemist at the Rensselaer Polytechnic Institute in Troy, New York, who did not participate in this study. These new discoveries, he says, also help solve the mystery of why plant eaters remained placed for a time as meat eaters roamed freely.
“This study reminds us that we cannot understand evolution without understanding climate and the environment,” says Steve Brusatte, a vertebrate paleontologist and evolutionary biologist at the University of Edinburgh, who is also not involved in the study. "Even the largest and most impressive creatures that have ever lived have been controlled by the vagaries of climate change."