A new paper refines estimates of when herbivorous dinosaurs must have crossed North America on a northern trek to reach Greenland, pointing out an exciting climate phenomenon that may have helped them on their journey.
The study, published today in Proceedings of the National Academy of Sciences, is the author of Dennis Kent, adjunct researcher at Columbia University’s Lamont-Doherty Earth Observatory, and Lars Clemmensen from the University of Copenhagen.
Previous estimates suggest that sauropodomorphs – a group of long-necked, herbivorous dinosaurs which eventually included Brontosaurus and Brachiosaurus – arrived in Greenland sometime between 225 and 205 million years ago. But by carefully matching old magnetism patterns in rock layers by fossil sites in South America, Arizona, New Jersey, Europe, and Greenland, the new study provides a more accurate estimate: It suggests that sauropodomorphs appeared in what is now Greenland about 214 million years ago. At that time, the continents were brought together and formed the supercontinent Pangea.
With this new and more accurate estimate, the authors faced another question. Fossil records show it sauropodomorph dinosaurs first appeared in Argentina and Brazil about 230 million years ago. So why did it take them so long to expand to the northern hemisphere?
“In principle, the dinosaurs could have gone from almost one pole to the other,” Kent explained. “There was no sea in between. There were no big mountains. And yet it took 15 million years. It’s as if snails could have done it faster.” He calculates that if a dinosaur herd only walked one kilometer a day, it would take less than 20 years to travel between South America and Greenland.
Exciting was the Earth in the middle of a huge dip in atmospheric CO2 just around the time the sauropodomorphs would have migrated 214 million years ago. Until about 215 million years ago, the Triassic period had experienced extremely high CO2 levels of about 4,000 parts per million – about 10 times higher than today. But between 215 and 212 million years ago, CO2 the concentration halved and dropped to approx. 2,000 ppm.
Although the timing of these two events – the declining CO2 and the migration of sauropodomorphs – could be pure coincidence, Kent and Clemmensen think they may be related. In the paper, they suggest that the milder levels of CO2 may have helped remove climatic barriers that may have captured the sauropodomorphs of South America.
On Earth, areas around the equator are hot and humid, while adjacent areas in low latitudes tend to be very dry. Kent and Clemmensen say that on a planet supercharged with CO2, the differences between these climate belts may have been extreme – perhaps too extreme for sauropodomorphic dinosaurs to cross.
“We know it with higher CO2, the dry gets dry and the wet gets wetter, “said Kent. 230 million years ago, the high CO2 conditions could have made the dry belts too dry to support the movements of large herbivores who need to eat a lot of vegetation to survive. The tropics may also have been locked inside rainy, monsoon-like conditions that might not have been ideal for sauropodomorphs. There is little evidence that they ventured out from the temperate habitats of the middle latitude to which they were adapted in Argentina and Brazil.
But when CO2 levels dipped 215-212 million years ago, perhaps tropical regions became milder and the arid regions became less arid. There may have been some passages, such as along rivers and lake strings, that would have helped sustain herbivores along the 6,500-mile journey to Greenland, where their fossils are now abundant. Back then, Greenland would have had one temperate climate similar to the climate of the New York state today, but with much milder winters because there was no isis at that time.
“When they arrived in Greenland, it looked like they were settling in,” Kent said. “They hung around like a long fossil record after that.”
The idea that a dip in CO2 could have helped these dinosaurs overcome a climate barrier is speculative but plausible, and it appears to be supported by the fossil record, Kent said. Sauropodomorph body fossils have not been found in the tropical and arid regions during this time period – although their footprints occasionally appear – suggesting that they do not dwell in these areas.
Next, Kent hopes to continue working to better understand the great CO2 dip, including what caused it and how fast CO2 levels fell.
Dennis V. Kent et al., “Northward distribution of dinosaurs from Gondwana to Greenland in the middle of the Nordic (215-212 Ma, late Triassic) dip in atmospheric pCO2,” PNAS (2021). www.pnas.org/cgi/doi/10.1073/pnas.2020778118
Earth Institute at Columbia University
Citation: CO2-dip may have helped dinosaurs go from South America to Greenland (2021, 15 February) retrieved 16 February 2021 from https://phys.org/news/2021-02-co2-dip-dinosaurs-south- america.html
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