The new article clarifies estimates of when herbivorous dinosaurs safely crossed North America by the northern route to reach Greenland and points to an intriguing climatic phenomenon that could have helped them travel.
The authors of the study, published today in the Proceedings of the National Academy of Sciences, are Dennis Kent, an assistant researcher at the Lamont-Doherty Earth Observatory at Columbia University, and Lars Clemmensen of the University of Copenhagen.
Previous estimates have suggested that sauropodomorphs – a group of long-necked, herbivorous dinosaurs that eventually included Brontosaurus and Brachiosaurus – arrived in Greenland approximately between 225 and 205 million years ago. But by painstakingly matching ancient patterns of magnetism in rock layers at fossil sites across South America, Arizona, New Jersey, Europe, and Greenland, the new study offers a more accurate estimate: It suggests that sauropodomorphs appeared in present-day Greenland, about 214 million years ago. At that time, all the continents merged, forming the supercontinent of Pangea.
With this new and more accurate assessment, the authors faced another question. Fossil records show that sauropodomorphic dinosaurs first appeared in Argentina and Brazil about 230 million years ago. So why did it take them so long to expand into the northern hemisphere?
“Basically, dinosaurs could walk from almost one pole to the other,” Kent explained. “There were no oceans in between. There were no big mountains. Still, it took 15 million years. As if snails could do it faster.” He calculated that if a herd of dinosaurs walked only one mile a day, it would take him less than 20 years to travel between South America and Greenland.
Interestingly, the Earth is in the midst of a terrible fall in atmospheric CO2 sometime at a time when sauropodomorphs would have migrated 214 million years ago. Until about 215 million years ago, the Triassic period had extremely high levels of CO2, about 4,000 parts per million – about 10 times more than today. But between 215 and 212 million years ago, the concentration of CO2 halved and dropped to about 2,000 ppm.
While the timing of these two events – CO2 depletion and sauropodomorph migration – might be pure coincidence, Kent and Clemmensen think they may be related. The paper suggests that milder CO2 levels may have helped remove climate barriers that may have trapped sauropodomorphs in South America.
On Earth, the areas around the equator are hot and humid, while neighboring areas in low latitudes tend to be very dry. Kent and Clemmensen say that on a planet charged with CO2, the differences between these climate zones could be extreme – perhaps too extreme for sauropodomorphic dinosaurs to cross.
“We know that with higher CO2 CO2 dries out and wet humidifies,” Kent said. 230 million years ago, conditions with high CO2 could have made dry belts too dry to support the movement of large herbivores that need to eat a lot of vegetation to survive. The tropics could also be locked in rainy, monsoon conditions that may not have been ideal for sauropodomorphs. There is little evidence that they emerged from temperate mid-latitude habitats to which they were adapted in Argentina and Brazil.
But when CO2 levels fell 215-212 million years ago, perhaps the tropics became milder and the arid regions less dry. There may have been some passages, such as rivers and rows of lakes, that would help keep herbivores on their 6,500-mile journey to Greenland, where their fossils are abundant today. Then Greenland would have a temperate climate similar to the climate of New York State today, but with much milder winters because there were no polar ice sheets at the time.
“Once they arrived in Greenland, it looked like they had settled in,” Kent said. “After that, they hung around like a long fossil record.”
The idea that a drop in CO2 could have helped these dinosaurs overcome climate barriers is speculative, but plausible, and seems to be backed by fossil data, Kent said. Fossils of sauropodomorphic bodies were not found in tropical and arid regions of this time period – although their footprints occasionally appear – suggesting that they did not remain in these areas.
Further, Kent hopes to continue working on a better understanding of the big drop in CO2, including what caused it and how quickly the CO2 level dropped.
Reference: Kent DV, Clemmensen LB. The scattering of dinosaurs north from Gondwana to Greenland by the Middle Norian (215–212 Ma, late Triassic) plunged the atmosphere into pCO2. PNAS. 2021. doi: 10.1073 / pnas.2020778118
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