Researchers have found evidence of dinosaur dispersal that, interestingly, coincided with a decline in atmospheric carbon dioxide concentrations 215 million years ago. This suggests that there was previously a climatic barrier on the supercontinent of Pangea that blocked the early representatives of the long-necked dinosaurs from moving from the south to the north.
When did the long-necked dinosaurs (Sauropodomorpha) from the south also reach the northern latitudes of the earth? This question was initially at the center of research by Dennis Kenta from Columbia University in Palisades and Lars Clemmensen from the University of Copenhagen. Before that, the history of the spread of the ancestors of Brontosaurus, Brachiosaurus and Co was only roughly known: On the basis of fossil finds in Greenland, paleontologists assumed that the first representatives of this important dinosaur group also entered the north of the supercontinent Pangea sometime ago 225 and 205 million years ago achieved. At that time, this area was at latitudes that allowed a moderate climate. There the Sauropodomorpha found conditions as they also prevailed in their original homeland, which was in what is now South America – across the equator.
As part of their study, the two researchers were able to show by dating the magnetism in the rock layers of fossil sites in South America, Arizona, New Jersey, Europe and Greenland: The first representatives of the sauropodomorpha first reached the area of today’s Greenland about 214 million years ago. As they report, however, this more precise estimate raised another question: According to fossil finds, the sauropodomorpha were already common in Argentina and Brazil about 230 million years ago. So why did it take them so long to get to the northern hemisphere? Because it is assumed that the supercontinent Pangea formed a continuous land mass at that time.
What prevented the spread?
“In principle, the dinosaurs could have migrated from one pole to another,” explains Kent. “There was no ocean in between and no big mountains either. And yet it took 15 million years. ”The researcher calculates that if a herd of dinosaurs had walked just over a kilometer a day, it would have taken less than 20 years to travel between South America and Greenland. Apparently something kept them from migrating north for a long time, which then fell away. But what?
As the researchers report, an explanation emerged: As is known from previous analyzes of rock formations, there was a sharp drop in the carbon dioxide concentration in the earth’s atmosphere at precisely the time when the sauropods were advancing north. Until about 215 million years ago, in the Triassic era there was a CO2 content that was about ten times higher than today. However, research results show that the greenhouse gas concentration halved in the period from 215 to 212 million years ago. Kenta and Clemmensen believe that the parallel between the decrease in CO2 and the spread of the sauropodomorpha was not a coincidence. They suspect that the lower CO2 levels helped remove climatic barriers that previously blocked the dinosaurs’ path from the southern hemisphere to the northern.
Climatic barriers disappeared
As they explain, basic climatic principles were probably already in effect on the earth at that time as they are today: The areas around the equator were hot and humid, while the areas adjacent to the south and north were dry. Only in the middle latitudes were there climatic conditions that could offer the sauropods good living conditions. With the high CO2 values over 215 million years ago, these climate zones could have been even more pronounced, the researchers explain: “We know that the higher the CO2 content in the atmosphere, the dry becomes even drier and the wet becomes even more humid”, says Kent. 230 million years ago, conditions could have made the dry belts too dry for large herbivores to pass through. At the equator, too, there may have been very unstable and extremely rainy conditions that were unfavorable for the sauropodomorphic dinosaurs, the researchers explain.
But when the CO2 levels fell 215 to 212 million years ago, the tropical regions may have become calmer and the dry regions less harsh. This may have resulted in passages, such as along rivers, that could have helped the herbivores bridge the 10,000-kilometer distance to what is now Greenland. There they found similar mild conditions as in their southern region of origin. “As soon as they reached what is now Greenland, they did well there, as is evident from the numerous fossil records,” said Kent.
So far it has not been proven that the drop in CO2 levels helped the sauropodomorphic dinosaurs to overcome a climatic barrier, but the evidence strongly supports this approach, the researchers sum up. Kenta and Clemmensen now also want to stay on the ball: In addition to further investigations into the spread of dinosaurs, they want to investigate the question of what could have led to the great CO2 dip 215 million years ago and how quickly the values fell back then.
Source: Earth Institute at Columbia University, Article: PNAS, doi: 10.1073 / pnas.2020778118