Why Did Dinosaurs Avoid The Tropics For So Long?

New Mexico's environment 212 million years ago was too hot and dry for large dinosaurs so phytosaurs and aetosaurs dominated. Victor Leshyk.

It took thirty million years from the first appearance of the giant dinosaurs known as sauropodomorphs until they maintained a sustained presence in the tropics. Paleontologists have been puzzled by this observation from the fossil record. A new theory suggests it was all about carbon dioxide.

The burning of fossil fuels is sending the climate in a direction that humans, or our near relatives, have never experienced before. However, for Earth itself high levels of greenhouse gasses are nothing new. In the late Triassic period, about 215-200 million years ago, carbon dioxide levels were at least 1200 parts per milion. This made the tropics an uncomfortable place.

Not only was the average temperature higher than today, but the climate was unpredictable with long dry spells punctuated by wet years. These conditions might have suited the small and adaptable species found in the area, or those that could migrate easily, but they were lethal to the huge long-necked herbivores that were the ancestors of famous species, such as Diplodocus and Brontosaurus.

According to Dr Jessica Whiteside of the University of Southampton, tropical latitudes during the Triassic were a terrifying place. “The conditions would have been something similar to the arid western United States today, although there would have been trees and smaller plants near streams and rivers and forests during humid times,” Whiteside says. When the rains came plant life would grow, but this would be followed by huge wildfires in subsequent dry years.

'The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive.' Whiteside says.

In a paper in the Proceedings of the National Academy of Sciences, Whiteside reports on rocks near the famous Ghost Ranch, New Mexico. When the rocks were deposited 205-215 million years ago, the location was at 12° N.

The rocks not only provided information about local vegetation and carbon dioxide levels in the atmosphere at the time, but they also contained traces of charcoal that allowed the researchers to estimate the wildfire temperatures at 600° C (1100°F). "Each dataset complements the others, and they all point towards similar conditions," Whiteside says.

We know large dinosaurs did colonize the tropics briefly during the Triassic, because they managed to cross it. We have found fossils from huge sauropodomorphs 230 million years old from Argentina, which even then was well south of the equator. Relatives were found not much later in Wales, indicating that, at some point, conditions were congenial enough for these big beasts to lumber across. However, no signs of a longlasting presence have been found.

“Throughout this period, levels of CO2 were four to six times higher than the levels we observe today,” says co-author Dr Randall Irmis of the Natural History Museum, Utah. “But the findings do indicate that if we continue our present course of human-caused climate change, similar conditions could develop and suppress equatorial ecosystems.”

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