Missing Dinosaur Mystery Solved: Extreme Climate Shifts in the Tropics

Dinosaurs couldn't stand living near the tropics, and now scientists may know why. Researchers may have discovered why dinosaurs remained inexplicably rare near the equator for more than 30 million years.

The earliest known dinosaur fossils, which can be found in Argentina, date from around 230 million years ago. Within 15 million years, species with different diets and body sizes had evolved and were abundant-just not in tropical latitudes. In the tropics, in contrast, the only dinosaurs present were small carnivores.

In this latest study, the researchers pieced together a remarkably detailed picture of the climate and ecology more than 200 million years at in northern New Mexico. They focused on Chinle Formation rocks, which were deposited by rivers and streams between 205 and 215 million years ago. The scientists analyzed fossils, charcoal left by ancient wildfires and stable isotopes from organic matter and carbonate nodules that formed in ancient soils.

So what did they find? As before the researchers found little evidence for dinosaurs, except for small, carnivorous theropods. They also found evidence for abrupt changes in climate that left a record in the shifting abundance of different types of pollen and fern spores between sediment layers.

The researchers discovered that wildfire burn temperatures varied drastically, consistent with a fluctuating environment in which the amount of combustible plant material rose and fell over time. The researchers estimated the intensity of wildfires using bits of charcoal in the sediment layers. The scientists also found atmospheric carbon dioxide levels rose from 1,200 parts per million (ppm) to about 2,400 ppm. At these high concentrations, there would be more frequent and extreme weather fluctuations.

"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," said Jessica Whiteside, one of the researchers, in a news release. "The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelphysis, could survive."

The findings are published in the journal Proceedings of the National Academy of Sciences.

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