For millions of years, the biggest dinosaurs on the planet steered clear of the tropics, and now scientists think they know why.
By the mid-Triassic period, some 212 million years ago, the massive long-necked plant-eaters known as sauropodomorphs were commonly found in northern latitudes (present-day Europe) and southern latitudes (Argentina), but according to the fossil record, they were absent in the tropics. It wasn’t until well into the early Jurassic period, about 30 million years after the origin of dinosaurs, that these big guys started to show up closer to the equator.
A broad group of researchers has concluded that high atmospheric carbon dioxide levels in the Triassic lead to a volatile and inhospitable environment in the low latitudes on the great continent of Pangaea.
“It was not a happy place for them to be hanging out,” said Nathan Smith, curator of the Natural History Museum of Los Angeles County’s Dinosaur Institute.
Smith was part of an international team that created the first snapshot of the paleoenvironment of Pangea’s tropics from fossils found in the Chama Basin in north-central New Mexico. In the Triassic, the land mass that is now New Mexico was closer to the equator, he said.
The researchers analyzed pollen grains to see what plants were there, looked at fossilized charcoal to see how hot it burned, collected fossilized bones to see what animals were present and collected data on how much atmospheric carbon there was.
They learned that in the Triassic, the area frequently alternated between extreme humidity and drought. Wildfires wiped out vegetation, leading to changes in plant life. They also report that atmospheric carbon dioxide was four to six times modern levels.
Despite the living conditions, the scientists did find evidence of animal life: small-bodied, meat-eating dinosaurs and an abundance of reptiles, relatives of modern crocodiles.
The researchers write that the reptiles were more resistant than dinosaurs to fluctuation in the environment, perhaps because they did not need as much food.
“Dinosaurs are fast-growing and have high metabolic needs,” Smith said. “It makes sense that if you are going to have these wild swinging fluctuations, that the animals that are biggest and growing fastest are the ones that are suffering.”
Paleontologists say they are not certain why larger dinosaurs were able to flourish in this area millions of years later, although a mass extinction event that wiped out the large-bodied crocodiles and climate change may lend a clue.
