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 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 completely 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 in lower latitudes closer to the equator.
So why did it take them so many tens of millions of years to venture toward the middle of our planet? A new study suggests that the answer may have been the weather.
In a paper published Monday in PNAS, a broad group of researchers concluded that high atmospheric carbon dioxide levels during the Triassic period 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, the new curator of the Natural History Museum of Los Angeles County's Dinosaur Institute.
Smith was part of an international team of scientists that created the first high-resolution snapshot of the paleoenvironment of Pangea's tropics from fossils found in the Chama Basin in north-central New Mexico. Back in the Triassic, the land mass that is now New Mexico was closer to the equator, he explained.
The researchers analyzed tiny pollen grains to see what plants were there at the time, looked at fossilized charcoal to see how hot it burned, collected fossil bones to see what animals were present, and collected data on how much atmospheric carbon there was at the time.
They learned that back in the Triassic, the area frequently alternated between extreme humidity and drought-like conditions. Hot-burning wildfires wiped out vegetation, leading to big changes in the abundance of plants. They also report that atmospheric carbon dioxide was four to six times modern levels.
Despite the uncertain living conditions, the scientists did find evidence of animal life: small-bodied meat-eating dinosaurs and an abundance of reptiles that are relatives of modern-day crocodiles.
The researchers write that the reptiles were more resistant than dinosaurs to fluctuation in their 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 are still 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 have something to do with it.
"We don't have a great explanation of what allowed them to springboard into the tropics," Smith said. "In all likelihood, it was a combination of factors."