For at least 30 million years after dinosaurs first appeared on the scene, they avoided the Earth's tropical regions even as they became abundant farther north and south of the equator - and researchers say they think they now know why.

An international group of scientists says their studies suggest it was an exceptionally unpredictable hot and dry climate with high levels of CO2 that kept large herbivore dinosaurs from inhabiting the tropics for a good 10 to 15 million years after they had populated northern and southern regions of the Earth with a more amenable climate.

While smaller meat-eating dinosaurs moved into the tropics around 200 million years ago, the big plant-eaters stayed away for millennia, they say.

That was because climate swung erratically between wet periods and periods of extremes of drought and intense heat, and wildfires swept over equatorial landscapes during those dry times and altered the vegetation that might feed plant-eating creatures in those regions, the paleontologists report in the Proceedings of the National Academy of Sciences.

"Our data suggest it was not a fun place," says Randall Irmis of the University of Utah.

"Large, warm-blooded dinosaurian herbivores weren't able to exist close to the equator - there was not enough dependable plant food," he says.

The tropics 200 million years ago weren't like the tropics of today, the researchers explain.

"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," says study lead author Jessica Whiteside of the University of Southampton in the U.K. "The fluctuating and harsh climate with widespread wildfires meant that only small two-legged carnivorous dinosaurs could survive."

The researchers analyzed rock samples taken from a location in northern New Mexico, rocks that were deposited by streams and rivers between 205 million and 215 million years ago.

The samples allowed the researchers to track changes in ecosystem productivity and estimate atmospheric CO2 levels.

The study, although looking million of years into the past when atmospheric CO2 levels were four to six times current levels, could be taken as a warning about our current environment, the researchers say.

If humans keep pumping carbon dioxide into the atmosphere, the impact on ecosystems could "fundamentally reshape" many animal communities, an effect humans might not escape, they say.

"If we continue along our present course, similar conditions in a high-CO2 world may develop, and suppress low-latitude ecosystems," Irmis says.