An Allosaurus replica greets visitors at Backyards Terrors Dinosaur Park, in Bluff City, Tenn. (Jessica Fuller/Johnson City Press via AP)

For years, scientists have puzzled over an odd gap in the fossil record for early dinosaurs: Even as dinosaurs spread through the most of the world 200 million years ago, some yet-unknown force kept them from colonizing the tropics.

Now researchers may have found the explanation: Climate change. Exceedingly high levels of greenhouse gases appear to have rendered the Earth’s equatorial zones all but uninhabitable for the planet’s largest terrestrial beasts.

That’s the core finding of a study that compares the geographic distribution of Triassic-era dinosaurs with new data that details the environmental conditions of the time. The report, published Monday in the Proceedings of the National Academy of Sciences, seeks to account for the pronounced absence of large dinosaur remains in Triassic rocks from tropical regions. During that early dinosaur epoch, massive plant-eating reptiles roamed the Earth’s middle- and upper latitudes, but only a handful of small, meat-eating dinosaurs colonized the tropics.

The reason appears to be related to harsh climactic conditions that made it difficult for many species to survive, according to the study by researchers from Britain’s University of Southampton and eight other institutions. Clues gleaned from a variety of fossils point to dramatic shifts in climate, particularly in the tropical region. Extreme heat and drought unleashed epic wildfires that destroyed plants and made it hard for grazing dinosaurs to find sufficient food, the scientists found.

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“Dinosaurs were fast-growing and they required a lot of resources,” said co-author Randall Irmis of the University of Utah’s Department of Geology and Geophysics. “With unpredictable, hot and dry conditions, plant populations were changing all the time and you might not have the dependable food supply you need.”

Among the more dramatic impacts were frequent and intense wildfires that swept through large areas, further cutting into food supplies, he said.

The weather extremes resulted in part from high levels of greenhouse gases — caused mostly by volcanic activity —which at that time were more than four times the historically high levels seen in this century. With so much carbon in the atmosphere, the Earth was a “greenhouse world,” Irmis said, with higher sea levels and “without ice caps at the poles.”  Eventually dinosaurs were able to adapt, colonizing the entire world, but only after a delay of a least 15 million years, Irmis said.

The study’s authors said their conclusions carry a warning for humans, as rapidly rising greenhouse gas levels raise questions about the future habitability of parts of the planet.

“We show that the climatic effects of increased CO2 significantly reshaped the land plant communities that are at the foundation of terrestrial food webs,” said lead author Jessica Whiteside of Southampton University’s National Oceanography Centre. For modern humans, rising carbon levels risk “considerable effects on the composition of ecosystems” that could survive on land, she said..

“Our data reflect that there are possibly substantial hurdles to human sustainability in the future if we undergo the high CO2 levels predicted to occur in the coming 100-200 years,” she said.

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