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Why big dinosaurs avoided the tropics like the plague

For most people, the tropics represent a sort of paradise – continuously warm weather, ample sunshine, scenic beaches, that sort of thing. But for the 30 million years they walked the Earth, the big behemoth herbivorous dinosaurs were nowhere to be found near the equator. Now, a study published in the Proceedings of the National Academy of Sciences explains why you’re unlikely to see a brontosaurus on your next cruise to the Caribbean: They couldn’t handle the climate extremes.

“Our data suggest it was not a fun place,” says study co-author Randall Irmis, curator of paleontology at the Natural History Museum of Utah and assistant professor at the University of Utah. “It was a time of climate extremes that went back and forth unpredictably and large, warm-blooded dinosaurian herbivores weren’t able to exist nearer to the equator – there was not enough dependable plant food.”

For the study, the authors managed to piece together an incredibly detailed image of the climate and environment more than 200 million years ago at Ghost Ranch, New Mexico. The site is rich in fossils from the Triassic Period, but suspiciously light on specimens of large, plant-eating dinosaurs.

It turns out that the weather in ancient times was exponentially moodier than it is today. In wet periods the environment would likely have been exceptionally lush and hospitable, but when things became hot and dry, they got really hot and dry. Vegetation would whither and dry out, and wildfires were common – further decreasing the plant life demanded by large herbivores.

Ghost Ranch, which at the time would have occupied a latitude similar to India today, is home to many Chinle Formation rocks. Within the rocks, they analyzed fossils, charcoal left by ancient wildfires, and stable isotopes from organic matter and carbonate nodules that formed in ancient soils – all clues about the past climate deposited within the rocks.

Small carnivores would have found enough prey to survive, but larger animals were out of luck.

“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 geochemist Jessica Whiteside, lecturer at the University of Southampton. “The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive.”