When dinosaurs first emerged on Earth, they became the world's top predators and conquered the land. That is, apparently everywhere except in the tropics. New research indicates that extreme climate swings - lasting tens of millions of years - were too much for the dinos and kept them out of the tropics, solving one of science's longstanding mysteries.

For more than 30 million years after dinosaurs first emerged, they remained inexplicably rare near the equator, where only a few small-bodied meat-eating dinosaurs could be found. But after creating a remarkably detailed picture of the tropical climate and ecology more than 200 million years ago, a team of scientists from the University of Utah believes they have finally explained the age-long absence of big plant-eaters at these low latitudes.

The new findings, published in the journal PNAS, show that the tropical climate wildly fluctuated, swinging from periods of extreme drought to intense heat. In addition, wildfires swept the landscape during these arid regimes, constantly reshaping the vegetation available for plant-eating animals.

"Our data suggest it was not a fun place," study co-author Randall Irmis, curator of paleontology at the Natural History Museum of Utah, said in a news release. "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."

This study is the first to provide a detailed look at the climate and ecology during the rise of the dinosaurs.

Drought and Fire

The researchers focused on Ghost Ranch in northern New Mexico, a site rich with fossils from the Late Triassic Period, to get a better idea of the tropical climate that kept certain dinosaurs at bay. Ghost Ranch stood close to the equator at roughly the same latitude as present-day southern India. Here, they analyzed fossils, charcoal left by ancient wildfires, and stable isotopes from organic matter and carbonate nodules that formed in ancient soils, in order to reconstruct the climate between 205 and 215 million years ago.

"Each dataset complements the others, and they all point towards similar conditions," said geochemist Jessica Whiteside from the University of Southampton, who led the study.

Fossilized bones, pollen grains and fern spores revealed the types of animals and plants living at different times, marked by layers of sediment. According to the results, dinosaurs made up less than 15 percent of the vertebrate animal remains found at Ghost Ranch. Instead, reptiles known as Pseudosuchian archosaurs - the ancestors of modern-day crocodiles and alligators - were abundant in the region.

What few dinosaurs that did inhabit this landscape were mostly small, carnivorous theropods. Big, long-necked dinosaurs, or sauropodomorphs, on the other hand, did not exist at the study site. Though that's not to say that the carnivores bullied them out of this landscape, as they were the dominant plant-eaters at other higher latitudes.

According to the study, the region's extreme and unstable ecosystem suppressed these herbivores from inhabiting the tropics for 30 million years. (Scroll to read on...)

Abrupt changes in climate left a record in the shifting abundance of different types of pollen and fern spores between sediment layers. Fossilized organic matter from decaying plants provided another window on climate shifts. Changes in the ratio of stable isotopes of carbon in the organic matter bookmarked times when plant productivity declined during extended droughts.

Not only did drought plague the tropics millions of years ago, but wildfires of varying intensity also made this land inhabitable for plant-eaters. That is, extreme shifts in precipitation caused plant die-offs that fueled hotter fires, which in turn killed more plants, damaged soils and increased erosion.

By limiting the amount of available plant resources, large-bodied herbivores could not thrive.

"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," explained Whiteside. "The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive."

Interestingly, these findings not only solve a longstanding mystery about the rise of the dinosaurs, but in addition have important implications for understanding current man-made climate change. Atmospheric carbon dioxide (CO2) levels during the Late Triassic were four to six times current levels. And if CO2 emissions continue at their current rate, we are likely to see more extreme weather fluctuations like those that kept plant-eating dinosaurs out of the tropics.

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