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Study: Tropical climate extremes kept dinosaurs at bay

SALT LAKE CITY — Intense heat. Drought. Wildly extreme weather accompanied by wildfires.

Those climate factors are enough to keep even the most hardy of creatures from settling in, even the gargantuan, plant-eating dinosaurs that roamed the earth more than 200 million years ago.

The first reconstruction of the climate and ecology of the Late Triassic Period has been put together by an international team of scientists, including Randall Irmis, assistant professor and curator of paleontology at the Natural History Museum of Utah.

“Our data suggest it was not a fun place,” Irmis said. "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.”

The study, funded with grants from the National Science Foundation, was published Monday in the Proceedings of the National Academy of Sciences.

Irmis said the study is significant because it represents the most comprehensive examination to date of a variety of evidence — such as fossil bones and charcoal formed by ancient fires — detailing the climatology and ecology that would have been in play between 205 million and 215 million years ago.

"We looked at a variety of lines of evidence that told us about the climate and the environment at the time, as well as what plants were living in the area," he said. "It was not a desert by any means, but it was really dry. There were good times and bad times for the plants, and would have been incredibly tough to get good resources in climate like this, which is why we think they stayed away."

The study, led by geochemist Jessica Whiteside, lecturer at the University of Southampton, tapped into nearly a decade's worth of field work in an area called the Ghost Ranch in northern New Mexico, which is rich with fossils from the Late Triassic Period.

Researchers focused on the multicolored rocks of the Chinle Formation that are a common sight on the Colorado Plateau. During the Late Triassic Period, North America and other land masses were all part of the supercontinent Pangea, and the Ghost Ranch site would been close to the equator at roughly the same latitude as present-day southern India.

"Our group has been working in this area called Ghost Ranch for almost 10 years now, and most of the data that was assembled in the new study took five years or a little longer," Irmis said.

He said scientists have been wondering why the equatorial tropics lacked the big dinosaurs for about the first 30 million years when dinosaurs first emerged.

The earliest known dinosaur fossils, found in Argentina, date from around 230 million years ago. Within 15 million years, multitudes of species of varying shapes, sizes and diets became abundant in areas beyond tropical latitudes. The only dinosaurs present in the tropics, however, were the small carnivores.

The new findings show that the tropical climate swung wildly with extremes of drought and intense heat. Wildfires swept the landscape during arid times, constantly reshaping the amount and kind of vegetation available.

Irmis said the study's results also contain important implications for climate change, with carbon dioxide levels during the Late Triassic that were four to six times current levels.

Those atmospheric carbon dioxide levels, calculated from stable isotope analyses of soil carbonate and preserved organic matter, rose from about 1,200 parts per million at the base of the section of organic matter, to about 2,400 parts per million near the top. At these high CO2 concentrations, climate models predict more frequent and more extreme weather fluctuations consistent with the fossil and charcoal evidence.

Authors say the overall picture is one of a climate that contained extreme precipitation shifts fueling more intense fires. In their wake, there were barren landscapes prone to greater erosion.

"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,” Whiteside said. “The fluctuating and harsh climate with widespread wildfires meant that only small, two-legged carnivorous dinosaurs, such as Coelophysis, could survive.”

The other study authors are Sofie Lindström, Ian Glasspool, Morgan Schaller, Maria Dunlavey, Sterling Nesbitt, Nathan Smith and Alan Turner.

Email: amyjoi@deseretnews.com, Twitter: amyjoi16