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The study, published July 1 in the journal Science Advances, examines the circumstances surrounding the Triassic-Jurassic extinction event 202 million years ago, which killed off a number of large reptiles and eventually led to the dinosaurs taking over.
During the extinction event, researchers said cold snaps killed many cold-blooded reptiles.
By examining footprints and rock fragments in a remote desert of northwest China’s Junggar Basin, the researchers say Triassic dinosaurs, a relatively small group that inhabit Earth’s polar regions, “survived the evolutionary bottleneck and spread.”
“Dinosaurs were under the radar all the time during the Triassic,” Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study, said in a statement.
“The key to their eventual dominance was very simple. They were basically cold-adapted animals. When it got cold everywhere, they were ready, other animals were not.”
Dinosaurs are thought to have first appeared in southern temperate latitudes about 231 million years ago during the Triassic period, the researchers say.
At that time, most of the earth was joined together into one giant continent known as Pangea.
Dinosaurs made their way to the far north about 214 million years ago, and until the mass extinction, reptiles ruled the tropical and subtropical regions of the planet.
While atmospheric carbon dioxide concentrations then were at or above 2,000 parts per million, or five times what they are today, resulting in “intense” temperatures, the researchers say climate models suggest higher latitudes experienced seasonal temperature declines and little for most of the year would have received sunlight.
The researchers say that by the end of the Triassic period, massive volcanic eruptions, possibly lasting for hundreds of years, killed more than three-quarters of all land and sea life on the planet.
The eruptions would also have caused carbon dioxide levels in the atmosphere to rise, causing deadly temperature spikes and making ocean water too acidic for many life forms.
But the researchers say the eruptions also released sulfur aerosols that deflect sunlight and could cause repeated “global volcanic winters” lasting a decade and possibly longer.
Not only were Triassic dinosaurs able to survive in these conditions, the researchers say evidence has shown that many, if not all, non-avian dinosaurs also had primitive feathers that would have been used primarily as insulation. It is also believed that many dinosaurs were warm-blooded and possessed high metabolisms.
“There is a stereotype that dinosaurs have always lived in lush tropical jungles, but this new research shows that the higher latitudes would have been frozen and even covered with ice for parts of the year,” says Stephen Brusatte, professor of paleontology and evolution at the University of Edinburgh said.
“Dinosaurs that live at high latitudes happened to already have winter coats [while] Many of their Triassic competitors died out.”
As for the physical evidence supporting their study, the researchers examined fine-grained sandstone and siltstone formations left in the sediments of shallow ancient lake floors in the Junggar Basin, formed 206 million years ago during the Late Triassic. At that time, the basin would have been above the Arctic Circle.
Footprints show that dinosaurs were present along shorelines, while pebbles about 1.5 centimeters wide found far from any obvious shoreline offered evidence of “ice-raised debris,” they say.
Ice-drifted debris forms when ice builds up against a coastal landmass and picks up underlying boulders, the researchers say.
Eventually the ice breaks loose and drifts away. As they melt, the stones fall off and mix with the sediment.
The researchers say the pebbles were likely collected in winter when the lake’s water froze and floated away as the weather warmed.
“This shows that these areas froze regularly and the dinosaurs did well,” said study co-author Dennis Kent, a geologist at Lamont-Doherty.
The researchers say more work is needed to find fossils in former polar areas like the Junggar Basin.
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