A new study has offered what it says is the first physical evidence showing that Triassic period dinosaurs regularly withstood freezing conditions, allowing them to survive and eventually displace other species on the planet.
The study, published in the journal Science Advances on July 1, examines the circumstances surrounding the Triassic-Jurassic extinction event 202 million years ago, which killed off a number of large reptiles and led to the eventual takeover of the dinosaurs.
During the extinction event, researchers say the cold killed many cold-blooded reptiles.
By studying footprints and rock fragments in a remote desert of the Junggar Basin in northwestern China, researchers say Triassic dinosaurs, a relatively small group that inhabited Earth’s polar regions, survived the “evolutionary hurdle and spread.”
“Dinosaurs were there during the Triassic under the radar the whole time,” 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 fundamentally cold-adapted animals. When it got cold everywhere, they were ready and other animals weren’t.”
Dinosaurs are thought to have first appeared around 231 million years ago during the Triassic period in temperate southern latitudes, the researchers said.
At that time, most of Earth’s landmass was united as one giant continent known as Pangea.
Dinosaurs reached the far north about 214 million years ago, and until the mass extinction, reptiles dominated the tropical and subtropical regions of the planet.
While atmospheric concentrations of carbon dioxide then were at or above 2,000 parts per million, or five times today’s levels, leading to “intense” temperatures, the researchers say climate models suggest that higher latitudes experienced seasonal temperature drops and would receive little sunlight most of the year.
By the end of the Triassic period, massive volcanic eruptions potentially lasting hundreds of years had killed more than three-quarters of all land and sea life on the planet, researchers say.
Eruptions would also cause atmospheric carbon dioxide levels to rise, creating deadly temperature spikes and making ocean waters too acidic for many life forms.
But the researchers say the eruptions would also release sulfur aerosols capable of deflecting sunlight and causing recurring “global volcanic winters” lasting a decade and possibly more.
Not only were Triassic dinosaurs able to survive in these conditions, the researchers say evidence shows that many, if not all, non-avian dinosaurs also had primitive feathers that would have been used mainly as insulation. It is also believed that many dinosaurs were warm-blooded and had high metabolisms.
“There’s a stereotype that dinosaurs always lived in lush tropical jungles, but this new research shows that higher latitudes would have been frozen and even ice-covered during some parts of the year,” Stephen Brusatte, professor of paleontology and evolution at The University of Edinburgh said.
“Dinosaurs living at high latitudes happened to already have winter coats [while] many of their Triassic competitors died out.”
As for the physical evidence to support their study, the researchers looked at formations of fine-grained sandstone and siltstone left in sediments at the bottom of shallow ancient lakes in the Dzungar Basin, formed 206 million years ago during the Late Triassic. At that time, the basin should be above the Arctic Circle.
The footprints show that dinosaurs were present along the shores, while pebbles about 1.5 centimeters wide, found far from any visible shoreline, offer evidence of “debris from ice rafts,” they said.
Debris ice forms when ice accumulates on a coastal land mass and absorbs pieces of underlying rock, the researchers said.
The ice eventually separates and drifts away. As it melts, the rocks fall and mix with the sediment.
Researchers say the pebbles were likely collected during the winter when the lake waters 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, such as the Junggar Basin.
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