'Cold-adapted' dinosaurs survived mass extinction event to achieve dominance, study finds
A new study has offered what it says is the first physical evidence showing dinosaurs from the Triassic period regularly endured freezing conditions, allowing them to survive and eventually supersede other species on the planet.
The study, published in the journal Science Advances on July 1, looks at the circumstances surrounding the Triassic-Jurassic Extinction 202 million years ago, which killed off a number of large reptiles and led to the eventual takeover of dinosaurs.
During the extinction event, researchers say cold snaps killed off many cold-blooded reptiles.
Through studying footprints and rock fragments in a remote desert of the Junggar Basin in northwest China, the researchers say Triassic dinosaurs, a relatively minor group populating Earth's polar regions, survived the "evolutionary bottleneck and spread out."
"Dinosaurs were there during the Triassic under the radar all the 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 about 231 million years ago during the Triassic period in temperate southern latitudes, the researchers say.
At the time, most of Earth's land was joined together as one giant continent known as Pangaea.
Dinosaurs made it to the far north about 214 million years ago and until the mass extinction, reptiles dominated the planet's tropical and subtropical regions.
While atmospheric concentrations of carbon dioxide then were at or above 2,000 parts per million or five times today's levels resulting in "intense" temperatures, the researchers say climate models suggest higher latitudes did experience seasonal temperature declines and would have received little sunlight much of the year.
By the end of the Triassic period, the researchers say massive volcanic eruptions potentially lasting hundreds of years killed more than three-quarters of all terrestrial and marine life on the planet.
The eruptions also would have caused carbon dioxide levels in the atmosphere to rise, creating deadly temperature spikes and making ocean waters too acidic for many lifeforms.
But the researchers say the eruptions also would have released sulfur aerosols, capable of deflecting sunlight and causing repeated "global volcanic winters" lasting a decade and possibly longer.
Not only were Triassic dinosaurs able to survive under these conditions, the researchers say evidence has shown many if not all non-avian dinosaurs also had primitive feathers that would have been used mainly as insulation. Many dinosaurs also are believed to have been warm blooded and possessed high metabolisms.
"There is a stereotype that dinosaurs always lived in lush tropical jungles, but this new research shows that the higher latitudes would have been freezing and even covered in ice during parts of the year," Stephen Brusatte, a professor of paleontology and evolution at the University of Edinburgh, said.
"Dinosaurs living at high latitudes just so happened to already have winter coats [while] many of their Triassic competitors died out."
As for the physical evidence supporting their study, the researchers looked at fine-grained sandstone and siltstone formations left behind in the sediments of shallow ancient lake bottoms in the Junggar Basin, formed 206 million years ago during the late Triassic. At the time, the basin would have been located above the Arctic Circle.
Footprints show dinosaurs were present along the shorelines, while pebbles about 1.5 centimetres wide, found far from any apparent shoreline, offered evidence of "ice-rafted debris," they say.
Ice-rafted debris forms when ice builds against a coastal landmass and takes in bits of underlying rock, the researchers say.
The ice eventually detaches and drifts away. As it melts, the rocks fall off and mix in with the sediment.
The researchers say the pebbles were likely picked up during the winter when lake waters froze and floated away as the weather warmed.
"This shows that these areas froze regularly, and the dinosaurs did just fine," study co-author Dennis Kent, a geologist at Lamont-Doherty, said.
The researchers say more work is needed to find fossils in former polar areas, such as the Junggar Basin.