science

Dinosaurs took over planet amid cold that other reptiles couldn’t survive 200 million years ago, study finds



A mass extinction event about 202 million years ago that wiped out big reptiles of the time, coinciding with a series of sudden global chills, paved the way for dinosaurs to take over the planet, a new study has suggested.

The research, based on recent excavations in the remote desert of northwest China’s Junggar Basin found the first physical evidence that during the Triassic period (252 million to 201 million years ago), dinosaur species of the time – a minor group relegated to the polar regions – regularly endured freezing conditions.

“Dinosaurs were there during the Triassic under the radar all the time. 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,” study lead author Paul Olsen from Columbia University said.

In the study, published last week in the journal Science Advances, scientists analysed fossil dinosaur footprints along with odd rock fragments that only could have been deposited by ice.

They noted that atmospheric concentrations of carbon dioxide during the Triassic, and for most of the Jurassic ranged at or above 2000 parts per million – five times today’s levels with temperatures likely being intense.

Earth’s North and South poles were likely free of ice caps at this point with paleontological excavations showing deciduous forests grew in these polar regions during this time.

Then in a geologically brief period of about a million years at the end of the Triassic, evidence reported in previous studies points to a mass extinction of more than three quarters of all terrestrial and marine species on the planet, including shelled creatures, corals and all sizable reptiles.

While research points to the survival of animals living in burrows, such as turtles as well as a few early mammals, scientists are unsure exactly what happened.

Many scientists suspect the extinctions are linked to a series of massive volcanic eruptions that could have lasted hundreds of years at a stretch.

Researchers suspect the eruptions could have caused atmospheric carbon dioxide levels to skyrocket beyond its already high levels, causing deadly temperatures spikes on land, and turning ocean waters too acid for many creatures to survive.

Previous studies have also shown that at this time, the Pangaea supercontinent started to split apart, opening what is now the Atlantic Ocean, and separating what are now the Americas from Europe, Africa and Asia.

The new study finds the posibility of a third factor.

Scientists say sulphur aerosols released during the eruptions’ fiercest phases deflected so much sunlight that they caused repeated global volcanic winters that overpowered high greenhouse-gas levels.

These winters, they say, may have lasted a decade or more with even the tropics likely experiencing freezing conditions, that eventually killed uninsulated reptiles.

However, the study theorises cold-adapted, insulated dinosaurs could hang on.

Complimenting this theory, researchers found evidence in the form of fine-grained sandstone and siltstone formations left by sediments in shallow ancient lake bottoms in the Junggar Basin, which at the time of the extinction event was well above the Arctic circle.

The sediments, according to the scientists, formed 206 million years ago during the late Triassic, through mass extinction and beyond.

Researchers also found abundant pebbles at the site up to about 1.5 cm across which they say the pebbles had “no business being there.”

The only plausible explanation for the pebbles, according to the scientists, is that they were ice-rafted debris (IRD).

IRD, researchers say, are created when ice forms against a coastal landmass and incorporates bits of underlying rock.

They say the pebbles were likely picked up during winter, when lake waters froze along pebbly shorelines, and as warm weather returned, chunks of the ice may have floated off with samples of the pebbles in tow, and later dropped them.

“This shows that these areas froze regularly, and the dinosaurs did just fine,” study co-author Dennis Kent said.

“Severe wintery episodes during volcanic eruptions may have brought freezing temperatures to the tropics, which is where many of the extinctions of big, naked, unfeathered vertebrates seem to have occurred. Whereas our fine feathered friends acclimated to colder temperatures in higher latitudes did OK,” Dr Kent added.

Contrary to a widely held assumption that dinosaurs mostly lived in lush tropical jungles, the new research shows that the higher latitudes where some dinosaurs lived would have been freezing and even covered in ice during parts of the year.

“Dinosaurs living at high latitudes just so happened to already have winter coats [while] many of their Triassic competitors died out,” Stephen Brusatte, another co-author of the study said.

“This is the first detailed evidence from the high paleolatitudes, the first evidence for the last 10 million years of the Triassic Period, and the first evidence of truly icy conditions,” Randall Irmis, curator of paleontology at the Natural History Museum of Utah, said.

“People are used to thinking of this as being a time when the entire globe was hot and humid, but that just wasn’t the case,” Dr Irmis said.



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