Many of us are familiar with the popular theory of how the dinosaurs died 66 million years ago: a violent collision between Earth and a meteorite, followed by a global winter caused by dust and debris that choked the atmosphere. But there was a much more mysterious and less-discussed earlier extinction: that of 202 million years ago, which wiped out the great reptiles that had ruled the planet until then, and apparently paved the way for the dinosaur takeover.
What caused the so-called Triassic-Jurassic extinction event, and why did dinosaurs thrive when other creatures died out? We know that the world was generally hot and muggy during the Triassic period that preceded the extinction, and there were similar conditions during the following Jurassic that ushered in the age of the dinosaurs. But new research is turning the notion of thermophilic dinosaurs on its head: it provides the first physical evidence that Triassic dinosaur species, then a small group mostly relegated to the polar regions, were regularly exposed to freezing conditions there.
The telltale indicators are dinosaur footprints along with strange rock fragments that could only have been deposited by ice. The authors of the study explain that during the extinction, cold snaps already occurring at the poles spread to lower latitudes and killed the cold-blooded reptiles. Dinosaurs that had already adapted survived the evolutionary bottleneck and spread.
The rest is ancient history. Dinosaurs were under the radar all along the Triassic, said Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. The key to their eventual dominance was very simple.
They were basically cold-adapted animals. When it got cold everywhere, they were ready and other animals were not. The study, based on recent excavations in the remote desert of northwest China’s Junggar Basin, was published on (July 1, 2022) in the journal Science Advances.
Dinosaurs are believed to have first appeared in southern temperate latitudes during the Triassic period, about 231 million years ago, when most of the planets were joined together to form a vast continent that geologists call Pangea. They made it to the far north about 214 million years ago. Until the extinction event 202 million years ago, the broader tropical and subtropical regions in between were dominated by reptiles, including relatives of crocodiles and other fearsome creatures at over 2,000 parts per million, five times today’s levels, so temperatures must have been extreme.
There is no evidence of polar ice caps at that time, and excavations have shown that deciduous forests grew in polar regions. However, some climate models indicate that it was intermittently cool in the high latitudes; Even with all that CO2, they would have received little sunlight for most of the year, and temperatures would drop, at least seasonally. But until now, no one has presented physical evidence that they froze.
At the end of the Triassic, a geologically brief period of perhaps a million years, more than three-quarters of all terrestrial and marine species on the planet, including shelled creatures, corals, and all major reptiles, became extinct. Some cave-dwelling animals, like turtles, made it, as did some early mammals.
It’s unclear exactly what happened, but many scientists link it to a series of massive volcanic eruptions that may have lasted hundreds of years at a time. At this time, Pangea began to split, opening what is now the Atlantic Ocean and separating what is now America from Europe, Africa, and Asia. Among other things, the eruptions would have caused atmospheric carbon dioxide to skyrocket above its already high levels, causing deadly temperature spikes on land and making seawater too acidic for many creatures to survive.
The authors of the new study cite a third factor: During the most violent phases of the eruptions, they spewed sulfur aerosols that deflected so much sunlight that they caused repeated global volcanic winters that overwhelmed high greenhouse gas levels.
Those winters might have lasted a decade or more; even in the tropics, prolonged freezing conditions may have occurred. This didn’t kill isolated reptiles, but cold-adapted, isolated dinosaurs were able to hold on, the scientists say. Researchers demonstrate: fine-grained sandstone and siltstone formations left behind by sediments in shallow ancient lake floors in the Junggar Basin.
The sediments formed 206 million years ago during the late Triassic, through the mass extinction and beyond. At that time, before the land masses realigned, the basin was about 71 degrees north, well above the Arctic Circle. Footprints found by the authors and others indicate that dinosaurs were present along shorelines.
In the lakes themselves, meanwhile, the researchers found copious amounts of pebbles, up to about 1.5 centimeters in diameter, in the normally fine sediments. Far from an obvious shoreline, the pebbles had no business. The only plausible explanation for their presence: they were debris from Ice Fleets (IRD).
In short, IRD is formed when ice forms against a coastal landmass and traps underlying boulders. Eventually, the ice loosens and drifts off into the adjacent body of water. When it melts, the rocks fall to the ground and mix with normal fine sediments.
Geologists have extensively studied ancient IRD in the oceans, where it is supplied by glacial icebergs but rarely in lake beds; The discovery of the Junggar Basin adds to the sparse records. The authors say the pebbles were likely collected in winter when the lake’s waters froze along the pebbly shores. As warm weather returned, chunks of this ice drifted away with samples of the pebbles in tow and later dropped them.
This shows that these areas froze regularly and the dinosaurs fared well, said study co-author Dennis Kent, a geologist at Lamont-Doherty. How did you do that? Since the 1990s, evidence has been accumulating that many, if not all, non-avian dinosaurs, including tyrannosaurs, had primitive feathers. If not for flight, some covers could have been used for mating purposes, but the researchers say their primary purpose was insulation.
There is also good evidence that many dinosaurs possessed warm-blooded systems with high metabolisms, in contrast to cold-blooded reptiles. Both properties would have helped dinosaurs in cold conditions. Severe wintry episodes during volcanic eruptions may have brought freezing temperatures to the tropics, where many of the extinctions of large, naked, featherless vertebrates appear to have occurred, Kent said. Whereas our fine feathered friends, who have adapted to colder temperatures at higher latitudes, have been OK.
The results defy conventional dinosaur imagery, but some prominent specialists say they’re convinced. There’s 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 during parts of the year, said Stephen Brusatte, a professor of paleontology and evolution at the University of Edinburgh. Dinosaurs that lived at high latitudes happened to already have winter coats [while] many of their Triassic competitors became extinct.
Randall Irmis, curator of paleontology at the Natural History Museum of Utah and a specialist in early dinosaurs, agrees. This is the first detailed evidence from the high paleo-elevations, the first evidence for the last 10 million years of the Triassic period, and the first evidence of truly icy conditions, he said. People are used to thinking of this as a time when the entire globe was hot and humid, but that just wasn’t the case. Olsen says the next step in understanding this period better is for more researchers to look for fossils in what was once the polar region, the Junggar Basin. The fossil record is very poor and nobody is doing research, he said. Those rocks are gray and black, and it’s a lot harder to look [for fossils] in those layers. Most paleontologists are drawn to the late Jurassic, which is known to have many large skeletons. The Paleo-Arctic is basically ignored.
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