Many of us know the traditional theory of how dinosaurs died out 66 million years ago: in a horrific collision of Earth with a meteorite, and the following global winter as dust and debris engulfed the atmosphere . But there was a previous extinction, far more mysterious and less discussed: one 202 million years ago, which killed off the large reptiles that had ruled the planet until then, and apparently cleared the way for dinosaurs. What caused the so-called Triassic-Jurassic extinction, and why did dinosaurs thrive when other creatures died out?
We know that the world was generally hot and steamy during the Triassic period, which preceded the extinction, and during the following Jurassic, which kicked off the age of the dinosaurs. However, a new study turns the idea of heat-loving dinosaurs on its head: It offers the first physical evidence that Triassic dinosaur species—then a minor group that largely migrated to the polar regions—regularly There he tolerated cold conditions. Telltale Indicators: Dinosaur footprints as well as strange rock fragments that could only have been deposited by ice. The study authors say that during the extinction, cold snaps already occurring at the poles spread to lower latitudes, killing off the cold-blooded reptiles. Dinosaurs, already adapted, survived the evolutionary bottleneck and spread. The rest is ancient history.
“Dinosaurs were under the radar all the time during the Triassic,” said Paul Olson, 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 animals adapted to the cold. When it was freezing everywhere, they were ready, and other animals were not.”
The study, based on recent excavations in the remote desert of the Jungar Basin in northwestern China, was recently published in the journal Science. Science Advances.
Dinosaurs are thought to have first appeared in temperate southern latitudes during the Triassic period, about 231 million years ago, when much of the planet’s land was joined together into a vast continent that geologists called Pangaea. They made it to the far north about 214 million years ago. The more expansive tropical and subtropical regions in between were dominated by reptiles, including crocodiles and relatives of other fearsome creatures, until the mass extinction in 202 million years.
During the Triassic, and for most of the Jurassic, atmospheric concentrations of carbon dioxide were 2000 parts per million or more – five times the level today – so temperatures must have been intense. Then there is no evidence of polar ice caps, and excavations have shown that deciduous forests have developed in the polar regions. However, some climate models suggest that the higher latitudes were sometimes colder; Even with that much CO2, they received little sunlight for most of the year, and temperatures dropped at least seasonally. But so far, no one has provided any physical evidence that they have froze.
At the end of the Triassic, a geologically brief period of perhaps one million years saw the extinction of more than three quarters of all terrestrial and marine species on the planet, including roundworms, coral and all sizable reptiles. Some animals that lived in the burrow, such as turtles, made it, as did some early mammals. It is not clear what exactly happened, but many scientists link it to a series of massive volcanic eruptions that could have lasted hundreds of years. At this time, Pangea began to separate from what is now the Atlantic Ocean, separating what is now the Americas from Europe, Africa, and Asia. Among other things, the eruptions would have caused atmospheric carbon dioxide to rise beyond its already high levels, raising deadly temperatures on land, and also turning ocean water into acid for many organisms to survive. will be given.
The authors of the new study cite a third factor: During the fierce phases of eruptions, they may have stirred up sulfur aerosols that deflected so much sunlight, they caused repeated global volcanic winters that led to higher greenhouse-gas levels. dominated. These winters must have lasted a decade or more; Even in the tropics, continuous cold conditions can be observed. Scientists say it killed the untouched reptiles, but the cold-adapted, untouched dinosaurs were able to hang on.
Researchers’ Evidence: Formations of fine-grained sandstone and siltstone left by sediment in a shallow ancient lake floor in the Jünger Basin. The sediment was formed 206 million years ago during the late Triassic, through the mass extinction and beyond. At that time, before the landmasses rearranged themselves, the basin was about 71 degrees north, well above the Arctic Circle. Footprints found by the authors and others suggest that dinosaurs existed along the shoreline. Meanwhile, in the lakes, the researchers found abundant pebbles up to about 1.5 centimeters deep within the normally fine sediments. Away from any clear shoreline, Pebbles had no business there. The only plausible explanation for their presence: they were ice-covered debris (IRD).
In a nutshell, IRD is created when ice builds up against a coastal terrain and includes pieces of underlying rock. At some point the ice becomes colorless and flows into the nearby water body. When it melts, the rocks mix with the normal finer sediments and fall to the bottom. Geologists have studied the ancient IRD extensively in the oceans, where it is distributed by glacial icebergs, but rarely in lake beds; The discovery of the Janggar Basin has added to the record. The authors state that the pebbles were probably picked up during the winter, when the lake’s waters froze along the pebbled shorelines. When warm weather returned, that iceberg swam with samples of pebbles in tow, and later dropped them.
“This suggests that these areas froze regularly, and the dinosaurs did just fine,” said study co-author Dennis Kent, a geologist at Lamont-Doherty.
How did they do it? Evidence has been building since the 1990s that many non-avian dinosaurs, including tyrannosaurs, had primitive feathers. If not for flight, some of the covers could have been used for demonstration purposes, but the researchers say their main purpose was insulation. There is also good evidence that unlike cold-blooded reptiles, many dinosaurs had warm-blooded, highly metabolic systems. Both qualities may have helped dinosaurs in colder conditions.
“Severe winter episodes during volcanic eruptions may have brought tropical temperatures to cooler temperatures, where many of the extinctions of large, naked, un-feathered vertebrates occurred,” Kent said. “While our fine feathered friends adapted to the colder temperatures in the higher latitudes, it was fine.”
The findings defy the traditional imagination of dinosaurs, but some leading experts say they are convincing. “There is a stereotype that dinosaurs have always lived in lush tropical forests, but this new research suggests that the higher latitudes may have been cold and even snow-covered in some parts of the year,” said Stephen Brusette. , Professor of Paleontology and Evolution. University of Edinburgh. “Dinosaurs living at high latitudes already have winter coats” [while] Many of his Triassic competitors died. ,
Randall Irmis, curator of paleontology at the Natural History Museum of Utah and an expert on early dinosaurs, agrees. “This is the first detailed evidence from the Upper Palaeolithic, the first evidence for the Triassic period of the last 10 million years, and the first evidence of truly icy conditions,” he said. “People are used to thinking of it as a time when the whole world was hot and humid, but it was not.”
Olsen says the next step to better understand this period is for more researchers to look for fossils in east polar regions such as the Junggar Basin. “The fossil record is very poor, and no one is doing prospecting,” he said. “These rocks are brown and black, and it’s too hard to prospect” [for fossils] In these classes. Most paleontologists are drawn to the late Jurassic, where it is known that there are many large skeletons. The Paleo-Arctic is basically ignored.”
The study was co-authored by Jingheng Sha and Yanan Feng of the Nanjing Institute of Geology and Paleontology; Clara Chang and Sean Kinney of the Lamont-Doherty Earth Observatory; Jessica Whiteside of the University of Southampton; The Smithsonian Institution’s Hans-Dieter lawsuit; Morgan Scholar of Rensselaer Polytechnic Institute; and Viv Vajda of the Swedish Museum of Natural History.