Dinosaurs emerged around 231 million years ago during the Triassic period of geological time. They went on to rule the Earth for nearly the next 170 million years.
Everyone knows about the mass extinction spurred on by the asteroid impact 66 million years ago that saw the end of the reign of the “terrible lizards”. But what about the earlier mass extinction 202 million years ago, which solidified the dinosaur dynasty?
New research has illuminated aspects of this more mysterious and less discussed extinction event, answering questions about what caused the extinction and why the dinosaurs survived while other animals did not.
Dinosaurs are associated with hot, humid climes. And that’s because the periods in which they lived – the Triassic, Jurassic and Cretaceous – were generally steamy.
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But dinosaurs are more complicated than that. It’s now well established that dinosaurs had colonised the cold, icy Arctic later in their time on Earth. But research published in Science Advances suggests that dinosaurs’ adaptability to colder climates began much sooner and may have been the key to their initial success over other creatures.
The findings show for the first time that Triassic dinosaurs – bit-part players at the time, largely relegated to the polar regions – endured freezing conditions regularly.
The study was based on recent excavations in the remote northwest Chinese desert of the Junggar Basin. Researchers found dinosaur footprints along with odd rock fragments that only could have been deposited by ice.
The researchers say that, during the extinction that marked the end of the Triassic and the beginning of the Jurassic, cold snaps already common in the polar regions were starting to become more frequent in lower, warmer latitudes. The chill would have been lethal for many of the cold-blooded reptile species that had dominated over the dinosaurs, which had not yet evolved into the giants that we’ve come to know and love.
Dinosaurs, on the other hand, were already adapted to the cold and were able to spread into the ecological niches left behind by the extinction.
“Dinosaurs were there during the Triassic under the radar all the time,” says lead author Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory, in the US. “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.”
By about 214 million years ago, dinosaurs had made it to the far north of the supercontinent Pangaea. The tropical and subtropical regions in between the poles was dominated by giant relatives of crocodiles and other reptiles.
Atmospheric CO2 concentration in the Triassic and Jurassic was five times today’s levels at 2000 parts per million. The greenhouse effect would, therefore, have seen intense heat and humidity. In fact, there is no evidence that there even existed polar ice caps at the time.
However, the higher latitudes would have received little sunlight and been seasonally cold. Until now, though, there has been no evidence of freezing – even toward the far north of Pangaea.
The Triassic-Jurassic extinction may have lasted a million years and seen the demise of three-quarters of Earth’s land and marine species. Apart from the dinosaurs, animals that survived the extinction included small burrowing animals like turtles and early mammals.
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While it’s not entirely clear what caused the extinction, scientists believe massive tectonic shifts were breaking Pangaea apart and causing heightened volcanic activity.
Apart from pumping even more carbon dioxide into the atmosphere and turning oceans acidic, these eruptions would have sent huge amounts of sulphur aerosols into the air. These gases would have deflected sunlight, causing “volcanic winters” that cancelled out the greenhouse effect. The resultant winters may have lasted years and killed off the less cold-adapted reptiles while dinosaurs survived.
On an ancient lake bottom in what is now northwest China, the researchers found 206-million-year-old dinosaur footprints. The dinosaurs would then have been living above the Arctic Circle at around 71°N. But the team also found pebbles up to 1.5 centimetres across in the otherwise fine lakebed sediment.
Far from the prehistoric lake shore, the pebbles shouldn’t be there. The realistic explanation: they are ice-rafted debris (IRD). IRDs are rocks picked up by ice formed against a coastal land mass that drifts and melts, depositing the pebbles.
“This shows that these areas froze regularly, and the dinosaurs did just fine,” says co-author Dennis Kent, also a Lamont-Doherty geologist.
Many, if not all, dinosaur species are now known to have been feathered, potentially including iconic species such as Tyrannosaurus rex. Some dinosaurs were pioneers of flight, giving rise to their bird descendants. Others used their feathers in mating and other displays. But it seems that the down’s main purpose would have been insulation. Most dinosaurs were also warm-blooded. Both of these characteristics would have been useful adaptations during wintery conditions thrown up amid the Triassic-Jurassic extinction.
“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,” says Kent. “Whereas our fine feathered friends acclimated to colder temperatures in higher latitudes did OK.”
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“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,” says Stephen Brusatte, a professor of paleontology and evolution at the University of Edinburgh, in Scotland. “Dinosaurs living at high latitudes just so happened to already have winter coats [while] many of their Triassic competitors died out.”
Randall Irmis, curator of paleontology at the Natural History Museum of Utah, US, and early dinosaur specialist, agrees. “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,” he explains. “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.”
To build a better picture of the period and understand how early dinosaurs made it while other animals died out, the researchers say there needs to be more exploration of sites in formerly polar areas, like the Junggar Basin.
“The fossil record is very bad, and no one is prospecting,” laments Olsen. “These rocks are gray and black, and it is much harder to prospect [for fossils] in these strata. Most paleontologists are attracted to the late Jurassic, where it’s known there are many big skeletons to be had. The paleo-Arctic is basically ignored.”