(CN) — When we think of dinosaurs, most of us think of large, lumbering lizards trudging through hot, tropical climates. We think of massive teeth and thick, scaly skin. Perhaps we even think of the meteorite that crashed to earth, triggering the ice age and killing off the wondrous beasts.
But a new study, published Friday in the journal Science Advances, presents a counterintuitive theory: that dinosaurs rose to global prominence because they were adapted not to the heat, but to the cold.
"We got dinosaurs all wrong," says Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory, and lead author of the study. "Dinosaurs are fundamentally cold-adapted animals."
He adds: "They were always living in these cold areas. When it got cold everywhere, they were ready. And other animals weren’t.”
The study centers around a mass extinction event — not the famous meteor, but a lesser-known global catastrophe that occurred at the end of the Triassic period. A myriad of volcanic eruptions sent lava flowing out for hundreds of years and spewed out tons of carbon dioxide into the atmosphere — to levels ten times of what they are now — drastically altering life on Earth.
"Traditionally, people thought, maybe that global warming had to do with mass extinction," says Olsen, since more carbon dioxide typically means a hotter planet.
But the new study argues just the opposite: that the volcanic event made the planet colder. Although carbon dioxide heats the atmosphere, the presence of such a high quantity of sulfur dioxide would have blocked the sunlight, leading to a sort of "volcanic winter" that enveloped the Earth for hundreds of years.
The key evidence the new paper presents is based on recent excavations in the Junggar Basin, a remote desert in northwest China. Archeologists there found an abundance of tiny pebbles, about 1.5 centimeters across, within fine sediments. Olsen and the other scientists believe these tiny pebbles are evidence of "ice rafting" — that is, seasonal sheets of ice rubbing up against the shore. These sediments date back to the late Triassic period, around the time of the mass extinction event.
"I picture it as a frozen lake," says Dennis Kent, a geologist at the Columbia Climate School, one of the study's co-authors. "Over the winter, the lake freezes. In the spring it starts melting, the ice breaks up, and pieces near the shore float out in the center of the lake, they melt, and drop sediment."
Archeologists also found dinosaur footprints in the same area, dating back to the same time.
Dinosaurs first showed up in the Southern Hemisphere, and had migrated to the Northern Hemisphere roughly 15 million years later. During the Triassic period, most dinosaurs were confined to higher latitudes, closer to the North and South Poles. Olsen, Kent and co-authors believe that the volcanic winter wiped out at least half of all species, but spared the dinosaurs, precisely because they were already adapted to living in the cold, in higher latitudes.
"When you look at who went extinct, the crocodile relatives that lived in the tropics were almost all wiped out," says Olsen. "All large reptiles on land were wiped out. And the dinosaurs and their pterosaur relatives, not only were they not intensely bothered, but they expanded in diversity and range immediately, and they filled in the tropics, where they hadn’t existed before."
Which brings us to feathers. For the last three decades, evidence has been mounting that many species of dinosaurs — including tyrannosaurs, a relative of the famous Tyrannosaurus Rex — had some form of feathers. Some small dinosaurs used the feathers to glide. But they may also have been for insulation, and they may have been the crucial trait that helped dinosaurs make it through the volcanic winter.
"Feathers, or things that have elements of feathers in fossils, are there across all these groups of what became dinosaurs," says Kent.
"We suspect all the small dinosaurs had feathers," says Olsen. Later on, in the Jurassic period, many dinosaurs evolved past the point of needing feathers, in much the same way mammals evolved past the point of needed hair all over their bodies. But many dinosaurs, as well as the flying pterosaurs, continued to have porto-feathers — thin hairs, a bit like an African elephant might have.
Though the findings in the new paper run counter to the conventional wisdom, a number of researchers, having seen the study, are convinced.
“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,” said Randall Irmis, curator of paleontology at the Natural History Museum of Utah, in a written statement. “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.”
Most dinosaurs didn't make it past the next extinction event, the meteor, which sent so much dust into the atmosphere that it effectively shut down photosynthesis, killing off most plants, starving large herbivores and then the large carnivores that fed off of them. But some dinosaurs did survive.
"We call them birds," says Olsen. "Birds are the surviving dinosaurs." Today, Olsen points out, there are still roughly 10,000 species of dinosaurs living on earth, about twice the number of mammal species.
Says Olsen: "We are still living in the age of dinosaurs."
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