How Dinosaurs Thrived in the Snow

Imagine a tyrannosaur striding through the snow, leaving three-toed footprints in the powder as flurries fall on the fuzz along the dinosaur’s back. The vision might seem fit for fantasy, vastly different than the steamy and plant-choked settings we typically think of dinosaurs inhabiting. Yet such scenes truly transpired millions of years ago, with an entire spiky, feathery and beaked menagerie of dinosaurs thriving in polar habitats marked by greater swings between the seasons and prolonged winter darkness.

The finds are coming fast and furious. A tiny jaw found in Alaska’s ancient rock record, and written about in July, indicates that dinosaurs nested in these places and stayed year-round. In 2018, paleontologists published a study describing how microscopic details of polar dinosaur bones show that some dinosaurs slowed their growth during harsh seasons to get by with less. The ongoing identification of new species, not found anywhere else, highlighted how some dinosaurs adapted to the cold. Each thread comes together to underscore how wonderfully flexible dinosaur species were, adapting to some of the harshest habitats of their time.

Understanding when and where polar dinosaurs roamed takes a little geological imagination. Earth’s continents are always shifting, so the climates where fossils are found were once different. The environments recorded in the strata of southern Australia, for example, were further south and within the Antarctic Circle when dinosaurs thrived there in the Cretaceous. But in reconstructing the tectonic jigsaw and tracking where fossils have been uncovered, paleontologists have found dinosaurs that lived near both the northern and southern poles at different times.

Some of the oldest polar dinosaurs are found among the rocks of southern Australia’s aptly-named Dinosaur Cove. Over 110 million years ago, says Monash University paleontologist Patricia Rich, this area was a temperate rainforest carpeted with ferns and bushy-looking conifers called podocarps. And while the Cretaceous world was a bit warmer, with no polar icecaps, winter could still be harsh. “There would have been ice and snow in the three-month-long, dark winters,” Rich says. Still, a variety of dinosaurs thrived here, including small, feathery predators, parrot-like oviraptors and Leaellynasaura, a small herbivore that walked on two legs and had one of the longest tails for its body size of any dinosaur.

Some dinosaurs might have dug in to survive the harshest months. Paleontologists working in southern Australia’s strata have found burrow-like structures from the age of Leaellynasaura, and elsewhere these structures actually contain small, herbivorous dinosaurs. “It’s possible that dinosaurs might have burrowed as a way to escape the cold,” says paleontologist Adele Pentland of the Australian Age of Dinosaurs Museum of Natural History.

“The clearest evidence we have of polar adaptations, or not, is the composition of the fauna,” adds Monash University paleontologist Steve Poropat. Which types of dinosaurs are found in cooler places, as opposed to those that are missing, offers some insights into which dinosaurs were better able to cope with or adapt to the long polar nights. “Theropods, ornithopods, ankylosaurs? No problem. You find them at heaps of sites throughout Victoria,” Poropat notes, referencing the state in southeastern Australia. These types of dinosaurs could withstand the cold and dark months. But long-necked, plant-eating dinosaurs called sauropods that lived at the same time are missing from the same sites, which suggests that they were not able to survive or adapt to the colder environments.

The Antarctic Circle wasn’t the only place to host chill-adapted dinosaurs. The 70 million-year-old rock of Alaska’s Prince Creek Formation contains the fossils of horned dinosaurs, tyrannosaurs, duckbilled dinosaurs, raptors and more that lived within the Arctic Circle. And when these dinosaurs began to catch researcher’s attention during the 1980s, they presented some challenges to what paleontologists thought about dinosaur lives.

“When dinosaurs were first found in the Arctic, they presented some serious problems to our understanding of dinosaurian physiology,” Perot Museum of Nature and Science paleontologist Tony Fiorillo says. Even as paleontologists considered that dinosaurs might keep warmer body temperatures, the harshness of the Arctic cold was thought to be too much. Some experts proposed that dinosaurs might migrate, drawing an analogy to modern-day caribou, which don’t migrate long distances north and south, Fiorillo says. Various lines of evidence indicate that the dinosaurs remained in their home habitat through the winter. Just this past year, Fiorillo and colleagues were the ones who published on a jaw from a very young raptor—evidence that dinosaurs were nesting in the region and not just passing through.

The landscape would have looked a little familiar. At the time the Prince Creek Formation was being laid down, Fiorillo says, the area was similar to what it’s like today—a coastal plain dominated by stands of conifers and flowering plants low to the ground. And while overall warmer than the same spot today, it still got cold enough to snow during the winters.

Alaska’s dinosaurs had to contend with some of the same stresses as their southern counterparts—such as harsher changes in seasons and months of darkness—but evidence from their bones indicate that these dinosaurs stayed year-round. Much like their relatives elsewhere, polar dinosaurs grew fast when they were young but switched to more of a stop-and-start growth pattern as they got older. This means that polar dinosaurs were already biologically predisposed to surviving on less during the cold months, with the dinosaurs growing faster again during the lush summers. While certainly chilly during the winter, the ground did not freeze in these places, providing enough vegetation to support an ecosystem of resident dinosaurs.

There may have been no one way that dinosaurs adjusted to the comparative harshness of life near the pole. The local tyrannosaur in the Prince Creek Formation was not a familiar species seen elsewhere, but a unique and smaller predator—roughly the size of a polar bear— that Fiorillo and colleagues dubbed Nanuqsaurus. The comparatively small stature of this dinosaur, as well as the downsized species of horned dinosaur called Pachyrhinosaurus in the area, hints that types of dinosaurs that grew big elsewhere adapted to become smaller and thereby get by on less food in the cool of ancient Alaska.

But some polar dinosaurs truly thrived. The raptor-relative Troodon was a feathery, eight-foot-long dinosaur with large eyes. While rare elsewhere, Fiorillo says, “it is the overwhelmingly abundant theropod dinosaur.” The small-carnivore’s large eyes may have given it an advantage, especially during the dark months.

Our visions of polar dinosaurs are still relatively new. Determining which species lived in cooler areas is part of that task. Some, like Nanuqsaurus which was named in 2014, are new. Others turn out to be familiar—a duckbill dinosaur previously thought to be a new species has turned out to be Edmontosaurus, a wide-ranging hadrosaur found elsewhere. “For me,” Fiorillo says, “the story is even more fascinating knowing that some Arctic dinosaurs became specialists within the ancient north while others were generalists capable of surviving a wide array of environmental conditions.”

Many finds are left to be made, not just among the dinosaurs but about the big picture of the habitats where they lived. “Discoveries are being made every day,” Rich says, noting that fieldwork just this year in the time of Leaellynasaura has uncovered dinosaur tracks, turtle shells, tree trunks with termite damage and more, all parts of a lost polar world. Finds like these will continue to highlight just how successful dinosaurs were, a testament to their prehistoric versatility. In virtually any ancient landscape, dinosaurs found a way.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.