The hot dinosaur has captured the public imagination. Our favorite prehistoric giants are often depicted roaming arid savannahs or chewing on tropical leaves.

But some dinosaurs had cooler lifestyles. Deposits of baby dinosaur bones and teeth in northern Alaska, reported on Thursday in Current Biology, suggest a number of species survived over the Arctic Circle year-round – they endured sub-zero temperatures, food shortages, and four consecutive months of darkness and the occasional snowstorm.

The first polar dinosaur footprints were found in 1960 while stomping in the Svalbard Archipelago. In the decades that followed, evidence of dinosaurs surfaced in both Arctic and Antarctic latitudes. This sparked curiosity about how they did it – especially in the northern regions, which, although warmer than they are today, turned into frozen landscapes of eternal night in winter.

“How the hell did you do that?” said Patrick Druckermiller, a paleontologist at the University of Alaska Museum of the North who contributed to the new study. Perhaps the dinosaurs were snow birds that spent the summers up there before migrating south, he said. “Or did you hold out somehow? Or do something crazy like hibernating? “

For this study, the researchers headed to the Prince Creek Formation in northern Alaska, above the Arctic Circle and home to “the most polar dinosaurs we know of,” said Dr. Druckmiller.

While the popular fossil hunting image also comes from warmer climates – suntanned paleontologists scratching for clues, thighbones sticking out of the ground like hambones – Prince Creek is less glamorous. “The ground is frozen, the bones are frozen,” said Tyler Hunt, a Ph.D. Florida State University student who worked on the project. “Everything is muddy.”

Over the course of several trips, Mr. Hunt, Dr. Druckmiller and other researchers sand and mud from the hills of Prince Creek and exposed the frozen layer of fossil material below. After thawing the treasure in the sun, they dug it up and sifted it through finer and finer meshes to keep anything larger than a poppy seed. The loot was brought back to the laboratory, where dozens of students and volunteers combed tablespoon after tables under microscopes, looking for tiny fragments of bones and teeth called microfossils.

Although microfossils are often overlooked in favor of “larger, more noticeable” remains, “they provide a window into the life of baby dinosaurs” and the animals and plants they may have coexisted with, said Holly Woodward Ballard, a paleontologist at Oklahoma State University who was not involved in the study.

The team found microfossils from at least seven different species of late Cretaceous dinosaurs, including tyrannosaurs, comb-headed ceratopsians, and duck-billed hadrosaurs.

Finding evidence of such young dinosaurs – including some that may not have hatched yet – will help our understanding of the polar dinosaur lifestyle, said Dr. Druckmiller. Recent estimates of dinosaur egg incubation times – it may have taken up to six months in some cases – make the winter migration schedule way too tight, he said, adding that they will “have to step out of the egg and run south.” . “

Instead, the dinosaurs must have found ways to keep themselves and their offspring warm and fed. It’s likely that some of them had downy feathers, said Dr. Druckmiller. The finding also supports the increasingly popular idea that dinosaurs weren’t “cold-blooded, lizard-like creatures” but were endotherms that are able to generate their own body heat, he said.

Other behaviors, such as hibernation, are “entirely possible,” said Dr. Ballard, but has not yet been substantiated by evidence from fossil sites.

“It’s exciting to see the whole story come together so nicely,” said David Fastovsky, a paleontologist at the University of Rhode Island, who was also not involved.

The team plans to return to Prince Creek to uncover more details and look for fossilized feathers and signs of cozy dinosaur burrows. When Mr. Hunt travels from sunny Florida to the cool arctic dig site, he thinks of the unlikely pioneers who came before him.

“You don’t imagine dinosaurs in the snow, you know?” He said. “But obviously they could thrive up there.”