The discovery in the 1950s of dinosaur bones in the Arctic set off a debate as to whether there was a permanent dino-presence at high latitudes, or if great beasts migrated there for the summer. Subsequent evidence supported the idea some dinosaur species survived the long nights in both the Arctic and Antarctic, but a new paper in Current Biology reveals even more: many species nested there.

Paleontologists have previously found evidence of nesting at sites close to the Arctic Circle. However, the species involved have not been identified. Dr Patrick Druckenmiller of the University of Alaska has easily eclipsed such finds. "We didn't just demonstrate the presence of perinatal remains – in the egg or just hatched – of one or two species, rather we documented at least seven species of dinosaurs reproducing in the Arctic," he said in a statement.

“This is the first time that anyone has ever demonstrated that dinosaurs could reproduce at these high latitudes,” Druckenmiller said. 

The Prince Creek Formation site sits above Alaska's Colville River and dates to the Late Cretaceous when it was 5-10 degrees from the pole.

The world was a warmer place then, but the average temperature year-round at the site is still thought to have been about 6ºC (42ºF). There would have been almost no Sun and precious little warmth for four months every winter.

Despite this, the location supported an abundance of life. Druckenmiller and colleagues have found hundreds of bones and teeth from baby dinosaurs. Some never left the egg, while others suffered unfortunate fates shortly after hatching. Rather than reflecting a variety of closely related species, their finds include hadrosaurids, ceratopsians (a family that includes triceratops), thescelosaurs, and even tyrannosaurs and other carnivores. Although the families are familiar, most of the individual species are new to science.

"It wasn't that long ago that the idea of finding any dinosaurs in such extreme latitudes and environments was a surprise," Druckenmiller said. "To then find out that most if not all of those species also reproduced in the Arctic is really remarkable. We have long been asked, 'Have you found any eggs?' [We] still answer 'no.' But, we have something much better: the actual baby dinosaurs themselves."

Lacking flight these journeys would have been beyond adults of the smaller species, let alone hatchlings. Co-author Professor Gregory Erickson of Florida State University has previously shown these sorts of dinosaur eggs took 2.5-6 months to hatch. Even if laid in early spring, the young would have been in no state to almost immediately migrate south after breaking free from their shells.

The finding is also possibly the best evidence yet on the question of whether dinosaurs were warm-blooded.

"Cold-blooded terrestrial vertebrates like amphibians, lizards, and crocodilians have yet to be found [so far north], only warm-blooded birds and mammals – and dinosaurs,” said Erickson, although he admits we still don't know how they survived the cold and limited food of the winters. Most likely smaller species hibernated and larger ones made do with tree bark and other low-quality forage. However, "I think that this is some of the most compelling evidence that dinosaurs were in fact warm-blooded," he said.