Until now, no such hatchlings had been found, and the only known polar dinosaur eggs were from the Kakanaut formation of north-eastern Russia, which was only just within the Arctic Circle when its rocks were laid down. Dr Druckenmiller's discoveries are from the Prince Creek Formation of northern Alaska, which may have been as close as 5° of latitude from the North Pole when its rocks formed 70m years ago.
The fossils themselves come from a range of dinosaur groups, including ceratopsians (related to the likes of Triceratops), duck-billed hadrosaurs, large carnivores related to Tyrannosaurus and smaller velociraptor-like predators. This suggests a diverse and flourishing ecosystem, despite the fact that Prince Creek was continuously dark for 120 days a year and had an average annual temperature of 6°C—meaning snow would have been common in winter.
How all these creatures survived those conditions was, Dr Druckenmiller suggests, a consequence of dinosaurs' warmbloodedness and the downy feathers many of them are now known to have sported. No direct evidence of feathers has yet been found among the Alaskan fossils, but their ubiquity elsewhere makes it likely they had them.