On a cool summer"s day in the Late Cretaceous, a mega-herd of hadrosaurs crossed a muddy floodplain in the Arctic. It was around 10-12C (50-54F), and the cow-like herbivores – equipped with toothless beaks for grinding vegetation and massive, fleshy tails – had just survived a harsh winter in which temperatures dipped to almost freezing. There were thousands of individuals of all ages – juveniles, teenagers and adults.