One of the greatest mysteries associated with dinosaurs is the rarity of juveniles. With the notable exception of the Djadochta and Iren Dabasu Formations of Mongolia and the Two Medicine Formation of Montana, skeletal elements of very young dinosaurs are exceedingly uncommon1-3, leaving a gap in the understanding of dinosaur social structures1,3,4. The most popular explanation, based chiefly on negative evidence, is that the eggs and young were `kept' in the uplands where they were rarely covered by sediments and as a result were destroyed by erosion2. The discovery of fifteen 1-m long hadrosaurian (`duck-billed') skeletons together in a nest-like structure offers the first tangible evidence of the social behaviour of at least one group of dinosaurs. They were discovered by Marion Brandvold in terrestrial sediments of the Two Medicine Formation (Upper Cretaceous) near Choteau, Teton County, Montana.