English: Hadrosaur skulls on display at the Museum of the Rockies in Bozeman, Montana. The higher land vertebrates (amniotes) are generally classed into four categories of individual development (ontogeny): hatchling (“baby”), juvenile, subadult, and adult. The smallest skull, to the right, is a restoration of a juvenile skull collected from a bonebed at the North Slope of Alaska. This Alaska material was previously assigned to Edmontosaurus but now it is classified as a distinct species (Ugrunaaluk kuukpikensis). The upper fossil skull is an adult specimen of Edmontosaurus annectens collected in Garfield County, Montana. The lower left skull (UCMP 128374) is a large adult of the same species, collected in Yellowstone County, Montana. All skulls probably are replicas.