A display of Triceratops horridus skulls, from baby to adult, at the Museum of the Rockies in Bozeman, Montana. Dinosaurs are classified into four growth stages: baby, juvenile, subadult, and adult. The skull to the far right is a baby, collected in Garfield County, Montana. (This is a cast.) The second and third skulls from the right are juveniles, one larger than the other. (The one second from the right was found in Garfield County, Montana. The one third from the right was found in McCone County, Montana.) Note the triangular frill nodules (not yet fused with the frill), the backward-curving horns, the longer snouts, and the lack of holes in the frill. The fourth from the right is a subadult (teenager). Although the horns still curve backward, the frill nodules are fusing with the frill and the snout is showing large "excavation" (areas of no bone). The final skull on the far left is an adult.