These fossilized skulls are all from Ceratopsians, a group of dinosaurs best known for their horns. This dinosaur group crossed a land bridge between East Asia and Alaska during the Cretaceous Period and lived in North America, where they flourished. A wealth of well-preserved fossils found across North America show that Ceratopsians split off into species with different types of horns, spikes, lumps, knobs, and other means of protecting themselves from predators.