During the Early Cretaceous period, many of the Jurassic period dinosaurs disappeared. They were replaced by new, more diverse forms. Toward the end of the Cretaceous, the diversity of dinosaurs dropped dramatically. Of the saurischian sauropods, only the titanosaurids (Late Jurassic or Early Cretaceous period) remained as a major group—and these herbivores were mostly found on the landmasses of Gondwana until the end of the Cretaceous period. In fact, through fossil evidence, it is thought that the titanosaurids replaced other sauropods, like the diplodocids and brachiosaurids, both of which died out around the Late Jurassic to Middle Cretaceous periods. Many of the saurischian theropods also became extinct during the Cretaceous period, while others diversified into a wide range of animals, from large carnivores like Tyrannosaurus, to speedy, agile predators like Velociraptor.

The ornithischian dinosaurs were the most numerous and diverse of all the dinosaurs in the Cretaceous period. They included the ornithopods, such as the Iguanodon, and the duck-billed dinosaurs, like the Edmontosaurus and Maiasaura; the armored ankylosaurs, including the Ankylosaurus, with its protective plating and tail-club; the thick-headed pachycephalosaurs, thought to engage in head butting, such as the Stegoceras; and the ceratopsians, four-legged animals with long, bony frills and horns, like the Triceratops.