Late Cretaceous Dinosaurs in North America
Dinosaurs remained the main large land animals. Smaller land animals included turtles, crocodiles, snakes, lizards, frogs, and salamanders. Mammals remained small, but mammals that gave birth to live young appeared for the first time.
Almost all Late Cretaceous flying birds were tiny. But pterosaurs became the largest flying creatures ever known. Ichthyosaurs became extinct, and mosasaurs became the main marine predators. These evolved from small-to-medium-size monitor lizards. They shared the seas with plesiosaurs.
As many dinosaur species are known from the Late Cretaceous as are known from all the other periods together. In North America, the duckbilled hadrosaurs diversified into nearly two dozen known types, including Brachylophosaurus, Prosaurolophus, and Saurolophus. The horned ceratopsians divided into at least a dozen varieties, including the most famous, Triceratops.
Although the hadrosaurids are best known for their broad, horny, ducklike beaks, they are also famous for the different shaped crests on their heads. Perhaps used for vocalizing, the crests may also have identified male and female animals during mating season. Horns and frills evolved for the same reason in ceratopsians, with horns also used for combat and defense.
Plant-eaters were eaten by fierce tyrannosaurids. Most of those known from North America were 25 to 35 feet long, such as Albertosaurus. But the latest Late Cretaceous saw one of the smallest, Nanotyrannus, about 18 feet long, and the largest, Tyrannosaurus, 40 feet long. Ankylosaurs and nodosaurs were heavily armored plant-eaters that did not need the protection of a herd to avoid being eaten.
There were many small predators in North America. Among these were Aublysodon, Chirostenotes, and Troodon. They ate the small plant-eating dinosaurs, such the hypsilophodontids (Orodromeus, Parksosaurus, and Thescelosaurus), the small protoceratopsids (Leptoceratops and Montanoceratops), and the smaller dome-headed dinosaurs (Stegoceras and Stygimoloch). The swift, ostrichlike ornithomimids outran their predators.