The Cretaceous Period is a geologic period that stretches from 145.5 to 65.5 million years ago. With a total duration of 80 million years, the Cretaceous is the longest geologic period in the last 542 million years. It is famous for being dominated by dinosaurs and other large reptiles, such as pterosaurs (winged reptiles), mosasaurs, plesiosaurs, ichthyosaurs, and pliosaurs (all marine reptiles). Mammals were present, as tiny scavengers in the night, as well as some giant amphibians that survived through geographic isolation. Birds began to diversify and compete with pterosaurs for the skies.
During the Cretaceous, the climate was warm and sea levels were high. Much of this warmth came from abundant volcanic activity that released large amounts of the greenhouse gas carbon dioxide into the atmosphere. Forests, made of conifers and/or cycads, covered the planet, including the South Pole. Much of North America was flooded by an epicontinental sea called the Western Interior Seaway, and water covered much of modern-day India, Africa, and Europe. Because Australia was still connected to Antarctica, there was no frigid circumpolar current, and the Antarctic continent was warm and lush.
Dinosaurs were at their most diverse during the Cretaceous, especially at the very end of the period, and included ceratopsians like Triceratops, the heavily armored ankylosaurs, the duck-billed hadrosaurs, carnivorous theropods like Tyrannosaurus rex, giant sauropods, diverse herbivores called ornithopods, small, feathered, egg-stealing dinosaurs, and many more. The vertebrate terrestrial biomass was huge, probably more than twice that of today's. If the dinosaurs were not wiped out at the end of the Cretaceous, they would have diversified even more and produced additional novel forms.
The seas were occupied and dominated by the usual marine reptiles: plesiosaurs and pliosaurs. Ichthyosaurs lived throughout the Cretaceous, going extinct about 25 million years before the dinosaurs did. They were among the only major reptile groups to go extinct in the middle of the Cretaceous and not in the mass extinction event at its end. Around the same time that ichthyosaurs went extinct, large serpent-like marine reptiles called mosasaurs evolved, growing up to 17.5 m (57 ft) in length, among the largest marine reptiles of all time.
65.5 million years ago, a massive asteroid hit the Earth, causing a rain of magma, blocking out the sun with dust, and killing pretty much all of the animals discussed in this article. The main vertebrate groups that survived were birds, mammals, crocodilians, and of course, fish and sharks.