The Cretaceous was a period dominated by dinosaurs and other large reptiles, with warm climates and high sea levels due to volcanic activity. Terrestrial vertebrate biomass was enormous, and the seas were occupied by marine reptiles. However, a massive asteroid struck the Earth 65.5 million years ago, causing the extinction of most animals, except for birds, mammals, crocodiles, fish, and sharks.
The Cretaceous is a geological period extending from 145.5 to 65.5 million years ago. With a total duration of 80 million years, the Cretaceous is the longest geological period of 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 small scavengers in the night, as were some giant amphibians that survived the geographical isolation. Birds began to diversify and compete with the pterosaurs for the skies.
During the Cretaceous, the climate was warm and sea levels were high. Much of this heat came from abundant volcanic activity that released large quantities of carbon dioxide, a greenhouse gas, into the atmosphere. Forests, made of conifers and/or cycads, covered the planet, including the South Pole. Much of North America was inundated by an epicontinental sea called the Western Interior Seaway, and water covered much of India, the modern Africa and Europe. Because Australia was still connected to Antarctica, there was no freezing circumpolar current, and the Antarctic continent was warm and lush.
Dinosaurs were more diverse during the Cretaceous, especially at the end of the period, and included ceratopsians such as Triceratops, the heavily armored ankylosaurs, duck-billed hadrosaurs, carnivorous theropods such as Tyrannosaurus rex, giant sauropods, several herbivores called ornithopods, small dinosaurs feathered birds stealing eggs and many more. Terrestrial vertebrate biomass was enormous, probably more than double that of today. Had dinosaurs not been wiped out at the end of the Cretaceous, they would have diversified even more and produced additional new forms.
The seas were occupied and dominated by the usual marine reptiles – plesiosaurs and pliosaurs. Ichthyosaurs lived throughout the Cretaceous period, becoming extinct about 25 million years before the dinosaurs. They were among the only major reptile groups to go extinct in the middle Cretaceous and not in the mass extinction event at its end. Around the same time that ichthyosaurs went extinct, large snake-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 struck the Earth, causing a shower of magma, blocking the sun with dust, and killing virtually all of the animals discussed in this article. The main groups of surviving vertebrates were birds, mammals, crocodiles and, of course, fish and sharks.
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