Middle Cretaceous

The Cretaceous (pronunciation: /krᵻˈteɪʃəs/, krə-TAY-shəs) is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago (Mya) to the beginning of the Paleogene Period 66 Mya. It is the last period of the Mesozoic Era. The Cretaceous Period is usually abbreviated K, for its German translation Kreide (chalk).

The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared. The Cretaceous ended with a large mass extinction, the Cretaceous–Paleogene extinction event, in which many groups, including non-avian dinosaurs, pterosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt K–Pg boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.