The Cretaceous, usually abbreviated K for its German translation Kreide, is a geologic period and system from circa 145.5 ± 4 to 65.5 ± 0.3 million years ago (Ma). In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period. It is the youngest period of the Mesozoic era, and at 80 million years long, the longest period of the Phanerozoic eon. The end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras.
The Cretaceous was a period with a relatively warm climate and high eustatic sea level. The oceans and seas were populated with now extinct marine reptiles, ammonites and rudists; and the land by dinosaurs. At the same time, new groups of mammals and birds as well as flowering plants appeared. The Cretaceous ended with one of the largest mass extinctions in Earth history, the Cretaceous-Tertiary extinction, when many species, including the dinosaurs, pterosaurs, and large marine reptiles, disappeared.