Cretaceous Cretaceous The Cretaceous (, ) is a geologic period and system that spans 79 million years from the end of the Jurassic Period million years ago (mya) to the beginning of the Paleogene Period mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation "Kreide" (chalk, "creta" in Latin). 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,
Mesozoic by an asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are also believed to have contributed to the Cretaceous–Paleogene extinction event. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs. The Triassic ranges roughly from 252 million to 201 million years ago, preceding the Jurassic Period. The period is bracketed between the Permian–Triassic extinction event and the Triassic–Jurassic extinction event, two of the "big five", and it is divided into three major epochs: Early, Middle, and Late Triassic. The Early Triassic, about 252 to 247 million years