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,
Scleractinia on which to base a hypothesis about the origin of the scleractinians; plenty is known about modern species but very little about fossil specimens, which first appeared in the record in the Middle Triassic (). It was not until 25 million years later that they became important reef builders, their success perhaps a result of teaming up with symbotic algae. Nine of the sub-orders were in existence by the end of the Triassic and three more had appeared by the Jurassic (200 million years ago), with a further suborder appearing in the Middle Cretaceous (100 million years ago). Some may