Cretaceous

The Cretaceous ( /krɪˈteɪʃəs/, kri-TAY-shəs) is a geologic period and system that spans 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, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide (chalk, creta in Latin).Tyrannosaurus rex, one of the largest land predators of all time, lived during the late Cretaceous.Up to 2 m long and 0.5 m high at the hip, Velociraptor was feathered and roamed the late Cretaceous.Triceratops, one of the most recognizable genera of the CretaceousA pterosaur, Anhanguera piscatorConfuciusornis, a genus of crow-sized birds from the Early CretaceousIchthyornis was a toothed seabird-like ornithuran from the late Cretaceous periodA scene from the early Cretaceous: a Woolungasaurus is attacked by a Kronosaurus.Tylosaurus was a large mosasaur, carnivorous marine reptiles that emerged in the late Cretaceous.Strong-swimming and toothed predatory waterbird Hesperornis roamed late Cretacean oceans.The ammonite Discoscaphites iris, Owl Creek Formation (Upper Cretaceous), Ripley, MississippiA plate with Nematonotus sp., Pseudostacus sp. and a partial Dercetis triqueter, found in Hakel, LebanonCretoxyrhina, one of the largest Cretaceous sharks, attacking a Pteranodon in the Western Interior SeawayNumerous borings in a Cretaceous cobble, Faringdon, England; these are excellent examples of fossil bioerosion.Cretaceous hardground from Texas with encrusting oysters and borings. The scale bar is 10 mm.Rudist bivalves from the Cretaceous of the Omani Mountains, United Arab Emirates. Scale bar is 10 mm.Inoceramus from the Cretaceous of South Dakota The Cretaceous ( /krɪˈteɪʃəs/, kri-TAY-shəs) is a geologic period and system that spans 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, 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, 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 (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction 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 Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras. The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta, meaning chalk. The Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian (lower/early), Gallic (middle) and Senonian (upper/late). A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use. As with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico. This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the usually accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a 'first step' toward formally changing the age in the International Union of Geological Sciences.