Cretaceous

EtymologyName formalityFormalUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionNot formally definedLower boundary definition candidates

• Magnetic—base of Chron M18r
• Base of Calpionellid zone B
• FAD of AmmoniteBerriasella jacobi

Lower boundary GSSP candidate section(s)NoneUpper boundary definitionIridium-enriched layer associated with a major meteorite impact and subsequent K-Pg extinction eventUpper boundary GSSPEl Kef Section, El Kef, Tunisia
36°09′13″N8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486Upper GSSP ratified1991

The Cretaceous (IPA: ) is a geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the MesozoicEra, as well as the longest. At around 77.1 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin , 'chalk', which is abundant in deposits from the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

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-extinctmarine reptiles, ammonites and rudists; while on land, dinosaurs continued to dominate. The world was largely ice-free – although there is some evidence of brief periods of glaciation during the cooler first half – and forests extended to the poles.

Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials (Eutheria and Metatheria respectively), with the earliest crown group birds appearing towards the end of the Cretaceous. Teleost fish, the most diverse group of modern vertebrates, continued to diversify during the Cretaceous with the appearance of their most diverse subgroup Acanthomorpha during this period. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.

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; it is widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and CenozoicEras.