Cretaceous

Cretaceous
~145.0 – 66.0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Map of Earth as it appeared 100 million years ago during the mid-Cretaceous, Cenomanian Age[citation needed]
Chronology
−140 — – −130 — – −120 — – −110 — – −100 — – −90 — – −80 — – −70 — – Mesozoic C Z Jurassic Cretaceous Paleogene Early Late Berriasian Valanginian Hauterivian Barremian Aptian Albian Cenomanian Turonian Coniacian Santonian Campanian Maastrichtian         ← K-Pg mass extinction Subdivision of the Cretaceous according to the ICS, as of 2023.[1] Vertical axis scale: millions of years ago.
Etymology
Name formality	Formal
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Period
Stratigraphic unit	System
Time span formality	Formal
Lower boundary definition	Not formally defined
Lower boundary definition candidates	Magnetic—base of Chron M18r Base of Calpionellid zone B FAD of Ammonite Berriasella jacobi
Lower boundary GSSP candidate section(s)	None
Upper boundary definition	Iridium-enriched layer associated with a major meteorite impact and subsequent K-Pg extinction event
Upper boundary GSSP	El Kef Section, El Kef, Tunisia 36°09′13″N 8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486
Upper GSSP ratified	1991

The Cretaceous (IPA: /krɪˈteɪʃəs/ krih-TAY-shəss)^[2] is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, 'chalk', which is abundant in 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-extinct marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land. 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 & marsupials (Eutheria and Metatheria respectively), and the earliest crown group birds. Acanthomorph fish, the most diverse group of modern vertebrates, appeared in aquatic habitats around the middle of the Cretaceous. 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, 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 Cenozoic Eras.