Paleogene

EtymologyName formalityFormalAlternate spelling(s)Palaeogene, PalæogeneUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionIridium enriched layer associated with a major meteorite impact and subsequent K–Pg extinction event.Lower boundary GSSPEl Kef Section, El Kef, Tunisia
36°09′13″N8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486Lower GSSP ratified1991Upper boundary definition

• Base of magnetic polarity chronozone C6Cn.2n.
• Near first appearance of the Planktonic foraminiferan.

Upper boundary GSSPLemme-Carrosio Section, Carrosio, Italy
44°39′32″N8°50′11″E / 44.6589°N 8.8364°E / 44.6589; 8.8364Upper GSSP ratified1996Atmospheric and climatic dataMean atmospheric O₂ contentc. 26 vol %
(125% of modern)Mean atmospheric CO₂ contentc. 500 ppm
(1.8 times pre-industrial)Mean surface temperaturec. 18 °C
(4.5 °C above pre-industrial)

The Paleogene Period (IPA: ; also spelledPalaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 Ma (million years ago) to the beginning of the Neogene Period 23.04 Ma. It is the first period of the Cenozoic Era, the tenth period of the Phanerozoic and is divided into the Paleocene, Eocene, and Oligocene epochs. The earlier term Tertiary Period was used to define the time now covered by the Paleogene and subsequent Neogene periods; despite no longer being recognized as a formal stratigraphic term, Tertiary sometimes remains in informal use. Paleogene is often abbreviated Pg, although the United States Geological Survey uses the abbreviation Pe for the Paleogene on the Survey's geologic maps.

Much of the world's modern vertebrate diversity originated in a rapid surge of diversification in the early Paleogene, as survivors of the Cretaceous–Paleogene extinction event took advantage of empty ecological niches left behind by the extinction of the non-avian dinosaurs, pterosaurs, marine reptiles, and primitive fish groups. Mammals continued to diversify from relatively small, simple forms into a highly diverse group ranging from small-bodied forms to very large ones, radiating into multiple orders and colonizing the air and marine ecosystems by the Eocene. Birds, the only surviving group of dinosaurs, quickly diversified from the very few neognath and paleognath clades that survived the extinction event, also radiating into multiple orders, colonizing different ecosystems and achieving an extreme level of morphological diversity. Percomorph fish, the most diverse group of vertebrates today, first appeared near the end of the Cretaceous but saw a very rapid radiation into their modern order and family-level diversity during the Paleogene, achieving a diverse array of morphologies.

The Paleogene is marked by considerable changes in climate from the Paleocene–Eocene thermal maximum, through global cooling during the Eocene to the first appearance of permanent ice sheets in the Antarctic at the beginning of the Oligocene.