Paleogene

Paleogene
66.0 – 23.03 Ma PreꞒ Ꞓ O S D C P T J K Pg N
A map of Earth as it appeared during the Eocene epoch, c. 40 Ma.
Chronology
−65 — – −60 — – −55 — – −50 — – −45 — – −40 — – −35 — – −30 — – −25 — – M Z C e n o z o i c Cretaceous P a l e o g e n e Neogene P a l e o c e n e E o c e n e O l i g o c e n e Danian Selandian Thanetian Ypresian Lutetian Bartonian Priabonian Rupelian Chattian         ← PETM ← First Antarctic permanent ice-sheets[1] ← K-Pg mass extinction Subdivision of the Paleogene according to the ICS, as of 2021.[2] Vertical axis scale: millions of years ago
Etymology
Name formality	Formal
Alternate spelling(s)	Palaeogene, Palæogene
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	Iridium enriched layer associated with a major meteorite impact and subsequent K-Pg extinction event.
Lower 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
Lower GSSP ratified	1991[3]
Upper boundary definition	Base of magnetic polarity chronozone C6Cn.2n. Near first appearance of the Planktonic foraminiferan Paragloborotalia kugleri.
Upper boundary GSSP	Lemme-Carrosio Section, Carrosio, Italy 44°39′32″N 8°50′11″E / 44.6589°N 8.8364°E / 44.6589; 8.8364
Upper GSSP ratified	1996[4]
Atmospheric and climatic data
Mean atmospheric O2 content	c. 26 vol % (130 % of modern)
Mean atmospheric CO2 content	c. 500 ppm (2 times pre-industrial)
Mean surface temperature	c. 18 °C (4 °C above modern)

The Paleogene (IPA: /ˈpeɪli.ədʒiːn, -li.oʊ-, ˈpæli-/ PAY-lee-ə-jeen, -⁠lee-oh-, PAL-ee-; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the span of time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use.^[5] Paleogene is often abbreviated "Pg" (but the United States Geological Survey uses the abbreviation Pe for the Paleogene on the Survey's geologic maps).^[6][7]

During the Paleogene, mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period.^[8]

This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (56 Mya) was marked by the Paleocene–Eocene Thermal Maximum, one of the most significant periods of global change during the Cenozoic, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals. The term "Paleogene System" is applied to the rocks deposited during the Paleogene Period.