The Paleogene, 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 and subsequent Neogene periods; despite no longer being recognised as a formal stratigraphic term, 'Tertiary' is still widely found in earth science literature and remains in informal use.
The Paleogene is most notable for being the time during which 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.
The United States Geological Survey uses the abbreviation PE for the Paleogene, but the more commonly used abbreviation is PG with PE being used for Paleocene, an epoch within the Paleogene.
This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (55.5/54.8 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'.