Paleogene

The Paleogene is formally defined by the International Commission on Stratigraphy as the first period of the Cenozoic Era, spanning from the Cretaceous–Paleogene boundary at 66.0 Ma to the Neogene boundary at 23.03 Ma. It is subdivided into three epochs: the Paleocene (66.0–56.0 Ma), the Eocene (56.0–33.9 Ma), and the Oligocene (33.9–23.03 Ma), each further divided into stages such as the Danian, Ypresian, and Chattian¹. The base of the Paleogene is defined by the iridium anomaly at the K–Pg boundary, which marks a mass extinction event. The term "Paleogene" was coined in the 19th century, but its modern chronostratigraphic definition was formalized in the late 20th century.

The Paleogene witnessed dramatic climatic shifts, beginning with a warm greenhouse world and ending with the onset of Antarctic glaciation. The Paleocene–Eocene Thermal Maximum (PETM), about 56 Ma, was a rapid global warming event that lasted roughly 200,000 years, associated with massive carbon release and deep-sea acidification². Following the PETM, the Early Eocene Climatic Optimum was the warmest interval of the Cenozoic, with tropical conditions extending to high latitudes. After the Eocene, a long-term cooling trend led to the formation of the Antarctic ice sheet around 34 Ma, at the Eocene–Oligocene transition³. Life rebounded after the K–Pg extinction: mammals diversified rapidly, birds and flowering plants spread, and marine ecosystems recovered with new groups like foraminifera and corals.

During the Paleogene, Earth's continents continued to drift toward their present positions, with major consequences for ocean circulation and climate. The Tethys Ocean closed progressively as the Indian Plate collided with Asia, initiating the Himalayan orogeny in the Eocene⁴. The Atlantic Ocean widened, and the Drake Passage opened between South America and Antarctica, enabling the Antarctic Circumpolar Current and contributing to global cooling⁵. Volcanic activity was intense, including the North Atlantic Igneous Province, which may have contributed to the PETM. Sea levels were generally high early in the period, then fell as ice sheets grew.

Beyond the headline events, the Paleogene holds many niche details. The earliest known primates, such as Purgatorius, appeared in the Paleocene, but the first true euprimates evolved in the Eocene. The period also saw the rise of giant flightless birds like Gastornis in the Northern Hemisphere and terror birds in South America. The Paleogene was a time of rapid evolution in grasses, though widespread grasslands did not appear until the Neogene. The Azolla event, a massive bloom of the aquatic fern Azolla in the Arctic Ocean around 49 Ma, may have contributed to CO2 drawdown and cooling⁶. The term "Paleogene" was once used informally to include the Neogene, but modern usage restricts it to the earlier period.

K–Pg boundary

The stratigraphic boundary marking the Cretaceous–Paleogene extinction event, dated to 66 Ma.

PETM

Paleocene–Eocene Thermal Maximum, a rapid global warming event about 56 million years ago.

Epoch

A subdivision of a geologic period; the Paleogene contains three epochs.