Oligocene Epoch

A time of the first grasslands

The Oligocene is the third and last epoch of the Paleogene. It occurs after the Eocene, and before the Miocene (Neogene)

Flora

Angiosperms continued their expansion throughout the world
Tropical and sub-tropical forests were replaced by temperate deciduous woodlands
- The Antarctic forests of the early Paleogene had been replaced by thick continental ice sheets, similar to today's conditions (Lauretano et al. 2021)
Open plains and deserts became more common
- Indirect evidence for the first grasslands around 32.5 Ma.
Major evolution and dispersal of modern types of angiosperms occurred
- Grasses, which appeared for the first time as plants of water margins, became more common in open habitats
- In North America was mixture of subtropical elements, such as cashews and lychee trees, with temperate trees such as roses, beech, and pine
- Leguminous plants of the pea and bean family were common, as were sedges, bulrushes, and a variety of ferns

Global increase in ice volume and decrease in sea level with closely related temperature depression
Carbon dioxide level decline to ~470 ppm from a high of 900 ppm at the start of the epoch
Oxygen levels hover around 22.5%
"Mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary" (Lauretano et al. 2021)
- The long-term cooling trend is due to reduced carbon dioxide in the atmosphere