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)

Geologic Age

  • 33.923.0 million years ago

Eon / Era / Period / Epoch


What happened during this time?



  • 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    • The Antarctic forests of the early Paleogene had been replaced by thick continental ice sheets, similar to today's conditions (Lauretano et al. 2021)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