The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich[5][6] from his studies of marine beds in Belgium and Germany.

The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene.

[9] Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.

[10] The upper boundary of the Oligocene is defined by its GSSP at Carrosio, Italy, which coincides with the first appearance of the foraminiferan Paragloborotalia kugleri and with the base of magnetic polarity chronozone C6Cn.2n.

Australia had been very slowly rifting away from West Antarctica since the Jurassic, but the exact timing of the establishment of ocean channels between the two continents remains uncertain.

This initiated strike-slip movement along the San Andreas Fault and extensional tectonics in the Basin and Range province,[19] ended volcanism south of the Cascades, and produced clockwise rotation of many western North American terranes.

Huge ash deposits from these volcanoes created the White River and Arikaree Groups of the High Plains, with their excellent fossil beds.

[20] Between 31 and 26 mya, the Ethiopia-Yemen Continental Flood Basalts were emplaced by the East African large igneous province, which also initiated rifting along the Red Sea and Gulf of Aden.

The Obik Sea, which had separated Europe from Asia, retreated early in the Oligocene, creating a persistent land connection between the continents.

By the end of the transition, sea levels had dropped by 105 meters (344 ft), and ice sheets were 25% greater in extent than in the modern world.

[40] Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about 5 °C (9.0 °F) just prior to the Oi1 event.

[17][48][14] Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene.

[51][54][35] The pCO2 is estimated to have dropped just before the transition, to 760 ppm at the peak of ice sheet growth, then rebounded slightly before resuming a more gradual fall.

[66] Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau.

The cooling trend that began in the middle Eocene continued into the Oligocene, bringing both poles well below freezing for the first time in the Phanerozoic.

The cooling climate, together with the opening of some land bridges and the closing of others, led to a profound reorganization of the biosphere and loss of taxonomic diversity.

[43][44] Subtropical species dominated with cashews[73] and lychee trees present,[74] and temperate woody plants such as roses, beeches,[75] and pines[76] were common.

Carnivores such as dogs, nimravids, bears, weasels, and raccoons began to replace the creodonts that had dominated the Paleocene in the Old World.

[85] Many groups, such as equids,[86] entelodonts, rhinos, merycoidodonts, and camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded.

[90] There was significant diversification of mammals in Eurasia, including the giant indricotheres, that grew up to 6 meters (20 ft) at the shoulder and weighed up to 20 tons.

[85] The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover of gastropods, amphibians, and reptiles.

[43] The White River Fauna of central North America inhabited a semiarid prairie home and included entelodonts like Archaeotherium, camelids (such as Poebrotherium), running rhinoceratoids, three-toed equids (such as Mesohippus), nimravids, protoceratids, and early canids like Hesperocyon.

[96] Nevertheless, the Early Oligocene saw a major reduction in the diversity of many Afro-Arabian mammal clades, including hyaenodonts, primates, and hystricognath and anomaluroid rodents.

Other factors to their decline could include climate changes and competition with today's modern cetaceans and the requiem sharks, which also appeared in this epoch.

Even so, many theories agree that at the Eocene/Oligocene (E/O) boundary, a yet shallow flow existed between South America and Antarctica, permitting the start of an Antarctic Circumpolar Current.

If the opening occurred as late as hypothesized, then the Antarctic Circumpolar Current could not have had much of an effect on early Oligocene cooling, as it would not have existed.

[108] The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.

[109] The gradual separation of the clump of continental crust and the deepening of the tectonic ridge in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area.

[104][112] Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the North Pacific and the Southern Ocean.

[105] The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully.