The Oligocene is often considered an important time of transition, a link between "[the] archaic world of the tropical Eocene and the more modern-looking ecosystems of the Miocene. The Oligocene change in ecosystems is a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.
The start of the Oligocene is marked by a major extinction event, a faunal replacement of European with Asian fauna except for the endemic rodent and marsupial families called the Grande Coupure. The Oligocene-Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene [26,23 Ma] and the relatively cooler Miocene.
|Chattian or Late Oligocene||(28.4 ± 0.1 – 23.03 mya)|
|Rupelian or Early Oligocene||(33.9 ± 0.1 – 28.4 ± 0.1 mya)|
The Paleogene Period general temperature decline is interrupted by a Oligocene 7M-year stepwise climate change. A deeper 8.2oC 0.4M-year temperature depression leads the 2oC 7M-year stepwise climate change 33.5Ma. The stepwise climate spanned 7M-years 25.5Ma thru 32.5Ma as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55m decrease in sea level (35.7-33.5Ma) with a closely related (25.5-32.5Ma) temperature depression. The 7M-year depression abruptly terminated within 1-2M-year of the La Garita Caldera volcanism event 28-26 Ma. The Oligocene Miocene boundary climate was relatively stable.
Mountain building in western North America continued, and the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. Oligocene marine exposures are rare in North America. There appears to have been a land bridge in the early Oligocene between North America and Europe since the faunas of the two regions are very similar. During sometime in the Oligocene, South America was finally detached from Antarctica and drifted north towards North America. It also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the continent.
In North America, subtropical species dominated with cashews and lychee trees present, and temperate trees such as roses, beech and pine were common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent.
Important Oligocene land faunas are found on all continents except Australia. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleogene.(Haines) Marine faunas became fairly modern, as did terrestrial vertebrate faunas in the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving.
South America was apparently isolated from the other continents and evolved a quite distinct fauna during the Oligocene.
The Oligocene oceans resembled today's fauna, such as the bivalves. The baleen and toothed cetaceans (whales) just appeared, and their ancestors, the Archaeoceti cetaceans remained relatively common but their numbers were falling as Oligocene progressed because of climate changes and competition with today's modern cetaceans and the Charcharinid sharks, which also appeared in this epoch. Pinnipeds probably appeared near the end of the epoch from a bear-like or otter-like ancestor.