oligocene

The lower boundary of the Oligocene (its Global Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of the foraminiferan genus Hantkenina in a quarry at Massignano, Italy. However, this GSSP has been criticized as excluding the uppermost part of the type Eocene Priabonian Stage and because it is slightly earlier than important climate shifts that form natural markers for the boundary, such as the global oxygen isotope shift marking the expansion of Antarctic glaciation (the Oi1 event).{{sfn|Prothero|2005|pp=472–473}}

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.{{cite journal |last1=Steininger |first1=Fritz F. |last2=Aubry |first2=M.P. |last3=Berggren |first3=W.A. |last4=Biolzi |first4=M. |last5=M.Borsetti |first5=A. |last6=Cartlidge |first6=Julie E. |last7=Cati |first7=F. |last8=Corfield |first8=R. |last9=Gelati |first9=R. |last10=Iaccarino |first10=S. |last11=Napoleone |first11=C. |last12=Ottner |first12=F. |last13=Rögl |first13=F. |last14=Roetzel |first14=R. |last15=Spezzaferri |first15=S. |last16=Tateo |first16=F. |last17=Villa |first17=G. |last18=Zevenboom |first18=D. |title=The Global Stratotype Section and Point (GSSP) for the base of the Neogene |journal=Episodes |date=1 March 1997 |volume=20 |issue=1 |pages=23–28 |doi=10.18814/epiiugs/1997/v20i1/005|doi-access=free }}

Oligocene faunal stages from youngest to oldest are:{{cite journal|last1=Coccioni|first1=Rodolfo |last2=Montanari|first2=Alessandro |last3=Nice|first3=David|last4=Brinkhuis|first4=Henk|last5=Deino|first5=Alain|last6=Frontalini|first6=Fabrizio |last7=Liter|first7=Fabrizio|last8=Maiorano|first8=Patricia|last9=Monechi|first9=Simonetta|last10=Prods|first10=Jörg|last11=Rochette|first11=Pierre|last12=Sagnotti|first12=Leonardo|last13=Sideri|first13=Marianna|last14=Sprovieri|first14=Mario|last15=Tateo|first15=Fabio|last16=Touchard|first16=Yannick|last17=Can Simaeys|first17=Stefaan|last18=Williams|first18=Graham L.|title=The Global Stratotype Section and Point (GSSP) for the base of the Chattian stage (Paleogene System, Oligocene Series) at Monte Cagnero, Italy|journal=Episodes|date=1 March 2018|volume=41|issue=1|pages=17–32|doi= 10.18814/epiiugs/2018/v41i1/018003|doi-access=free|hdl=11573/1611823|hdl-access=free}}

File:Oligocene Chart.jpg

File:Mediterranean Rupelian.jpg during the Oligocene (Rupelian, 33.9–28.4 mya)]]

During the Oligocene Epoch, the continents continued to drift toward their present positions.{{sfn|Prothero|2005|pp=476–477}}{{cite book |last1=Torsvik |first1=Trond H. |last2=Cocks |first2=L. Robin M. |title=Earth history and palaeogeography |date=2017 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107105324 |pages=241–245}}

Antarctica became more isolated as deep ocean channels were established between Antarctica and Australia and South America. 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. However, one estimate is that a deep channel was in place between the two continents by the end of the early Oligocene.{{sfn|Torsvik|Cocks|2017|pp=251–252}} The timing of the formation of the Drake Passage between South America and Antarctica is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene),{{cite journal |last1=Scher |first1=H. D. |last2=Martin |first2=E. E. |title=Timing and Climatic Consequences of the Opening of Drake Passage |journal=Science |date=21 April 2006 |volume=312 |issue=5772 |pages=428–430 |doi=10.1126/science.1120044|pmid=16627741 |bibcode=2006Sci...312..428S |s2cid=19604128 }} but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene.{{sfn|Prothero|2005|pp=474, 476}}{{sfn|Torsvik|Cocks|2017|pp=251–252}} This may have been interrupted by a temporary constriction of the Drake Passage from sometime in the middle to late Oligocene (29 to 22 mya) to the middle Miocene (15 mya).{{cite journal |last1=Lagabrielle |first1=Yves |last2=Goddéris |first2=Yves |last3=Donnadieu |first3=Yannick |last4=Malavieille |first4=Jacques |last5=Suarez |first5=Manuel |title=The tectonic history of Drake Passage and its possible impacts on global climate |journal=Earth and Planetary Science Letters |date=30 March 2009 |volume=279 |issue=3–4 |pages=197–211 |doi=10.1016/j.epsl.2008.12.037|bibcode=2009E&PSL.279..197L |url=https://hal.archives-ouvertes.fr/hal-00413525 }}

The reorganization of the oceanic tectonic plates of the northeastern Pacific, which had begun in the Paleocene, culminated with the arrival of the Murray and Mendocino Fracture Zones at the North American subduction zone in the Oligocene. This initiated strike-slip movement along the San Andreas Fault and extensional tectonics in the Basin and Range province,{{sfn|Torsvik|Cocks|2017|p=245}} ended volcanism south of the Cascades, and produced clockwise rotation of many western North American terranes. The Rocky Mountains were at their peak. A new volcanic arc was established in western North America, far inland from the coast, reaching from central Mexico through the Mogollon-Datil volcanic field to the San Juan volcanic field, then through Utah and Nevada to the ancestral Northern Cascades. Huge ash deposits from these volcanoes created the White River and Arikaree Groups of the High Plains, with their excellent fossil beds.{{sfn|Prothero|2005|p=477}}

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.{{sfn|Torsvik|Cocks|2017|pp=241, 249}}

The Alps were rapidly rising in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea.{{sfn|Prothero|2005|pp=476–477}}{{sfn|Torsvik|Cocks|2017|pp=241–245}} Sea levels were lower in the Oligocene than in the early Eocene, exposing large coastal plains in Europe and the Gulf Coast and Atlantic Coast of North America. The Obik Sea, which had separated Europe from Asia, retreated early in the Oligocene, creating a persistent land connection between the continents.{{sfn|Prothero|2005|pp=476–477}} The Paratethys Sea stretched from what is now the Balkan Peninsula across Central Asia to the Tian Shan region of what is now Xinjiang.{{cite journal |last1=Li |first1=Qian |last2=Li |first2=Long |last3=Zhang |first3=Yuanyuan |last4=Guo |first4=Zhaojie |date=20 April 2020 |title=Oligocene incursion of the Paratethys seawater to the Junggar Basin, NW China: insight from multiple isotopic analysis of carbonate |journal=Scientific Reports |volume=10 |issue=1 |page=6601 |doi=10.1038/s41598-020-63609-0 |pmid=32313139 |pmc=7170927 |bibcode=2020NatSR..10.6601L }} 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.{{cite book |last1=Denk |first1=Thomas |last2=Grímsson |first2=Friðgeir |last3=Zetter |first3=Reinhard |last4=Símonarson |first4=Leifur A. |title=Late Cainozoic Floras of Iceland |chapter=The Biogeographic History of Iceland – the North Atlantic Land Bridge Revisited |series=Topics in Geobiology |date=2011 |volume=35 |pages=647–668 |doi=10.1007/978-94-007-0372-8_12|isbn=978-94-007-0371-1 }} However, towards the end of the Oligocene, there was a brief marine incursion in Europe.{{cite journal |last1=Rousse |first1=Stephane |last2=Duringer |first2=Philippe |last3=Stapf |first3=Karl R. G. |title=An exceptional rocky shore preserved during Oligocene (Late Rupelian) transgression in the Upper Rhine Graben (Mainz Basin, Germany): OLIGOCENE ROCKY SHORE |journal=Geological Journal |date=July 2012 |volume=47 |issue=4 |pages=388–408 |doi=10.1002/gj.1349|s2cid=129895800 }}{{cite journal |last1=Filek |first1=Thomas |last2=Hofmayer |first2=Felix |last3=Feichtinger |first3=Iris |last4=Berning |first4=Björn |last5=Pollerspöck |first5=Jürgen |last6=Zwicker |first6=Jennifer |last7=Smrzka |first7=Daniel |last8=Peckmann |first8=Jörn |last9=Kranner |first9=Matthias |last10=Mandic |first10=Oleg |last11=Reichenbacher |first11=Bettina |last12=Kroh |first12=Andreas |last13=Uchman |first13=Alfred |last14=Roetzel |first14=Reinhard |last15=Harzhauser |first15=Mathias |title=Environmental conditions during the late Oligocene transgression in the North Alpine Foreland Basin (Eferding Formation, Egerian) – A multidisciplinary approach |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=July 2021 |volume=580 |page=110527 |doi=10.1016/j.palaeo.2021.110527|bibcode=2021PPP...58010527F |doi-access=free }}

The rise of the Himalayas during the Oligocene remains poorly understood. One recent hypothesis is that a separate microcontinent collided with south Asia in the early Eocene, and India itself did not collide with south Asia until the end of the Oligocene.{{cite journal |last1=van Hinsbergen |first1=D. J. J. |last2=Lippert |first2=P. C. |last3=Dupont-Nivet |first3=G. |last4=McQuarrie |first4=N. |last5=Doubrovine |first5=P. V. |last6=Spakman |first6=W. |last7=Torsvik |first7=T. H. |title=Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia |journal=Proceedings of the National Academy of Sciences |date=15 May 2012 |volume=109 |issue=20 |pages=7659–7664 |doi=10.1073/pnas.1117262109|pmid=22547792 |pmc=3356651 |bibcode=2012PNAS..109.7659V |doi-access=free }}{{sfn|Torsvik|Cocks|2017|p=241}} The Tibetan Plateau may have reached nearly its present elevation by the late Oligocene.{{cite journal |last1=DeCelles |first1=Peter G. |last2=Quade |first2=Jay |last3=Kapp |first3=Paul |last4=Fan |first4=Majie |last5=Dettman |first5=David L. |last6=Ding |first6=Lin |title=High and dry in central Tibet during the Late Oligocene |journal=Earth and Planetary Science Letters |date=January 2007 |volume=253 |issue=3–4 |pages=389–401 |doi=10.1016/j.epsl.2006.11.001|bibcode=2007E&PSL.253..389D }}

The Andes first became a major mountain chain in the Oligocene, as subduction became more direct into the coastline.{{sfn|Prothero|2005|p=477}}{{Cite book

| last1 = Orme

| first1 = Antony R.

|editor-first1=Thomas T.

|editor-last1=Veblen

|editor-first2=Kenneth R.

|editor-last2=Young

|editor-first3=Anthony R.

|editor-last3=Orme

|editor-link1=Thomas T. Veblen

|chapter=The Tectonic Framework of South America

| title = Physical Geography of South America

| url = https://archive.org/details/physicalgeograph00vebl

| url-access = limited

| pages = [https://archive.org/details/physicalgeograph00vebl/page/n32 12]–17

| year = 2007

|publisher=Oxford University Press

| isbn = 978-0-19-531341-3

}}

Image:65 Myr Climate Change.png

Climate during the Oligocene reflected a general cooling trend following the Early Eocene Climatic Optimum. This transformed the Earth's climate from a greenhouse to an icehouse climate.{{sfn|Prothero|2005|p=473}}

The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere,{{cite book |last1=Berggren |first1=William A. |last2=Prothero |first2=Donald R. |title=Eocene-Oligocene Climatic and Biotic Evolution |chapter=Eocene-Oligocene climatic and biotic evolution: an overview |date=1992 |doi=10.1515/9781400862924.1 |publisher=Princeton University Press |page=1|isbn=9781400862924 }}{{cite book |last1=Coxall |first1=H.K. |last2=Pearson |first2=P.N. |year=2007 |chapter=The Eocene–Oligocene Transition |editor-last1=Williams |editor-first1=M. |editor-last2=Haywood |editor-first2=A.M. |editor-last3=Gregory |editor-first3=F.J. |editor-last4=Schmidt |editor-first4=D.N. |title=Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies |series=The Micropalaeontological Society, Special Publications |publisher=The Geological Society |location=London |pages=351–387}} being part of a broader trend of global cooling lasting from the Bartonian to the Rupelian.{{cite journal |last1=Lauretano |first1=Vittoria |last2=Kennedy-Asser |first2=Alan T. |last3=Korasidis |first3=Vera A. |last4=Wallace |first4=Malcolm W. |last5=Valdes |first5=Paul J. |last6=Lunt |first6=Daniel J. |last7=Pancost |first7=Richard T. |last8=Naafs |first8=B. David A. |date=2 August 2021 |title=Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2 |url=https://www.nature.com/articles/s41561-021-00788-z |journal=Nature Geoscience |volume=14 |issue=9 |pages=659–664 |doi=10.1038/s41561-021-00788-z |bibcode=2021NatGe..14..659L |s2cid=236781214 |access-date=8 December 2022|hdl=1983/45dea1c1-704b-469d-9fce-7d760100a309 |hdl-access=free }}{{cite journal |last1=Gutiérrez |first1=Néstor M. |last2=Pino |first2=Juan Pablo |last3=Le Roux |first3=Jacobus P. |last4=Petroza |first4=Viviana |last5=Orayzun |first5=José Luis |last6=Hinojosa |first6=Luis Felipe |date=15 August 2019 |title=An Oligocene microthermal forest dominated by Nothofagus in Sierra Baguales, Chilean Patagonia: Response to global cooling and tectonic events |url=https://www.sciencedirect.com/science/article/pii/S0031018218303961 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=528 |pages=1–13 |doi=10.1016/j.palaeo.2019.04.006 |bibcode=2019PPP...528....1G |s2cid=149478504 |access-date=4 December 2022}} The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago,{{cite journal |last1=Jovane |first1=Luigi |last2=Florindo |first2=Fabio |last3=Sprovieri |first3=Mario |last4=Pälike |first4=Heiko |date=27 July 2006 |title=Astronomic calibration of the late Eocene/early Oligocene Massignano section (central Italy) |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2005GC001195 |journal=Geochemistry, Geophysics, Geosystems |volume=7 |issue=7 |pages=1–10 |doi=10.1029/2005GC001195 |bibcode=2006GGG.....7.7012J |s2cid=127299427 |access-date=6 December 2022}} during which oxygen isotope ratios decreased by 1.3{{char|‰}}. About 0.3–0.4{{char|‰}} of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0{{char|‰}} was due to about {{convert|5 to 6|C-change|sigfig=1|sp=us}} of global cooling.{{sfn|Prothero|2005|p=473}} The transition likely took place in three closely spaced steps over the period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by {{convert|105|m||sp=us}}, and ice sheets were 25% greater in extent than in the modern world.{{cite journal |last1=Katz |first1=Miriam E. |last2=Miller |first2=Kenneth G. |last3=Wright |first3=James D. |last4=Wade |first4=Bridget S. |last5=Browning |first5=James V. |last6=Cramer |first6=Benjamin S. |last7=Rosenthal |first7=Yair |title=Stepwise transition from the Eocene greenhouse to the Oligocene icehouse |journal=Nature Geoscience |date=May 2008 |volume=1 |issue=5 |pages=329–334 |doi=10.1038/ngeo179|bibcode=2008NatGe...1..329K }}

The effects of the transition can be seen in the geological record at many locations around the world. Ice volumes rose as temperature and sea levels dropped.{{cite journal |last1=Miller |first1=K. G. |last2=Browning |first2=J. V. |last3=Aubry |first3=M.-P. |last4=Wade |first4=B. S. |last5=Katz |first5=M. E. |last6=Kulpecz |first6=A. A. |last7=Wright |first7=J. D. |title=Eocene-Oligocene global climate and sea-level changes: St. Stephens Quarry, Alabama |journal=Geological Society of America Bulletin |date=1 January 2008 |volume=120 |issue=1–2 |pages=34–53 |doi=10.1130/B26105.1|bibcode=2008GSAB..120...34M }} Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia.{{cite journal |last1=Dupont-Nivet |first1=Guillaume |last2=Krijgsman |first2=Wout |last3=Langereis |first3=Cor G. |last4=Abels |first4=Hemmo A. |last5=Dai |first5=Shuang |last6=Fang |first6=Xiaomin |title=Tibetan plateau aridification linked to global cooling at the Eocene–Oligocene transition |journal=Nature |date=February 2007 |volume=445 |issue=7128 |pages=635–638 |doi=10.1038/nature05516|pmid=17287807 |s2cid=2039611 }} Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about {{convert|5|C-change||sp=us}} just prior to the Oi1 event.{{cite journal |last1=Eldrett |first1=James S. |last2=Greenwood |first2=David R. |last3=Harding |first3=Ian C. |last4=Huber |first4=Matthew |title=Increased seasonality through the Eocene to Oligocene transition in northern high latitudes |journal=Nature |date=June 2009 |volume=459 |issue=7249 |pages=969–973 |doi=10.1038/nature08069|pmid=19536261 |bibcode=2009Natur.459..969E |s2cid=4365115 }} Borehole dating from the Southeast Faroes drift indicates that deep-ocean circulation from the Arctic Ocean to the North Atlantic Ocean began in the early Oligocene.{{cite journal |last1=Davies |first1=Richard |last2=Cartwright |first2=Joseph |last3=Pike |first3=Jennifer |last4=Line |first4=Charles |title=Early Oligocene initiation of North Atlantic Deep Water formation |journal=Nature |date=April 2001 |volume=410 |issue=6831 |pages=917–920 |doi=10.1038/35073551|pmid=11309613 |bibcode=2001Natur.410..917D |s2cid=4429436 }}

The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by {{convert|7 to 11|C-change||sp=us}} in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene paleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this.{{sfn|Prothero|2005|p=475}}{{cite journal|last1=Retallack |first1=G.J. |year=1983 |title=Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota |volume=193 |journal=Geological Society of America Special Paper |isbn=9780813721934}} In central North America, the cooling was by 8.2 ± 3.1 °C over a period of 400,000 years, though there is little indication of significant increase in aridity during this interval.{{cite journal |last1=Zanazzi |first1=Alessandro |last2=Kohn |first2=Matthew J. |last3=MacFadden |first3=Bruce J. |last4=Terry |first4=Dennis O. |title=Large temperature drop across the Eocene–Oligocene transition in central North America |journal=Nature |date=February 2007 |volume=445 |issue=7128 |pages=639–642 |doi=10.1038/nature05551|pmid=17287808 |s2cid=4301193 }} Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.{{cite journal |last1=Eldrett |first1=James S. |last2=Harding |first2=Ian C. |last3=Wilson |first3=Paul A. |last4=Butler |first4=Emily |last5=Roberts |first5=Andrew P. |title=Continental ice in Greenland during the Eocene and Oligocene |journal=Nature |date=March 2007 |volume=446 |issue=7132 |pages=176–179 |doi=10.1038/nature05591|pmid=17287724 |bibcode=2007Natur.446..176E |s2cid=4372596 }}

Continental ice sheets in Antarctica reached sea level during the transition.{{sfn|Coxall|Pearson|2007|pages=351–387}}{{sfn|Berggren|Prothero|1992|p=1}}{{cite journal |last1=O'Brien |first1=Charlotte L. |last2=Huber |first2=Matthew |last3=Thomas |first3=Ellen |author3-link=Ellen Thomas (scientist) |last4=Pagani |first4=Mark |last5=Super |first5=James R. |last6=Elder |first6=Leanne E. |last7=Hull |first7=Pincelli M. |title=The enigma of Oligocene climate and global surface temperature evolution |journal=Proceedings of the National Academy of Sciences |date=13 October 2020 |volume=117 |issue=41 |pages=25302–25309 |doi=10.1073/pnas.2003914117|pmid=32989142 |pmc=7568263 |bibcode=2020PNAS..11725302O |doi-access=free }} Glacially rafted debris of early Oligocene age in the Weddell Sea and Kerguelen Plateau, in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene.{{sfn|Berggren|Prothero|1992|pp=5–6}}

The causes of the Eocene-Oligocene transition are not yet fully understood.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}}

The timing is wrong for this to be caused either by known impact events or by the volcanic activity on the Ethiopean Plateau.{{sfn|Prothero|2005|p=474}} Two other possible drivers of climate change, not mutually exclusive, have been proposed.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}} The first is thermal isolation of the continent of Antarctica by development of the Antarctic Circumpolar Current.{{sfn|Prothero|2005|pp=474, 476}}{{sfn|Berggren|Prothero|1992|p=1}} Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene.{{sfn|Prothero|2005|p=474}} However, the timing of this event remains controversial.{{cite journal |last1=Lyle |first1=Mitchell |last2=Gibbs |first2=Samantha |last3=Moore |first3=Theodore C. |last4=Rea |first4=David K. |title=Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific |journal=Geology |date=2007 |volume=35 |issue=8 |page=691 |doi=10.1130/G23806A.1|bibcode=2007Geo....35..691L }} The other possibility, for which there is considerable evidence, is a drop in atmospheric carbon dioxide levels (pCO2) during the transition.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}}{{cite journal |last1=Francis |first1=J.E. |last2=Marenssi |first2=S. |last3=Levy |first3=R. |last4=Hambrey |first4=M. |last5=Thorn |first5=V.C. |last6=Mohr |first6=B. |last7=Brinkhuis |first7=H. |last8=Warnaar |first8=J. |last9=Zachos |first9=J. |last10=Bohaty |first10=S. |last11=DeConto |first11=R. |title=Chapter 8 From Greenhouse to Icehouse – The Eocene/Oligocene in Antarctica |journal=Developments in Earth and Environmental Sciences |date=2008 |volume=8 |pages=309–368 |doi=10.1016/S1571-9197(08)00008-6|isbn=9780444528476 }} 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.{{cite journal |last1=Pearson |first1=Paul N. |last2=Foster |first2=Gavin L. |last3=Wade |first3=Bridget S. |title=Atmospheric carbon dioxide through the Eocene–Oligocene climate transition |journal=Nature |date=October 2009 |volume=461 |issue=7267 |pages=1110–1113 |doi=10.1038/nature08447|pmid=19749741 |bibcode=2009Natur.461.1110P |s2cid=205218274 }} Climate modeling suggests that glaciation of Antarctica took place only when pCO2 dropped below a critical threshold value.{{sfn|Francis|Marenssi|Levy|Hambrey|2008}}

Brachiopod oxygen isotope ratios from New Zealand suggest that a proto-Subtropical Convergence developed during the Early Oligocene, with northern New Zealand being subtropical and southern and eastern New Zealand being cooled by cold, subantarctic water.{{Cite journal |last1=Buening |first1=Nancy |last2=Carlson |first2=Sandra J. |last3=Spero |first3=Howard J. |last4=Lee |first4=Daphne E. |date=April 1998 |title=Evidence for the Early Oligocene formation of a proto-Subtropical Convergence from oxygen isotope records of New Zealand Paleogene brachiopods |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018297001132 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=138 |issue=1–4 |pages=43–68 |doi=10.1016/S0031-0182(97)00113-2 |access-date=4 May 2024 |via=Elsevier Science Direct}}

Oligocene climate following the Eocene-Oligocene event is poorly known.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302–25309}} There were several pulses of glaciation in middle Oligocene, about the time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about {{convert|75|m|sp=us}}. This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.{{sfn|Prothero|2005|p=475}}

Some evidence suggests that the climate remained warm at high latitudes{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302–25309}}{{cite journal |last1=Lear |first1=C. H. |title=Cenozoic Deep-Sea Temperatures and Global Ice Volumes from Mg/Ca in Benthic Foraminiferal Calcite |journal=Science |date=14 January 2000 |volume=287 |issue=5451 |pages=269–272 |doi=10.1126/science.287.5451.269|pmid=10634774 |bibcode=2000Sci...287..269L }} even as ice sheets experienced cyclical growth and retreat in response to orbital forcing and other climate drivers.{{cite journal |last1=Pälike |first1=Heiko |last2=Norris |first2=Richard D. |last3=Herrle |first3=Jens O. |last4=Wilson |first4=Paul A. |last5=Coxall |first5=Helen K. |last6=Lear |first6=Caroline H. |last7=Shackleton |first7=Nicholas J. |last8=Tripati |first8=Aradhna K. |last9=Wade |first9=Bridget S. |title=The Heartbeat of the Oligocene Climate System |journal=Science |date=22 December 2006 |volume=314 |issue=5807 |pages=1894–1898 |doi=10.1126/science.1133822|pmid=17185595 |bibcode=2006Sci...314.1894P |s2cid=32334205 |url=https://eprints.soton.ac.uk/42935/2/Paelike_etal_Science2006_supplMat.pdf }} Other evidence indicates significant cooling at high latitudes.{{sfn|Coxall|Pearson|2007|pages=351–387}}{{cite journal |last1=Liu |first1=Z. |last2=Pagani |first2=M. |last3=Zinniker |first3=D. |last4=DeConto |first4=R. |last5=Huber |first5=M. |last6=Brinkhuis |first6=H. |last7=Shah |first7=S. R. |last8=Leckie |first8=R. M. |last9=Pearson |first9=A. |title=Global Cooling During the Eocene-Oligocene Climate Transition |journal=Science |date=27 February 2009 |volume=323 |issue=5918 |pages=1187–1190 |doi=10.1126/science.1166368|pmid=19251622 |bibcode=2009Sci...323.1187L |s2cid=46623205 |url=http://doc.rero.ch/record/210392/files/PAL_E4399.pdf }} Part of the difficulty may be that there were strong regional variations in the response to climate shifts. Evidence of a relatively warm Oligocene suggests an enigmatic climate state, neither hothouse nor icehouse.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25303}}

The late Oligocene (26.5 to 24 mya) likely saw a warming trend in spite of low pCO2 levels, though this appears to vary by region.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302-25303}} However, Antarctica remained heavily glaciated during this warming period.{{cite journal |last1=Pekar |first1=Stephen F. |last2=DeConto |first2=Robert M. |last3=Harwood |first3=David M. |title=Resolving a late Oligocene conundrum: Deep-sea warming and Antarctic glaciation |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=February 2006 |volume=231 |issue=1–2 |pages=29–40 |doi=10.1016/j.palaeo.2005.07.024|bibcode=2006PPP...231...29P |url=http://doc.rero.ch/record/13512/files/PAL_E309.pdf }}{{cite journal |last1=Hauptvogel |first1=D. W. |last2=Pekar |first2=S. F. |last3=Pincay |first3=V. |title=Evidence for a heavily glaciated Antarctica during the late Oligocene "warming" (27.8–24.5 Ma): Stable isotope records from ODP Site 690: LATE OLIGOCENE STABLE ISOTOPE RECORD |journal=Paleoceanography |date=April 2017 |volume=32 |issue=4 |pages=384–396 |doi=10.1002/2016PA002972|doi-access=free }} The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that the South Asian Monsoon had already developed by the late Oligocene.{{cite journal |last1=Wu |first1=Fuli |last2=Miao |first2=Yunfa |last3=Meng |first3=Qingquan |last4=Fang |first4=Xiaomin |last5=Sun |first5=Jimin |title=Late Oligocene Tibetan Plateau Warming and Humidity: Evidence From a Sporopollen Record |journal=Geochemistry, Geophysics, Geosystems |date=January 2019 |volume=20 |issue=1 |pages=434–441 |doi=10.1029/2018GC007775|bibcode=2019GGG....20..434W |doi-access=free }} Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau.{{Cite journal |last1=Jin |first1=Chun-Sheng |last2=Xu |first2=Deke |last3=Li |first3=Mingsong |last4=Hu |first4=Pengxiang |last5=Jiang |first5=Zhaoxia |last6=Liu |first6=Jianxing |last7=Miao |first7=Yunfa |last8=Wu |first8=Fuli |last9=Liang |first9=Wentian |last10=Zhang |first10=Qiang |last11=Su |first11=Bai |last12=Liu |first12=Qingsong |last13=Zhang |first13=Ran |last14=Sun |first14=Jimin |date=11 April 2023 |title=Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene |journal=Proceedings of the National Academy of Sciences of the United States of America |language=en |volume=120 |issue=15 |pages=e2214558120 |doi=10.1073/pnas.2214558120 |issn=0027-8424 |pmc=10104490 |pmid=37011203 |bibcode=2023PNAS..12014558J }}

A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound and King George Island.{{cite journal |last1=Wilson |first1=G.S. |last2=Pekar |first2=S.F. |last3=Naish |first3=T.R. |last4=Passchier |first4=S. |last5=DeConto |first5=R. |title=Chapter 9 The Oligocene–Miocene Boundary – Antarctic Climate Response to Orbital Forcing |journal=Developments in Earth and Environmental Sciences |date=2008 |volume=8 |pages=369–400 |doi=10.1016/S1571-9197(08)00009-8|isbn=9780444528476 }}

File:Turtle Cove mural - Roger Witter.jpg (far left) and other animals from the Turtle Cove Formation]]

The early Eocene climate was very warm, with crocodilians and temperate plants thriving north of the Arctic Circle. 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. Land animals and marine organisms reached a Phanerozoic low in diversity by the late Oligocene, and the temperate forests and jungles of the Eocene were replaced by forest and scrubland. The closing of the Tethys Seaway destroyed its tropical biota.{{sfn|Prothero|2005|pp=473–477}}

The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving Nothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions.{{sfn|Francis|Marenssi|Levy|Hambrey|2008}}

Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced by temperate deciduous forests. Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts.{{sfn|Torsvik|Cocks|2017|p=255}} The decline in pCO2 favored C4 photosynthesis,{{cite journal |last1=Christin |first1=Pascal-Antoine |last2=Besnard |first2=Guillaume |last3=Samaritani |first3=Emanuela |last4=Duvall |first4=Melvin R. |last5=Hodkinson |first5=Trevor R. |last6=Savolainen |first6=Vincent |last7=Salamin |first7=Nicolas |title=Oligocene CO2 Decline Promoted C4 Photosynthesis in Grasses |journal=Current Biology |date=January 2008 |volume=18 |issue=1 |pages=37–43 |doi=10.1016/j.cub.2007.11.058|pmid=18160293 |hdl=2262/82791 |s2cid=16946058 |hdl-access=free }} which is found only in angiosperms and is particularly characteristic of grasses.{{cite journal | vauthors = Sage RF | title = A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame | journal = Journal of Experimental Botany | volume = 67 | issue = 14 | pages = 4039–56 | date = July 2016 | pmid = 27053721 | doi = 10.1093/jxb/erw156 | doi-access = free }} However, even at the end of the period, grass was not quite common enough for modern savannas.{{sfn|Torsvik|Cocks|2017|p=255}}

In North America, much of the dense forest was replaced by patchy scrubland with riparian forests.{{sfn|Prothero|2005|p=475}} Subtropical species dominated with cashews{{cite journal |last1=Méndez-Cárdenas |first1=Juliana P. |last2=Cevallos-Ferriz |first2=Sergio R.S. |last3=Calvillo-Canadell |first3=Laura |last4=Rodríguez-Yam |first4=Gabriel A. |last5=Borja |first5=Amparo M. |last6=Martínez-Cabrera |first6=Hugo I. |title=Loxopterygium wood in Coayuca de Andrade, Oligocene of Puebla, Mexico |journal=Review of Palaeobotany and Palynology |date=August 2014 |volume=207 |pages=38–43 |doi=10.1016/j.revpalbo.2014.04.004|bibcode=2014RPaPa.207...38M }} and lychee trees present,{{cite journal |last1=Buerki |first1=Sven |last2=Forest |first2=Félix |last3=Stadler |first3=Tanja |last4=Alvarez |first4=Nadir |date=July 2013 |title=The abrupt climate change at the Eocene–Oligocene boundary and the emergence of South-East Asia triggered the spread of sapindaceous lineages |journal=Annals of Botany |volume=112 |issue=1 |pages=151–160 |doi=10.1093/aob/mct106 |pmc=3690995 |pmid=23723259}} and temperate woody plants such as roses, beeches,{{cite journal |last1=Denk |first1=Thomas |last2=Grimm |first2=Guido W. |date=December 2009 |title=The biogeographic history of beech trees |url=https://www.sciencedirect.com/science/article/abs/pii/S0034666709001353 |journal=Review of Palaeobotany and Palynology |volume=158 |issue=1–2 |pages=83–100 |bibcode=2009RPaPa.158...83D |doi=10.1016/j.revpalbo.2009.08.007 |access-date=15 December 2023 |via=Elsevier Science Direct}} and pines{{sfn|Torsvik|Cocks|2017|p=254}} were common. The legumes spread,{{cite journal |last1=Herendeen |first1=Patrick S. |last2=Dilcher |first2=David L. |title=Fossil mimosoid legumes from the Eocene and Oligocene of southeastern North America |journal=Review of Palaeobotany and Palynology |date=March 1990 |volume=62 |issue=3–4 |pages=339–361 |doi=10.1016/0034-6667(90)90094-Y|bibcode=1990RPaPa..62..339H }} while sedges{{cite journal |last1=Escudero |first1=Marcial |last2=Hipp |first2=Andrew L. |last3=Waterway |first3=Marcia J. |last4=Valente |first4=Luis M. |title=Diversification rates and chromosome evolution in the most diverse angiosperm genus of the temperate zone (Carex, Cyperaceae) |journal=Molecular Phylogenetics and Evolution |date=June 2012 |volume=63 |issue=3 |pages=650–655 |doi=10.1016/j.ympev.2012.02.005|pmid=22366369 }} and ferns continued their ascent.{{cite journal |last1=Devore |first1=M.L. |last2=Pigg |first2=K.B. |title=Floristic composition and comparison of middle Eocene to late Eocene and Oligocene floras in North America |journal=Bulletin of Geosciences |date=22 March 2010 |pages=111–134 |doi=10.3140/bull.geosci.1135|doi-access=free }}

In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity.{{Cite journal |last1=Uhl |first1=Dieter |last2=Spiekermann |first2=Rafael |last3=Wuttke |first3=Michael |last4=Poschmann |first4=Markus J. |last5=Jasper |first5=André |date=1 February 2022 |title=Wildfires during the Paleogene (late Eocene–late Oligocene) of the Neuwied Basin (W-Germany) |url=https://www.sciencedirect.com/science/article/pii/S0034666721001895 |journal=Review of Palaeobotany and Palynology |volume=297 |pages=104565 |doi=10.1016/j.revpalbo.2021.104565 |bibcode=2022RPaPa.29704565U |s2cid=244364779 |issn=0034-6667 |access-date=15 December 2023 |via=Elsevier Science Direct}}

In Pakistan, the flora consisted mainly of dry but dense forests.{{Cite journal |last=Martin |first=C. |last2=Bentaleb |first2=I. |last3=Antoine |first3=P.-O. |date=15 October 2011 |title=Pakistan mammal tooth stable isotopes show paleoclimatic and paleoenvironmental changes since the early Oligocene |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018211003932 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=311 |issue=1-2 |pages=19–29 |doi=10.1016/j.palaeo.2011.07.010 |access-date=6 January 2025 |via=Elsevier Science Direct}} In northern China, there was a progressive ascendance of open, grassy environments.{{Cite journal |last=Gomes Rodrigues |first=Helder |last2=Marivaux |first2=Laurent |last3=Vianey-Liaud |first3=Monique |date=1 November 2012 |title=Expansion of open landscapes in Northern China during the Oligocene induced by dramatic climate changes: Paleoecological evidence |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018212004269 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=358-360 |pages=62–71 |doi=10.1016/j.palaeo.2012.07.025 |access-date=6 January 2025 |via=Elsevier Science Direct}} The Ha Long megafossil flora from the Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is now Vietnam was very similar to its present flora.{{cite journal |last1=Huang |first1=Jian |last2=Spicer |first2=Robert A. |last3=Li |first3=Shu-Feng |last4=Liu |first4=Jia |last5=Do |first5=Truong Van |last6=Nguyen |first6=Hung Ba |last7=Zhou |first7=Zhe-Kun |last8=Su |first8=Tao |date=1 May 2022 |title=Long-term floristic and climatic stability of northern Indochina: Evidence from the Oligocene Ha Long flora, Vietnam |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018222001006 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=593 |page=110930 |doi=10.1016/j.palaeo.2022.110930 |bibcode=2022PPP...59310930H |s2cid=247368063 |access-date=13 February 2023}}

Kelps make their first appearance in the fossil record during the earliest Oligocene.{{Cite journal |last1=Kiel |first1=Steffen |last2=Goedert |first2=James L. |last3=Huynh |first3=Tony L. |last4=Krings |first4=Michael |last5=Parkinson |first5=Dula |last6=Romero |first6=Rosemary |last7=Looy |first7=Cindy V. |date=16 January 2024 |title=Early Oligocene kelp holdfasts and stepwise evolution of the kelp ecosystem in the North Pacific |journal=Proceedings of the National Academy of Sciences of the United States of America |language=en |volume=121 |issue=4 |pages=e2317054121 |doi=10.1073/pnas.2317054121 |issn=0027-8424 |pmc=10823212 |pmid=38227671 }}

File:Ancient Mammal at Denver Museum of Nature and Science.jpg]]

File:Paraceratherium_transouralicum.jpg restored next to Hyaenodon]]

Most extant mammal families had appeared by the end of the Oligocene. These included primitive three-toed horses, rhinoceroses, camels, deer, and peccaries. Carnivores such as dogs, nimravids, bears, weasels, and raccoons began to replace the creodonts that had dominated the Paleocene in the Old World. Rodents and rabbits underwent tremendous diversification due to the increase in suitable habitats for ground-dwelling seed eaters, as habitats for squirrel-like nut- and fruit-eaters diminished. The primates, once present in Eurasia, were reduced in range to Africa and South America.{{sfn|Prothero|2005|p=476}} Many groups, such as equids,{{cite book |last1=Floyd |first1=Andrea E. |title=Equine podiatry |date=2007 |publisher=Elsevier Saunders |location=Philadelphia, Pa. |isbn=9781416064596 |chapter=Evolution of the equine digit and its relevance to the modern horse}} 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.{{cite book |last1=Prothero |first1=Donald R. |title=Horns, tusks, and flippers : the evolution of hoofed mammals |date=2002 |publisher=Johns Hopkins University Press |location=Baltimore |isbn=9780801871351 |page=19 |url=https://books.google.com/books?id=kWpQX-sfsLgC&q=running |access-date=10 August 2021 |chapter=Cloven hooves}} Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates, an ancient lineage of primitive mammals that originated back in the Jurassic, also became extinct in the Oligocene, aside from the gondwanatheres.{{cite journal |last1=Prothero |first1=Donald R. |title=North American mammalian diversity and Eocene–Oligocene extinctions |journal=Paleobiology |date=1985 |volume=11 |issue=4 |pages=389–405 |doi=10.1017/S0094837300011696|bibcode=1985Pbio...11..389P |s2cid=87346202 }}

The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure.{{Cite journal |last1=Mennecart |first1=Bastien |last2=Aiglstorfer |first2=Manuela |last3=Li |first3=Yikun |last4=Li |first4=Chunxiao |last5=Wang |first5=ShiQi |date=6 September 2021 |title=Ruminants reveal Eocene Asiatic palaeobiogeographical provinces as the origin of diachronous mammalian Oligocene dispersals into Europe |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=17710 |doi=10.1038/s41598-021-96221-x |issn=2045-2322 |doi-access=free |pmid=34489502 |pmc=8421421 |bibcode=2021NatSR..1117710M }} The lowering of sea levels closed the Turgai Strait across the Obik Sea, which had previously separated Asia from Europe. This allowed Asian mammals, such as rhinoceroses and ruminants, to enter Europe and drive endemic species to extinction.{{sfn|Prothero|2005|p=476}} Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene.{{cite journal |last1=Barberà |first1=X. |last2=Cabrera |first2=L. |last3=Marzo |first3=M. |last4=Parés |first4=J.M. |last5=Agustı́ |first5=J. |title=A complete terrestrial Oligocene magnetobiostratigraphy from the Ebro Basin, Spain |journal=Earth and Planetary Science Letters |date=April 2001 |volume=187 |issue=1–2 |pages=1–16 |doi=10.1016/S0012-821X(01)00270-9|bibcode=2001E&PSL.187....1B |url=http://doc.rero.ch/record/16006/files/PAL_E3760.pdf }} There was significant diversification of mammals in Eurasia, including the giant indricotheres, that grew up to {{convert|6|m|sigfig=1|sp=us}} at the shoulder and weighed up to 20 tons. Paraceratherium was one of the largest land mammals ever to walk the Earth.{{cite book | last = Prothero | first = D. | title = Rhinoceros Giants: The Palaeobiology of Indricotheres| publisher = Indiana University Press | year = 2013 | location = Indiana | isbn = 978-0-253-00819-0 |url=https://books.google.com/books?id=fbyn88ExO9IC&pg=PA87 |pages=87–106}} However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts.{{sfn|Torsvik|Cocks|2017|p=255}} The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia.{{sfn|Prothero|2005|p=476}} The first felid, Proailurus, originated in Asia during the late Oligocene and spread to Europe.{{cite web| last = Mott| first = Maryann| title = Cats Climb New Family Tree| publisher = National Geographic News| date = January 11, 2006 | url = http://news.nationalgeographic.com/news/2006/01/0111_060111_cat_evolution.html| access-date = 2006-07-15 |url-status=dead |archive-url=https://web.archive.org/web/20071012220411/http://news.nationalgeographic.com/news/2006/01/0111_060111_cat_evolution.html |archive-date=Oct 12, 2007 }}

File:Paraphysornis BW-2r.jpg]]

There was only limited migration between Asia and North America.{{sfn|Prothero|2005|p=476}} The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover of gastropods, amphibians, and reptiles. Mammals were much less affected. Crocodilians and pond turtles replaced by dry land tortoises. Molluscs shifted to more drought-tolerant forms.{{sfn|Prothero|2005|p=475}} 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.{{cite book |last1=Benton |first1=Rachel C. |last2=Terry | first2=Dennis O. Jr. |last3=Evanoff |first3=Emmett |last4=McDonald |first4=Hugh Gregory |title=The White River Badlands: Geology and Paleontology |publisher=Indiana University Press |date=2015 |location=Bloomington |isbn=9780253016089 |url=https://books.google.com/books?id=ZcFtCQAAQBAJ |access-date=10 August 2021}} Merycoidodonts, an endemic American group, were very diverse during this time.{{cite book |last1=Saarinen |first1=Juha |last2=Mantzouka |first2=Dimitra |last3=Sakala |first3=Jakub |title=Nature through Time |chapter=Aridity, Cooling, Open Vegetation, and the Evolution of Plants and Animals During the Cenozoic |series=Springer Textbooks in Earth Sciences, Geography and Environment |date=2020 |pages=83–107 |doi=10.1007/978-3-030-35058-1_3|isbn=978-3-030-35057-4 |s2cid=226435040 }}

File:Aegyptopithecus NT.jpg is an early fossil catarrhine that predates the divergence between hominoids (apes) and Old World monkeys]]

Australia and South America became geographically isolated and developed their own distinctive endemic fauna. These included the New World and Old World monkeys. The South American continent was home to animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchians, terror birds, and carnivorous metatheres, like the borhyaenids remained the dominant predators.{{cite journal |last1=Flynn |first1=John J |last2=Wyss |first2=André R |last3=Croft |first3=Darin A |last4=Charrier |first4=Reynaldo |title=The Tinguiririca Fauna, Chile: biochronology, paleoecology, biogeography, and a new earliest Oligocene South American Land Mammal 'Age' |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=June 2003 |volume=195 |issue=3–4 |pages=229–259 |doi=10.1016/S0031-0182(03)00360-2|bibcode=2003PPP...195..229F |doi-access=free }}

Africa was also relatively isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms.{{sfn|Prothero|2005|p=476}} Egypt in the Oligocene was an environment of lush forested deltas.{{cite book |last1=Benton |first1=M. J. |title=Cowen's history of life |date=2019 |publisher=John Wiley & Sons Ltd |location=Hoboken, NJ |isbn=9781119482215 |edition=Sixth |page=306}} 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.{{Cite journal |last1=de Vries |first1=Dorien |last2=Heritage |first2=Steven |last3=Borths |first3=Matthew R. |last4=Sallam |first4=Hesham M. |last5=Seiffert |first5=Erik R. |date=7 October 2021 |title=Widespread loss of mammalian lineage and dietary diversity in the early Oligocene of Afro-Arabia |journal=Communications Biology |language=en |volume=4 |issue=1 |page=1172 |doi=10.1038/s42003-021-02707-9 |issn=2399-3642 |doi-access=free |pmid=34621013 |pmc=8497553 }}

During the Oligocene, the Tethyan marine biodiversity hotspot collapsed as the Tethys Ocean contracted. The seas around Southeast Asia and Australia became the new dominant hotspot of marine biodiversity.{{cite journal |last1=Cowman |first1=P. F. |last2=Bellwood |first2=D. R. |date=10 October 2011 |title=Coral reefs as drivers of cladogenesis: expanding coral reefs, cryptic extinction events, and the development of biodiversity hotspots |journal=Journal of Evolutionary Biology |volume=24 |issue=12 |pages=2543–2562 |doi=10.1111/j.1420-9101.2011.02391.x |pmid=21985176 |doi-access=free }} At sea, 97% of marine snail species, 89% of clams, and 50% of echinoderms of the Gulf Coast did not survive past the earliest Oligocene. New species evolved, but the overall diversity diminished. Cold-water mollusks migrated around the Pacific Rim from Alaska and Siberia.{{sfn|Prothero|2005|p=476}} The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene.{{cite journal | author = Vinn, O. | year = 2009 | title = The ultrastructure of calcareous cirratulid (Polychaeta, Annelida) tubes | journal = Estonian Journal of Earth Sciences | volume = 58 | issue = 2 | pages = 153–156 | url = http://www.kirj.ee/public/Estonian_Journal_of_Earth_Sciences/2009/issue_2/earth-2009-2-153-156.pdf | access-date = 2012-09-16 | doi = 10.3176/earth.2009.2.06 | doi-access = free }}

The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into the Indo-Pacific.{{cite journal |last1=Siqueira |first1=Alexandre C. |last2=Bellwood |first2=David R. |last3=Cowman |first3=Peter F. |date=4 June 2019 |title=Historical biogeography of herbivorous coral reef fishes: The formation of an Atlantic fauna |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jbi.13631 |journal=Journal of Biogeography |volume=46 |issue=7 |pages=1611–1624 |doi=10.1111/jbi.13631 |bibcode=2019JBiog..46.1611S |s2cid=195431434 |access-date=9 May 2023}} The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen whales and toothed whales had just appeared, and their ancestors, the archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. 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. Early desmostylians, like Behemotops, are known from the Oligocene. Pinnipeds appeared near the end of the epoch from an otter-like ancestor.{{cite web| last = Handwerk| first = Brian| title = Seal with "Arms" Discovered| publisher = National Geographic News| date = 2009-03-22| url = http://news.nationalgeographic.com/news/2009/04/090422-seal-evolution-missing-link.html| archive-url = https://web.archive.org/web/20090425125042/http://news.nationalgeographic.com/news/2009/04/090422-seal-evolution-missing-link.html| url-status = dead| archive-date = April 25, 2009| access-date = 2014-12-31}}

File:NMNH-USNMV15917Poebrotherium.jpg|Poebrotherium

File:Merycoidodon Skull Oligocene Left Side.jpg|Merycoidodon

File:Hoplophoneus primaevus (fossil false sabertooth cat) (Middle Oligocene; Nebraska, USA) 3 (32791323412).jpg|Hoplophoneus

File:Mesohippus barbouri Harvard.jpg|Mesohippus

File:Paraceratherium transouralicum skull.jpg|Paraceratherium

File:Paleoparadoxia Natural History Museum of Los Angeles County 20110330.jpg|Paleoparadoxia

File:Protoceras skeleton.jpg|Protoceras

File:Archaeotherium mortoni.JPG|Archaeotherium

File:Janjucetus hunderi skull.jpg|Janjucetus

The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene/Oligocene boundary, and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in the Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute.

The opening and closing of ocean gateways: the opening of the Drake Passage; the opening of the Tasmanian Gateway and the closing of the Tethys seaway; along with the final formation of the Greenland–Iceland–Faroes Ridge; played vital parts in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation.

File:Eocene-Paleocene-circumpolar.svg

The Drake Passage is located between South America and Antarctica. Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the Southern Ocean was its connection to South America. As the South American continent moved north, the Drake Passage opened and enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept the cold waters of Antarctica circulating around that continent and strengthened the formation of Antarctic Bottom Water (ABW). With the cold water concentrated around Antarctica, sea surface temperatures and, consequently, continental temperatures would have dropped. The onset of Antarctic glaciation occurred during the early Oligocene, and the effect of the Drake Passage opening on this glaciation has been the subject of much research. However, some controversy still exists as to the exact timing of the passage opening, whether it occurred at the start of the Oligocene or nearer the end. 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.

Stemming from the issue of when the opening of the Drake Passage took place, is the dispute over how great of an influence the opening of the Drake Passage had on the global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation and subsequent global cooling, other studies have suggested that the δ¹⁸O signature is too strong for glaciation to be the main trigger for cooling. Through study of Pacific Ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300,000 years, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.

The latest hypothesized time for the opening of the Drake Passage is during the early Miocene. Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true 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.

The earliest hypothesized time for the opening of the Drake Passage is around 30 Ma.{{r|Lyle}} One of the possible issues with this timing was the continental debris cluttering up the seaway between the two plates in question. This debris, along with what is known as the Shackleton fracture zone, has been shown in a recent study to be fairly young, only about 8 million years old.{{r|Mack}} The study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would have facilitated an earlier onset of the Antarctic Circumpolar Current. There is some evidence that it occurred much earlier, during the early Eocene.{{cite journal |last1= van de Lagemaat |first1= S.H.A. |last2= Swart |first2= M.L.A.|display-authors=etal |date= April 2021|title= Subduction initiation in the Scotia Sea region and opening of the Drake Passage: When and why? |journal= Earth-Science Reviews |volume= 215 |pages= 103351 |doi= 10.1016/j.earscirev.2021.103551|bibcode= 2021ESRv..21503551V |s2cid= 233576410 |doi-access= free |hdl= 20.500.11850/472835 |hdl-access= free }}

The other major oceanic gateway opening during this time was the Tasman, or Tasmanian, depending on the paper, gateway between Australia and Antarctica. The time frame for this opening is less disputed than the Drake Passage and is largely considered to have occurred around 34 Ma. As the gateway widened, the Antarctic Circumpolar Current strengthened.

The Tethys Seaway was not a gateway, but rather a sea in its own right. Its closing during the Oligocene had significant impact on both ocean circulation and climate. The collisions of the African plate with the European plate and of the Indian subcontinent with the Asian plate, cut off the Tethys Seaway that had provided a low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.

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. More information about the evolution of North Atlantic Deep Water will be given a few sections down.

Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction and patterns of species migration can also be studied to gain insight into ocean conditions. For a while, it was thought that the glaciation of Antarctica may have significantly contributed to the cooling of the ocean, however, recent evidence tends to deny this.

File:Reconstruction of Aglaocetus moreni (fossil baleen whale) (Oligocene; Argentina) (32313481396).jpg moreni]]

Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the North Pacific and the Southern Ocean. As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of the North Atlantic began to come into play as well. Computer models suggest that once this occurred, a more modern in appearance thermo-haline circulation started.

Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts.

The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. Regardless of the time at which the opening of the Drake Passage occurred, the effect on the cooling of the Southern Ocean would have been the same.

Recorded extraterrestrial impacts:

• Haughton impact crater, Nunavut, Canada (23 Ma, crater {{convert|24|km|mi|abbr=on}} diameter; now considered questionable as an Oligocene event; later analyses have concluded the crater dates to 39 Ma, placing the event in the Eocene.){{cite Earth Impact DB |name=Haughton |access-date=2017-10-09}}{{cite journal |last1= Sherlock |first1= S.C. |last2= Kelley |first2= S.P. |display-authors=etal |date= 2005 |title= Re-evaluating the age of the Haughton impact event |journal= Meteoritics & Planetary Science |volume= 40 |issue= 12 |pages= 1777–1787|doi= 10.1111/j.1945-5100.2005.tb00146.x|bibcode= 2005M&PS...40.1777S |doi-access= free }}

• La Garita Caldera (28–26 million years ago){{cite book|last=Breining|first=Greg|title=Super Volcano: The Ticking Time Bomb Beneath Yellowstone National Park|publisher=Voyageur Press|location=St. Paul, MN|year=2007|pages=[https://archive.org/details/supervolcanotick0000brei/page/256 256 pg]|chapter=Most-Super Volcanoes|isbn=978-0-7603-2925-2|chapter-url-access=registration|chapter-url=https://archive.org/details/supervolcanotick0000brei/page/256}}
• Wah Wah Springs Caldera (30 million years ago)

• List of fossil sites (with link directory)
• Turgai Sea

{{Reflist|

refs=

{{cite book|last=Prothero|first=D.|title=Encyclopedia of Geology|chapter=TERTIARY TO PRESENT {{pipe}} Oligocene| date=May 2005 |pages=472–478|doi=10.1016/B0-12-369396-9/00056-3|isbn=978-0-12-369396-9}}

{{cite journal|last=Katz|first=M|author2=Cramer, B. |author3=Toggweiler, J. |author4=Esmay, G. |author5=Liu, C. |author6=Miller, K. |author7=Rosenthal, Y. |author8=Wade, B. |author9=Wright, J. |title=Impact of Antarctic Circumpolar Current development on late Paleogene ocean structure|journal=Science|date=May 2011|volume=332|issue=6033|pages=1076–1079|bibcode = 2011Sci...332.1076K |doi = 10.1126/science.1202122|pmid=21617074 |s2cid=22335538}}

{{cite journal|last=Mackensen|first=Andreas|title=Changing Southern Ocean palaeocirculation and effects on global climate|journal=Antarctic Science|date=Dec 2004|volume=16|issue=4|pages=369–389|doi=10.1017/S0954102004002202|bibcode=2004AntSc..16..369M|s2cid=127236104}}

{{cite journal|last=Via|first=Rachael|author2=Thomas, D.|title=Evolution of Antarctic thermohaline circulation: Early Oligocene onset of deep-water production in the North Atlantic|journal=Geology|date=June 2006|volume=34|issue=6|pages=441–444|bibcode = 2006Geo....34..441V |doi = 10.1130/G22545.1 }}

{{cite journal |last1=Lyle |first1=Mitchell |last2=Barron |first2=J. |last3=Bralower |first3=T. |last4=Huber |first4=M. |last5=Olivarez Lyle |first5=A. |last6=Ravelo |first6=A. C. |last7=Rea |first7=D. K. |last8=Wilson |first8=P. A. |title=Pacific Ocean and Cenozoic evolution of climate |journal=Reviews of Geophysics |date=April 2008 |volume=46 |issue=2 |pages=RG2002 |bibcode=2008RvGeo..46.2002L |doi=10.1029/2005RG000190|hdl=2027.42/95039 |s2cid=12384214 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/95039/1/rog1637.pdf |hdl-access=free }}

{{cite journal|last=Allen|first=Mark|author2=Armstrong, Howard|title=Arabia-Eurasia cooling and the forcing of mid-Cenozoic global cooling|journal=Palaeogeology, Palaeoclimatology, Palaeoecology|date=July 2008|volume=265|issue=1–2|series=1–2|pages=52–58|doi=10.1016/j.palaeo.2008.04.021|url=http://dro.dur.ac.uk/14557/1/14557.pdf}}

{{cite journal|last=Green|first=William|author2=Hunt, G. |author3=Wing, S. |author4=DiMichele, W. |title=Does extinction wield an axe or pruning shears? How interactions between phylogeny and ecology affect patterns of extinction|journal=Paleobiology|year=2011|volume=37|issue=1|pages=72–91|doi=10.1666/09078.1|bibcode=2011Pbio...37...72G |s2cid=55150020}}

{{cite journal|last=Bosellini|first=Francesca|author2=Perrin, Christine|title=Estimating Mediterranean Oligocene–Miocene sea surface temperatures: An approach based on coral taxonomic richness|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|date=February 2008|volume=258|issue=1–2|series=1–2|pages=71–88|doi=10.1016/j.palaeo.2007.10.028|bibcode=2008PPP...258...71B}}

{{cite journal|last=Hay|first=William|author2=Flogel, S. |author3=Soding, E. |title=Is initiation of glaciation on Antarctica related to a change in the structure of the ocean?|journal=Global and Planetary Change|date=September 2004|volume=45|issue=1–3|series=1–3|pages=23–33|doi=10.1016/j.gloplacha.2004.09.005|bibcode=2005GPC....45...23H}}

{{cite journal |last1=von der Heydt |first1=Anna |last2=Dijkstra |first2=Henk A. |title=The effect of gateways on ocean circulation patterns in the Cenozoic |journal=Global and Planetary Change |date=May 2008 |volume=62 |issue=1–2 |series=1–2 |pages=132–146 |bibcode=2008GPC....62..132V |doi=10.1016/j.gloplacha.2007.11.006}}

Pérez-Consuegra, N., Góngora, D. E., Herrera, F., Jaramillo, C., Montes, C., Cuervo-Gómez, A. M., Hendy, A., Machado, A., Cárdenas, D., Bayona, G. (2018). New records of Humiriaceae fossil fruits from the Oligocene and Early Miocene of the western Azuero Peninsula, Panamá. Boletín de la Sociedad Geológica Mexicana, 223.

}}

• Ogg, Jim (June 2004). [https://web.archive.org/web/20060423084018/http://www.stratigraphy.org/gssp.htm "Overview of Global Boundary Stratotype Sections and Points (GSSP's)"]. International Commission on Stratigraphy. Accessed April 30, 2006.

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{{EB1911 poster|Oligocene System}}

• [http://palaeos.com/cenozoic/oligocene/oligocene.html Palaeos: Oligocene]
• [http://www.ucmp.berkeley.edu/tertiary/oli.html UCMP Berkeley Oligocene Page]
• [http://www.copyrightexpired.com/earlyimage/index.html Prehistoric Pictures, in the Public Domain]
• [https://web.archive.org/web/20041211081307/http://paleodirect.com/pl-003.htm Oligocene Leaf Fossils]
• [https://web.archive.org/web/20041103215522/http://www.paleodirect.com/f1-001.htm Olicgocene Fish Fossils]
• [http://scotese.com/oligocen.htm PaleoMap Project: Oligocene]
• [http://www.foraminifera.eu/querydb.php?age=Oligocene&aktion=suche Oligocene Microfossils: 300+ images of Foraminifera]

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