The Oligocene ( ) is a geologic
epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
of the
Paleogene Period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist
Heinrich Ernst Beyrich
Heinrich Ernst Beyrich (31 August 1815 – 9 July 1896) was a German palaeontologist. Life
Born in Berlin, he was educated at the university in that city, and afterwards at Bonn, where he studied under Georg August Goldfuss and Johann Jakob N ...
from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek (''olígos'', "few") and (''kainós'', "new"),
and refers to the sparsity of
extant forms of
molluscs. The Oligocene is preceded by the
Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
Epoch and is followed by the
Miocene
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
Epoch. The Oligocene is the third and final epoch of the
Paleogene Period.
The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s of the Miocene. Major changes during the Oligocene included a global expansion of
grassland
A grassland is an area where the vegetation is dominated by grasses ( Poaceae). However, sedge ( Cyperaceae) and rush ( Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur na ...
s, and a regression of
tropical
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
broad leaf
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
s to the
equatorial belt
The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific,Bin WangThe Asian Monsoon.Retrieved 2008-05-03. as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such ...
.
The start of the Oligocene is marked by a notable
extinction event called the
Grande Coupure
Grande means "large" or "great" in many of the Romance languages. It may also refer to:
Places
* Grande, Germany, a municipality in Germany
* Grande Communications, a telecommunications firm based in Texas
* Grande-Rivière (disambiguation)
* Ar ...
; it featured the replacement of
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...
an fauna with
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
n
fauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''Biota (ecology ...
, except for the endemic
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
and
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...
families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.
Boundaries and subdivisions
The lower boundary of the Oligocene (its
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted b ...
or GSSP) is placed at the last appearance of the
foraminiferan
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an ...
genus ''
Hantkenina
''Hantkenina'' is a genus of planktonic foraminifera that lived from the Middle Eocene up to late Eocene, cirka 49 Ma-33.9 Ma. There have been 11 morphospecies described, including one of Cribrohantkenina Coxal, H.K. y Pearson, P.N. (2006). Chapt ...
'' in a quarry at
Massignano
Massignano is a ''comune'' (municipality) in the Province of Ascoli Piceno in the Italian region Marche, located about southeast of Ancona and about northeast of Ascoli Piceno. As of 31 December 2004, it had a population of 1,621 and an area of ...
,
Italy
Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...
. 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).
The upper boundary of the Oligocene is defined by its GSSP at
Carrosio
Carrosio is a ''comune'' (municipality) in the Province of Alessandria in the Italian region Piedmont, located about southeast of Turin and about southeast of Alessandria. As of 31 December 2004, it had a population of 468 and an area of .All dem ...
,
Italy
Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...
, which coincides with the
first appearance
In American comic books and other stories with a long history, first appearance refers to the first issue to feature a fictional character. These issues are often highly valued by collectors due to their rarity and iconic status.
Reader interes ...
of the foraminiferan ''
Paragloborotalia kugleri'' and with the base of
magnetic polarity chronozone C6Cn.2n.
Oligocene
faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convent ...
s from youngest to oldest are:
Tectonics and paleogeography
![Mediterranean Rupelian](https://upload.wikimedia.org/wikipedia/commons/1/12/Mediterranean_Rupelian.jpg)
During the Oligocene Epoch, the continents continued to
drift
Drift or Drifts may refer to:
Geography
* Drift or ford (crossing) of a river
* Drift, Kentucky, unincorporated community in the United States
* In Cornwall, England:
** Drift, Cornwall, village
** Drift Reservoir, associated with the village
...
toward their present positions.
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
became more isolated as deep ocean channels were established between Antarctica and
Australia and
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
. 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. The timing of the formation of the
Drake Passage
The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spanish-speaking countries) is the body of water between South America's Cape Horn, Chile and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atla ...
between South America and Antarctica is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene), but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene. 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).
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
The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...
and extensional tectonics in the
Basin and Range province
The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating ...
, 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
The Mogollon-Datil volcanic field is a large () silicic volcanic field in western New Mexico (Mogollon Mountains- Datil, New Mexico). It is a part of an extensive Eocene to Oligocene volcanic event which includes the San Juan volcanic field in s ...
to the
San Juan volcanic field
The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in t ...
, 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.
Between 31 and 26 mya, the
Ethiopia-Yemen Continental Flood Basalts
The Ethiopia-Yemen Continental Flood Basalts were erupted during the Oligocene. They cover an area of about 600,000 km2 in Yemen and Ethiopia, with an estimated volume of greater than 350,000 km3. They are associated with the Afar Plume ...
were emplaced by the East African
large igneous province
A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...
, which also initiated rifting along the
Red Sea
The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...
and
Gulf of Aden.
The
Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...
were rapidly rising in
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...
as the
African plate
The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate ...
continued to push north into the
Eurasian plate
The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...
, isolating the remnants of the
Tethys Sea
The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...
. 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. 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. However, towards the end of the Oligocene, there was a brief marine incursion in Europe.
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. The
Tibetan Plateau
The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the ...
may have reached nearly its present elevation by the late Oligocene.
The Andes first became a major mountain chain in the Oligocene, as subduction became more direct into the coastline.
Climate
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.
Eocene-Oligocene transition and Oi1 event
The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere, being part of a broader trend of global cooling lasting from the
Bartonian
The Bartonian is, in the ICS's geologic time scale, a stage or age in the middle Eocene Epoch or Series. The Bartonian Age spans the time between . It is preceded by the Lutetian and is followed by the Priabonian Age.
Stratigraphic defini ...
to the Rupelian.
The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago,
during which
oxygen isotope ratios decreased by 1.3. About 0.3–0.4 of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0 was due to about of
global cooling
Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing.
Some press reports in the 1970s specula ...
. 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 , and ice sheets were 25% greater in extent than in the modern world.
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.
Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia. Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about just prior to the Oi1 event. 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.
The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene
paleosols
In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science.
In geol ...
reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this.
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.
Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.
Continental ice sheets in Antarctica reached sea level during the transition. Glacially rafted debris of early Oligocene age in the
Weddell Sea
The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha ...
and
Kerguelen Plateau
The Kerguelen Plateau (, ), also known as the Kerguelen–Heard Plateau, is an oceanic plateau and a large igneous province (LIP) located on the Antarctic Plate, in the southern Indian Ocean. It is about to the southwest of Australia and is ...
, in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene.
The causes of the Eocene-Oligocene transition are not yet fully understood.
The timing is wrong for this to be caused either by known
impact events or by the volcanic activity on the Ethiopean Plateau. Two other possible drivers of climate change, not mutually exclusive, have been proposed. The first is thermal isolation of the continent of Antarctica by development of the
Antarctic Circumpolar Current.
[ Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene. However, the timing of this event remains controversial. The other possibility, for which there is considerable evidence, is a drop in atmospheric ]carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
levels (pCO2
''p''CO2, pCO2, or P_\ceis the partial pressure of carbon dioxide (CO2), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO2 as a function of its ...
) during the transition. 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. Climate modeling suggests that glaciation of Antarctica took placed only when pCO2 dropped below a critical threshold value.
Middle Oligocene climate and the Oi2 event
Oligocene climate following the Eocene-Oligocene event is poorly known. 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 . This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.
Some evidence suggests that the climate remained warm at high latitudes even as ice sheets experienced cyclical growth and retreat in response to orbital forcing
Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the Earth's orbit around the sun (see Milankovitch cycles). These orbital changes modify the total amount of sunlight reaching the Earth by up to ...
and other climate drivers. Other evidence indicates significant cooling at high latitudes. 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.
Late Oligocene warming
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. However, Antarctica remained heavily glaciated during this warming period. The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that the south Asian monsoon
The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year fro ...
had already developed by the late Oligocene.
A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound
McMurdo Sound is a sound in Antarctica. It is the southernmost navigable body of water in the world, and is about from the South Pole.
Captain James Clark Ross discovered the sound in February 1841, and named it after Lt. Archibald McMurdo ...
and King George Island.
Biosphere
The early Eocene climate was very warm, with crocodilians
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...
and temperate plants thriving above the Arctic Circle. The cooling trend that began in the middle Eocene continued into the Oligocene, bringing the 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.
Flora
The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving ''Nothofagus
''Nothofagus'', also known as the southern beeches, is a genus of 43 species of trees and shrubs native to the Southern Hemisphere in southern South America (Chile, Argentina) and Australasia (east and southeast Australia, New Zealand, New Gui ...
'' and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions.
Angiosperms continued their expansion throughout the world as tropical and sub-tropical forest
Tropical forests (a.k.a. jungle) are forested landscapes in tropical regions: ''i.e.'' land areas approximately bounded by the tropic of Cancer and Capricorn, but possibly affected by other factors such as prevailing winds.
Some tropical fore ...
s were replaced by temperate deciduous forest
Temperate deciduous or temperate broad-leaf forests are a variety of temperate forest 'dominated' by trees that lose their leaves each year. They are found in areas with warm moist summers and cool winters. The six major areas of this forest type ...
s. Open plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
s and deserts became more common and grass
Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...
es expanded from their water-bank habitat in the Eocene moving out into open tracts. The decline in pCO2 favored C4 photosynthesis
carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when suppl ...
, which is found only in angiosperms and is particularly characteristic of grasses. However, even at the end of the period, grass was not quite common enough for modern savanna
A savanna or savannah is a mixed woodland- grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to ...
s.
In North America, much of the dense forest was replaced by patchy scrubland with riparian forests. Subtropical species dominated with cashews and lychee
Lychee (US: ; UK: ; ''Litchi chinensis''; ) is a monotypic taxon and the sole member in the genus ''Litchi'' in the soapberry family, ''Sapindaceae''.
It is a tropical tree native to Southeast and Southwest China (the Guangdong, Fujian, Yun ...
trees present, and temperate woody plants such as rose
A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
s, beeches, and pine
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accepts ...
s were common. The legumes spread, while sedge
The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" genus '' Carex'' ...
s and fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...
s continued their ascent.
Fauna
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
Nimravidae is an extinct family of carnivorans, sometimes known as false saber-toothed cats, whose fossils are found in North America and Eurasia. Not considered to belong to the true cats (family Felidae), the nimravids are generally considered ...
(ancestor of cats), bears, weasels, and raccoons began to replace the creodonts
Creodonta ("meat teeth") is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ance ...
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. Many groups, such as equids, entelodont
Entelodontidae, the entelodonts, are an extinct family of pig-like artiodactyls (even-toed ungulates) which inhabited the Northern Hemisphere (Asia, Europe, and North America) from the late Eocene to the Middle Miocene epochs, about 38-19 millio ...
s, rhino
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...
s, merycoidodonts, and camelid
Camelids are members of the biological family Camelidae, the only currently living family in the suborder Tylopoda. The seven extant members of this group are: dromedary camels, Bactrian camels, wild Bactrian camels, llamas, alpacas, vicuñas, ...
s, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded. Brontotheres
Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like rhinos, although they were actually more closely related to ...
died out in the Earliest Oligocene, and creodonts
Creodonta ("meat teeth") is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ance ...
died out outside Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
and the Middle East
The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabia (including the Arabian Peninsula and Bahrain), Asia Minor (Asian part of Turkey except Hatay Province), East Thrace (Europ ...
at the end of the period. Multituberculates
Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...
, an ancient lineage of primitive mammals that originated back in the Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
, also became extinct in the Oligocene, aside from the gondwanathere
Gondwanatheria is an extinct group of mammaliaforms that lived in parts of Gondwana, including Madagascar, India, South America, Africa and Antarctica during the Upper Cretaceous through the Paleogene (and possibly much earlier, if '' Allostaffi ...
s.
The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure. 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
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species ...
and ruminants
Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
, to enter Europe and drive endemic species to extinction. Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene. There was significant diversification of mammals in Eurasia, including the giant indricotheres, that grew up to at the shoulder and weighed up to 20 tons. ''Paraceratherium
''Paraceratherium'' is an extinct genus of hornless rhinoceros. It is one of the largest terrestrial mammals that has existed and lived from the early to late Oligocene epoch (34–23 million years ago). The first fossils were discovered ...
'' was one of the largest land mammals ever to walk the Earth. However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts. The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia. The first felid
Felidae () is the family of mammals in the order Carnivora colloquially referred to as cats, and constitutes a clade. A member of this family is also called a felid (). The term "cat" refers both to felids in general and specifically to the dom ...
, ''Proailurus
''Proailurus'' is an extinct felid genus that lived in Europe and Asia approximately 25 million years ago in the Late Oligocene and Miocene. Fossils have been found in Mongolia, Germany, and Spain.
Etymology
The generic name ''Proailurus'' comes ...
'', originated in Asia during the late Oligocene and spread to Europe.
There was only limited migration between Asia and North America. 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. The White River Fauna
The White River Fauna are fossil animals found in the White River Badlands of South Dakota, Wyoming, Colorado and Nebraska in the United States including Badlands National Park.
The fossils have been found in the White River Formation, Chadron F ...
of central North America inhabited a semiarid prairie home and included entelodonts like ''Archaeotherium
''Archaeotherium'' ( grc, αρχαιοθήριον, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya), existing for approximately . ''Archa ...
'', camelids (such as ''Poebrotherium
''Poebrotherium'' ( ) is an extinct genus of camelid, endemic to North America. They lived from the Eocene to Miocene epochs, 46.3—13.6 mya, existing for approximately .
Discovery and history
''Poebrotherium'' was first named by scientist J ...
''), running rhino
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...
ceratoids, three-toed equids (such as ''Mesohippus
''Mesohippus'' (Greek: / meaning "middle" and / meaning "horse") is an extinct genus of early horse. It lived 37 to 32 million years ago in the Early Oligocene. Like many fossil horses, ''Mesohippus'' was common in North America. Its shoulder hei ...
''), nimravids
Nimravidae is an extinct family of carnivorans, sometimes known as false saber-toothed cats, whose fossils are found in North America and Eurasia. Not considered to belong to the true cats (family Felidae), the nimravids are generally considered ...
, protoceratid
Protoceratidae is an extinct family of herbivorous North American artiodactyls (even-toed ungulates) that lived during the Eocene through Pliocene at around 46.2—4.9 Mya, existing for about 41 million years.
Classification
Protoceratidae was ...
s, and early canid
Canidae (; from Latin, '' canis'', " dog") is a biological family of dog-like carnivorans, colloquially referred to as dogs, and constitutes a clade. A member of this family is also called a canid (). There are three subfamilies found withi ...
s like ''Hesperocyon
''Hesperocyon'' is an extinct genus of canids (subfamily Hesperocyoninae, family Canidae) that was endemic to North America, ranging from southern Canada to Colorado. It appeared during the Uintan age, – Bridgerian age (NALMA) of the Mid-Eocen ...
''. Merycoidodonts, an endemic American group, were very diverse during this time.
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 litoptern
Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until th ...
s and notoungulate
Notoungulata is an extinct order of mammalian ungulates that inhabited South America from the early Paleocene to the Holocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms resemb ...
s. Sebecosuchians, terror birds
Phorusrhacids, colloquially known as terror birds, are an extinct clade of large carnivorous flightless birds that were one of the largest species of apex predators in South America during the Cenozoic era; their conventionally accepted tempor ...
, and carnivorous metatheres, like the borhyaenids remained the dominant predators.
Africa was also relative isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms. Egypt in the Oligocene was an environment of lush forested deltas.
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. The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene. 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
The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales possessing teeth, such as the beaked whales and sperm whales. Seventy-three species of t ...
had just appeared, and their ancestors, the archaeocete
Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean Evolutionary radiation, radiation, t ...
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 shark
Requiem sharks are sharks of the family Carcharhinidae in the order Carcharhiniformes. They are migratory, live-bearing sharks of warm seas (sometimes of brackish or fresh water) and include such species as the tiger shark, bull shark, le ...
s, which also appeared in this epoch. Early desmostylia
The Desmostylia (from Greek δεσμά ''desma'', "bundle", and στῦλος ''stylos'', "pillar") are an extinct order of aquatic mammals that existed from the early Oligocene (Rupelian) to the late Miocene ( Tortonian) ().
Desmostylians are t ...
ns, like '' Behemotops'', are known from the Oligocene. Pinnipeds
Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walru ...
appeared near the end of the epoch from an otter-like ancestor.
File:NMNH-USNMV15917Poebrotherium.jpg, Poebrotherium
''Poebrotherium'' ( ) is an extinct genus of camelid, endemic to North America. They lived from the Eocene to Miocene epochs, 46.3—13.6 mya, existing for approximately .
Discovery and history
''Poebrotherium'' was first named by scientist J ...
File:Merycoidodon Skull Oligocene Left Side.jpg, Merycoidodon
''Merycoidodon'' ("ruminating teeth") is an extinct genus of herbivorous artiodactyl of the family Merycoidodontidae, more popularly known by the name ''Oreodon'' ("hillock teeth"). It was endemic to North America during the Middle Eocene to Mid ...
File:Hoplophoneus primaevus (fossil false sabertooth cat) (Middle Oligocene; Nebraska, USA) 3 (32791323412).jpg, Hoplophoneus
''Hoplophoneus'' (Greek: "murder" (phonos), "weapon" (hoplo)) is an extinct genus of the family Nimravidae, endemic to North America during the Late Eocene to Early Oligocene epochs (35–29 mya), existing for approximately .
Taxonomy
In 20 ...
File:Mesohippus barbouri Harvard.jpg, Mesohippus
''Mesohippus'' (Greek: / meaning "middle" and / meaning "horse") is an extinct genus of early horse. It lived 37 to 32 million years ago in the Early Oligocene. Like many fossil horses, ''Mesohippus'' was common in North America. Its shoulder hei ...
File:Paraceratherium transouralicum skull.jpg, Paraceratherium
''Paraceratherium'' is an extinct genus of hornless rhinoceros. It is one of the largest terrestrial mammals that has existed and lived from the early to late Oligocene epoch (34–23 million years ago). The first fossils were discovered ...
File:Paleoparadoxia Natural History Museum of Los Angeles County 20110330.jpg, Paleoparadoxia
File:Protoceras skeleton.jpg, Protoceras
File:Archaeotherium mortoni.JPG, Archaeotherium
''Archaeotherium'' ( grc, αρχαιοθήριον, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya), existing for approximately . ''Archa ...
File:Janjucetus hunderi skull.jpg, Janjucetus
Oceans
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 effects of oceanic gateways on circulation
The opening and closing of ocean gateways: the opening of the Drake Passage
The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spanish-speaking countries) is the body of water between South America's Cape Horn, Chile and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atla ...
; the opening of the Tasmanian Gateway and the closing of the Tethys seaway; along with the final formation of the Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
–Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
–Faroes
The Faroe Islands ( ), or simply the Faroes ( fo, Føroyar ; da, Færøerne ), are a North Atlantic island group and an autonomous territory of the Kingdom of Denmark.
They are located north-northwest of Scotland, and about halfway betw ...
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.
The Drake Passage
The Drake Passage is located between South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
and Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
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 late Oligocene opening of the Drake Passage
=
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 early Oligocene opening of the Drake Passage
=
The earliest hypothesized time for the opening of the Drake Passage is around 30 Ma. 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. 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.
Currently, an opening of the Drake Passage during the early Oligocene is favored.
The opening of the Tasman Gateway
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 closing
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.
Greenland–Iceland–Faroes
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.
Ocean cooling
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.
Deep water
Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the North Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
and the Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
. 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.
North Atlantic deep water
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.
South Ocean deep water
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.
Impact events
Recorded extraterrestrial impacts:
* Haughton impact crater
Haughton impact crater is located on Devon Island, Nunavut in far Northern Canada. It is about in diameter and was formed 31-32 million years ago during the Early Oligocene. The impacting object is estimated to have been approximately in diameter ...
, Nunavut, Canada (23 Ma, crater diameter) (now considered questionable as an Oligocene event; later analyses have concluded the crater dates to 39 Ma, placing the event in the Eocene.)
Supervolcanic explosions
*La Garita Caldera
La Garita Caldera is a large caldera in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States.
It is west of La Garita, Colorado. The eruption that created the La Garita Calde ...
(28–26 million years ago)
* Wah Wah Springs Caldera (30 million years ago)
See also
* List of fossil sites
This list of fossil sites is a worldwide list of localities known well for the presence of fossils. Some entries in this list are notable for a single, unique find, while others are notable for the large number of fossils found there. Many of t ...
''(with link directory)''
* Turgai Sea
The Turgai Sea, also known as the Turgay Sea, Turgai Strait, Obik Sea, Ural Sea or West Siberian Sea, was a large shallow body of salt water (an epicontinental or epeiric sea) during the Mesozoic through Cenozoic Eras. It extended north of the pre ...
References
* Ogg, Jim; June, 2004, ''Overview of Global Boundary Stratotype Sections and Points (GSSP's)'
Accessed April 30, 2006.
External links
Palaeos: Oligocene
Oligocene Microfossils: 300+ images of Foraminifera
{{Authority control
Oligocene,
Geological epochs
Paleogene geochronology