Eocene–Oligocene extinction event
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The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition or ''Grande Coupure'', is the transition between the end of the
Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
and the beginning of the
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period 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 ...
, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago marked by large-scale extinction and floral and faunal turnover (although minor in comparison to the largest mass extinctions). Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient
ungulates Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, cam ...
, the "
condylarths Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs. They are considered early, primitive ungulates. It is now largely considered to be a waste ...
". This was a time of major
climatic change ''Climatic Change'' is a biweekly peer-reviewed scientific journal published by Springer Science+Business Media covering cross-disciplinary work on all aspects of climate change and variability. It was established in 1978 and the editors-in-chie ...
, especially cooling, not clearly caused by any single major impact or
volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...
event. Extended volcanic activity is one possible cause. Another speculation points to several large
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
impacts near this time, including those of the Chesapeake Bay crater and the
Popigai impact structure The Popigai impact structure is the eroded remnant of an impact crater in northern Siberia, Russia. It is tied with the Manicouagan structure as the fourth largest verified impact structure on Earth. A large bolide impact created the diameter ...
of central
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
, which scattered debris perhaps as far as Europe. New dating of the Popigai meteor strengthens its association with the extinction. A leading model of climate cooling at this time predicts a decrease 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 transpar ...
, which slowly declined in the middle to late Eocene and possibly reached some threshold approximately 34 million years ago. This boundary is closely linked with the Oligocene Oi-1 event, an
oxygen isotope There are three known stable isotopes of oxygen (8O): , , and . Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a half-life of , while the shortest-lived isotope is ...
excursion that occurred around 33.55 million years ago, marking the formation of a large ice sheet in
East Antarctica East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying on the Indian Ocean side of the continent, separated from West Antarctica by the Transantarctic Mountains. It lies almost ...
and the beginning of permanent
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at Las ...
coverage on
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 contine ...
.


Grande Coupure

The ''Grande Coupure'', or "great break" in French, with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the
Priabonian The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene Epoch or Series. It spans the time between . The Priabonian is preceded by the Bartonian and is followed by the Rupelian, the lowest stage of t ...
, and the arrival in Europe of Asian species. The Grande Coupure is characterized by widespread extinctions and
allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
in small isolated
relict population In biogeography and paleontology, a relict is a population or taxon of organisms that was more widespread or more diverse in the past. A relictual population is a population currently inhabiting a restricted area whose range was far wider during a ...
s. It was given its name in 1910 by the
Swiss Swiss may refer to: * the adjectival form of Switzerland * Swiss people Places * Swiss, Missouri * Swiss, North Carolina *Swiss, West Virginia * Swiss, Wisconsin Other uses *Swiss-system tournament, in various games and sports *Swiss Internation ...
palaeontologist
Hans Georg Stehlin Hans Georg Stehlin (1870–1941) was a Swiss paleontologist and geologist. Stehlin specialized in vertebrate paleontology, particularly the study of Cenozoic mammals. He published numerous scientific papers on primates and ungulates. He was pre ...
, to characterise the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling". The Grande Coupure marks a break between
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
European faunas before the break and mixed faunas with a strong Asian component afterwards. J. J. Hooker and his team summarized the break: :"Pre-Grande Coupure faunas are dominated by the perissodactyl family
Palaeotheriidae Palaeotheriidae is an extinct family of herbivorous perissodactyl mammals related to equids. They ranged across Europe and Asia from the Eocene through to the early Oligocene 55–33  Ma, existing for approximately . Living in dense forests ...
(distant
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
relatives), six families of artiodactyls (cloven-hoofed mammals) (
Anoplotheriidae Anoplotheriidae is an extinct family of even-toed ungulates ( order Artiodactyla). They were endemic to Western Europe during the Eocene and Oligocene epochs about 48—23 million years ago (Mya), existing for about 25 million years. They disa ...
,
Xiphodontidae Xiphodontidae is an extinct family of even-toed ungulates (order Artiodactyla), endemic to Europe during the Eocene 40.4—33.9 million years ago, existing for about 7.5 million years. They were, most likely, all terrestrial herbivore ...
,
Choeropotamidae Choeropotamidae, also known as Haplobunodontidae, are a family of extinct mammals, extinct herbivores, belonging to artiodactyls. They lived between the lower/middle Eocene and lower Oligocene (about 48 - 30 million years ago) and their remai ...
, Cebochoeridae,
Dichobunidae Dichobunidae is an extinct family of basal artiodactyl mammals from the early Eocene to late Oligocene of North America, Europe, and Asia. The Dichobunidae include some of the earliest known artiodactyls, such as ''Diacodexis''. Description T ...
and Amphimerycidae), the rodent family Pseudosciuridae, the primate families
Omomyidae Omomyidae is a group of early primates that radiated during the Eocene epoch between about (mya). Fossil omomyids are found in North America, Europe & Asia making it one of two groups of Eocene primates with a geographic distribution spanning h ...
and
Adapidae Adapidae is a family of extinct primates that primarily radiated during the Eocene epoch between about 55 and 34 million years ago. Adapid systematics and evolutionary relationships are controversial, but there is fairly good evidence from the ...
, and the
archonta The Archonta are a now-abandoned group of mammals, considered a superorder in some classifications, which consists of these orders: *Primates * Plesiadapiformes (extinct primate-like archontans) * Scandentia (treeshrews) * Dermoptera (colugos) W ...
n family
Nyctitheriidae Nyctitheriidae is a family of extinct eulipotyphlan insectivores known from the Paleocene and Eocene epochs of North America and Asia and persisting into the Oligocene of Europe. Several genera, including '' Nyctitherium'', '' Paradoxonycteris' ...
. :"Post-Grande Coupure faunas include the true rhinoceros (family
Rhinocerotidae 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 ...
), three artiodactyl families ( Entelodontidae,
Anthracotheriidae Anthracotheriidae is a paraphyletic family of extinct, hippopotamus-like artiodactyl ungulates related to hippopotamuses and whales. The oldest genus, ''Elomeryx'', first appeared during the middle Eocene in Asia. They thrived in Africa and Eura ...
and
Gelocidae The Gelocidae are an extinct group of hornless ruminants, ruminantia that are estimated to have lived during the Eocene and Oligocene epochs, from 36 MYA (unit), MYA to 6 MYA. The family generally includes extinct hornless ruminants which do not ...
) related respectively to
pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus ...
s,
hippo The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extant ...
s and
ruminant Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...
s, the
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 na ...
families
Eomyidae Eomyidae is a family of extinct rodents from North America and Eurasia related to modern day pocket gophers and kangaroo rats. They are known from the Middle Eocene to the Late Miocene in North America and from the Late Eocene to the Pleistocene ...
,
Cricetidae The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, muskrats, and New World rats and mice. At almost 608 species, it is the second-largest family of mammals, and has m ...
(
hamster Hamsters are rodents (order Rodentia) belonging to the subfamily Cricetinae, which contains 19 species classified in seven genera.Fox, Sue. 2006. ''Hamsters''. T.F.H. Publications Inc. They have become established as popular small pets. The b ...
s) and
Castoridae The family Castoridae contains the two living species of beavers and their fossil relatives. A highly diverse group of rodents within this family once roamed the earth, but only a single genus is extant today, '' Castor''. Characteristics C ...
(
beaver Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers ar ...
s), and the lipotyphlan family
Erinaceidae Erinaceidae is a family in the order Eulipotyphla, consisting of the hedgehogs and moonrats. Until recently, it was assigned to the order Erinaceomorpha, which has been subsumed with the paraphyletic Soricomorpha into Eulipotyphla. Eulipotyphla ...
(
hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introducti ...
s). The speciose genus
Palaeotherium ''Palaeotherium'' (Ancient Greek for 'old beast') is an extinct genus of perissodactyl ungulate known from the Mid Eocene to earliest Oligocene of Europe. First described by French naturalist Georges Cuvier in 1804, ''Palaeotherium'' was among ...
plus ''
Anoplotherium ''Anoplotherium'' is an extinct genus of herbivorous artiodactyl mammal, possibly belonging to or a close relative of the suborder Tylopoda, which lived in Europe from the Late Eocene to the earliest Oligocene. Fossils of ''Anoplotherium'' were ...
'' and the families Xiphodontidae and Amphimerycidae were observed to disappear completely. :"Only the
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 po ...
family
Herpetotheriidae Herpetotheriidae is an extinct family of metatherians, closely related to marsupials. Species of this family are generally reconstructed as terrestrial, and are considered morphologically similar to modern opossums. Fossils of herpetotheriids com ...
, the artiodactyl family
Cainotheriidae Cainotheriidae is an extinct family of artiodactyls known from the Late Eocene to Middle Miocene of Europe. They are mostly found preserved in karstic deposits. These animals were small in size, and generally did not exceed in height at the ...
, and the rodent families Theridomyidae and Gliridae (
dormice A dormouse is a rodent of the family Gliridae (this family is also variously called Myoxidae or Muscardinidae by different taxonomists). Dormice are nocturnal animals found in Africa, Asia, and Europe. They are named for their long, dormant hibe ...
) crossed the faunal divide undiminished." It has been suggested that this was caused by
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
associated with the earliest polar glaciations and a major fall in sea levels, or by competition with taxa dispersing from Asia. However, few argue for an isolated single cause. Other possible causes are related to the impact of one or more large bolides in northern hemisphere at Popigai, Toms Canyon and
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...
. Improved correlation of
northwest Europe Northwestern Europe, or Northwest Europe, is a loosely defined subregion of Europe, overlapping Northern and Western Europe. The region can be defined both geographically and ethnographically. Geographic definitions Geographically, Northw ...
an successions to global events confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350
millennia A millennium (plural millennia or millenniums) is a period of one thousand years, sometimes called a kiloannum (ka), or kiloyear (ky). Normally, the word is used specifically for periods of a thousand years that begin at the starting point (ini ...
prior to the first record of post-Grande Coupure Asian immigrant taxa. An element of the paradigm of the Grande Coupure was the apparent
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
of all European primates at the Coupure. However, the 1999 discovery of a
mouse A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
-sized early Oligocene
omomyid Omomyidae is a group of early primates that radiated during the Eocene epoch between about (mya). Fossil omomyids are found in North America, Europe & Asia making it one of two groups of Eocene primates with a geographic distribution spanning h ...
, reflecting the better survival chances of small mammals, undercut the Grand Coupure
paradigm In science and philosophy, a paradigm () is a distinct set of concepts or thought patterns, including theories, research methods, postulates, and standards for what constitute legitimate contributions to a field. Etymology ''Paradigm'' comes f ...
. Evidence in the world's ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene–Oligocene boundary at 33.9 Ma. The remarkable cooling period in the ocean is correlated with pronounced mammalian faunal replacement within continental Asia as well. The Asian biotic reorganization events are comparable to the Grande Coupure in Europe and the Mongolian Remodeling of mammalian communities. The global cooling is also correlated with marked drying conditions in low-latitudes Asia.


References


External links


''Extinction'' an essay by Richard Cowen
{{DEFAULTSORT:Eocene-Oligocene Extinction Event Extinction events