Oligocene
Encyclopedia
The Oligocene is a geologic epoch
of the Paleogene
Period and extends from about 34 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly uncertain. The name Oligocene comes from the Greek (oligos, few) and (kainos, new), and refers to the sparsity of additional modern mammal
ian faunas after a burst of evolution
during the Eocene
. The Oligocene follows the Eocene Epoch and is followed by the Miocene
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-looking ecosystem
s of the Miocene." Major changes during the Oligocene included a global expansion of grassland
s, and a regression of tropical broad leaf forest
s to the equatorial belt.
The start of the Oligocene is marked by a major extinction event
, a faunal replacement of Europe
an with Asia
n fauna
except for the endemic rodent
and marsupial
families called the Grande Coupure. The Oligocene-Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.
s from youngest to oldest are:
The Paleogene Period general temperature decline is interrupted by an Oligocene 7M-year stepwise climate change. A deeper 8.2oC 0.4M-year temperature depression leads the 2oC 7M-year stepwise climate change 33.5Ma. The stepwise climate spanned 7M-years 25.5Ma through 32.5Ma as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55m decrease in sea level (35.7-33.5Ma) with a closely related (25.5-32.5Ma) temperature depression. The 7M-year depression abruptly terminated within 1-2M-year of the La Garita Caldera volcanism event 28-26 Ma. A deep 400 k-year glaciating Oligocene Miocene boundary event is recorded at McMurdo Sound
and King George Island
.
toward their present positions. Antarctica continued to become more isolated and finally developed a permanent ice cap
.(Haines)
Mountain building
in western North America
continued, and the Alps
started to rise in Europe
as the African plate
continued to push north into the Eurasian plate
, isolating the remnants of the Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. Oligocene marine exposures are rare in North America. There appears to have been a land bridge in the early Oligocene between North America
and Europe
since the faunas of the two regions are very similar. During sometime in the Oligocene, South America
was finally detached from Antarctica and drifted north towards North America
. It also allowed the Antarctic Circumpolar Current
to flow, rapidly cooling the continent.
es expanded from the water-bank habitat in the Eocene and moved out into open tracts; however even at the end of the period it was not quite common enough for modern savanna
.(Haines)
In North America, subtropical species dominated with cashew
s and lychee
trees were present, and temperate trees such as rose
s, beech
and pine
were common. The legumes of the pea and bean family spread, and sedge
s, bulrush
es and fern
s continued their ascent.
. Marine faunas became fairly modern, as did terrestrial vertebrate
faunas in the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as horse
s, entelodont
s, rhinoceros
es, oreodont
s, and camel
s, became more cursorial during this time, adapting to the plains that were spreading as the Eocene rainforests receded.
South America
was isolated from the other continents and evolved a quite distinct fauna during the Oligocene, home to strange animals such as pyrothere
s and astrapothere
s, as well as litopterns and notoungulates. Sebecosuchia
n crocodiles, terror birds, and carnivorous marsupials like the borhyaenids remained the dominant predators. 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, also went extinct in the Oligocene. The Oligocene was home to a wide variety of strange mammals. A good example of this would be in the White River Badlands of the United States, which were formerly a semi-arid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium
, camels (such as Poebrotherium
), running rhino
s, three-toed horses (such as Mesohippus
), nimravids, protoceratid
s, and early dogs like Hesperocyon
. Oreodont
s, an endemic American group, were very diverse during this time. In Asia during the Oligocene, a group of running rhinos gave rise to the indricotheres, like Indricotherium, which were the largest land mammals ever to walk the Earth.
The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. 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 and toothed cetaceans (whales) 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 carcharhinid shark
s, which also appeared in this epoch. Early desmostylia
ns, like Behemotops
, are known from the Oligocene. Pinnipeds probably appeared near the end of the epoch from a bear
-like or otter
-like ancestor.
Stemming from the DOP timing issue is dispute over the extent of the DPO’s influence on 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 δO18 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 ka, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.
The latest-hypothesized time for the Drake Passage Opening (DPO) 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 ACC. If the DPO occurred as late as hypothesized, then the ACC could not have had much of an effect on early Oligocene cooling, as it wouldn't have existed.
The earliest-hypothesized time for the DPO is around 30 Ma. One of the possible issues with this timing was the continental debris, as it were, 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 Ma. The aforementioned study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would facilitate an earlier onset of the ACC, but the rapidity of the
Currently, an early Oligocene DPO is favored.
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
of the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
Period and extends from about 34 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly uncertain. The name Oligocene comes from the Greek (oligos, few) and (kainos, new), and refers to the sparsity of additional modern mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian faunas after a burst of evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. The Oligocene follows the Eocene Epoch and is followed by the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
Epoch. The Oligocene is the third and final epoch of the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
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-looking ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s of the Miocene." Major changes during the Oligocene included a global expansion of grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
s, and a regression of tropical broad leaf forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
s to the equatorial belt.
The start of the Oligocene is marked by a major extinction event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
, a faunal replacement of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an with Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
n fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
except for the endemic rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
and marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
families called the Grande Coupure. The Oligocene-Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.
Subdivisions
Oligocene faunal stageFaunal 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 convention have the same name, and the same boundaries.Rock...
s from youngest to oldest are:
| Chattian Chattian The Chattian is, in the geologic timescale, the youngest of two ages or upper of two stages of the Oligocene epoch/series. It spans the time between and . The Chattian is preceded by the Rupelian and is followed by the Aquitanian .-Stratigraphic definition:The Chattian was introduced by Austrian... or Late Oligocene |
( – ) |
| Rupelian Rupelian The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene epoch/series. It spans the time between and . It is preceded by the Priabonian stage and is followed by the Chattian stage.... or Early Oligocene |
( – ) |
Climate
McMurdo Sound
The ice-clogged waters of Antarctica's McMurdo Sound extend about 55 km long and wide. The sound opens into the Ross Sea to the north. The Royal Society Range rises from sea level to 13,205 feet on the western shoreline. The nearby McMurdo Ice Shelf scribes McMurdo Sound's southern boundary...
and King George Island
King George Island
King George Island is the largest of the South Shetland Islands, situated at , off the coast of Antarctica in the Southern Ocean. The Island was named after King George III...
.
Paleogeography
During this period, the continents continued to driftContinental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
toward their present positions. Antarctica continued to become more isolated and finally developed a permanent ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....
.(Haines)
Mountain building
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
in western North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
continued, and the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
started to rise in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
as the African plate
African Plate
The African Plate is a tectonic plate which includes the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges.-Boundaries:...
continued to push north into the Eurasian plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...
, isolating the remnants of the Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. Oligocene marine exposures are rare in North America. There appears to have been a land bridge in the early Oligocene between North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
since the faunas of the two regions are very similar. During sometime in the Oligocene, South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
was finally detached from Antarctica and drifted north towards North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. It also allowed the Antarctic Circumpolar Current
Antarctic Circumpolar Current
The Antarctic Circumpolar Current is an ocean current that flows from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current...
to flow, rapidly cooling the continent.
Flora
Angiosperms continued their expansion throughout the world; tropical and sub-tropical forests were replaced by temperate deciduous woodlands. Open plains and deserts became more common. GrassGrass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
es expanded from the water-bank habitat in the Eocene and moved out into open tracts; however even at the end of the period it was not quite common enough for modern savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
.(Haines)
In North America, subtropical species dominated with cashew
Cashew
The cashew is a tree in the family Anacardiaceae. Its English name derives from the Portuguese name for the fruit of the cashew tree, caju, which in turn derives from the indigenous Tupi name, acajú. It is now widely grown in tropical climates for its cashew nuts and cashew apples.-Etymology:The...
s and lychee
Lychee
The lychee is the sole member of the genus Litchi in the soapberry family, Sapindaceae. It is a tropical and subtropical fruit tree native to Southern China and Southeast Asia, and now cultivated in many parts of the world...
trees were present, and temperate trees such as rose
Rose
A rose is a woody perennial of the genus Rosa, within the family Rosaceae. There are over 100 species. They form a group of erect shrubs, and climbing or trailing plants, with stems that are often armed with sharp prickles. Flowers are large and showy, in colours ranging from white through yellows...
s, beech
Beech
Beech is a genus of ten species of deciduous trees in the family Fagaceae, native to temperate Europe, Asia and North America.-Habit:...
and pine
Pine
Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...
were common. The legumes of the pea and bean family spread, and sedge
Cyperaceae
Cyperaceae are a family of monocotyledonous graminoid flowering plants known as sedges, which superficially resemble grasses or rushes. The family is large, with some 5,500 species described in about 109 genera. These species are widely distributed, with the centers of diversity for the group...
s, bulrush
Typha
Typha is a genus of about eleven species of monocotyledonous flowering plants in the family Typhaceae. The genus has a largely Northern Hemisphere distribution, but is essentially cosmopolitan, being found in a variety of wetland habitats...
es and fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s continued their ascent.
Fauna
Important Oligocene land faunas are found on all continents at this time. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the PaleogenePaleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
. Marine faunas became fairly modern, as did terrestrial vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
faunas in the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, entelodont
Entelodont
Entelodonts, sometimes nicknamed hell pigs or terminator pigs, is an extinct family of pig-like omnivores endemic to forests and plains of North America, Europe, and Asia from the middle Eocene to early Miocene epochs , existing for approximately .-Taxonomy:Entelodontidae was named by Richard...
s, rhinoceros
Rhinoceros
Rhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia....
es, oreodont
Oreodont
Oreodons, sometimes called prehistoric "ruminating hogs," were a family of cud-chewing plant-eater with a short face and tusk-like canine teeth...
s, and camel
Camel
A camel is an even-toed ungulate within the genus Camelus, bearing distinctive fatty deposits known as humps on its back. There are two species of camels: the dromedary or Arabian camel has a single hump, and the bactrian has two humps. Dromedaries are native to the dry desert areas of West Asia,...
s, became more cursorial during this time, adapting to the plains that were spreading as the Eocene rainforests receded.
South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
was isolated from the other continents and evolved a quite distinct fauna during the Oligocene, home to strange animals such as pyrothere
Pyrotheria
Pyrotheria is an order of extinct meridiungulate mammals. These mastodon-like ungulates include the genera Baguatherium, Carolozittelia, Colombitherium, Gryphodon, Propyrotherium, Proticia, and Pyrotherium....
s and astrapothere
Astrapotheria
Astrapotheria is an extinct order of South American hoofed animals. The history of this order is enigmatic, but it may taxonomically belong to Meridiungulata . In turn, Meridungulata is believed to belong to the extant superorder Laurasiatheria...
s, as well as litopterns and notoungulates. Sebecosuchia
Sebecosuchia
Sebecosuchia is an extinct group of mesoeucrocodylian crocodyliforms that includes the families Sebecidae and Baurusuchidae. The group first appeared in the Late Cretaceous with the baurusuchids and went extinct in the Miocene with the last sebecids. Fossils have been found primarily from South...
n crocodiles, terror birds, and carnivorous marsupials like the borhyaenids remained the dominant predators. 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, also went extinct in the Oligocene. The Oligocene was home to a wide variety of strange mammals. A good example of this would be in the White River Badlands of the United States, which were formerly a semi-arid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium
Archaeotherium
Archaeotherium is an extinct artiodactyl genus of the family Entelodontidae, endemic to North America during the Oligocene epoch , existing for approximately . Archaeotherium was about 1.2m tall at the shoulder and around 2m long and weighing around 270kg.It was a relative of javelinas and pigs...
, camels (such as Poebrotherium
Poebrotherium
Poebrotherium is an extinct genus of terrestrial herbivore the family Camelidae, endemic to North America from the Eocene through Oligocene 38—30.8 mya, existing for approximately .-Discovery and history:...
), running rhino
Rhinoceros
Rhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia....
s, three-toed horses (such as Mesohippus
Mesohippus
Mesohippus is an extinct genus of early horse. It lived some 40 to 30 million years ago from the late Eocene to the mid-Oligocene...
), nimravids, protoceratid
Protoceratidae
Protoceratidae is an extinct family of herbivorous North American artiodactyls that lived during the Eocene through Pliocene at around 46.2—4.9 Ma., existing for approximately .-Taxonomy:...
s, and early dogs like Hesperocyon
Hesperocyon
Hesperocyon is an extinct genus of canids, family of Canidae and subfamily Hesperocyoninae which was endemic to North America from southern Canada to appearing during the Uintan age-Bridgerian age of the Mid-Eocene 42.5 mya—31.0 Ma. . Hesperocyon existed for approximately .-Taxonomy:Hesperocyon...
. Oreodont
Oreodont
Oreodons, sometimes called prehistoric "ruminating hogs," were a family of cud-chewing plant-eater with a short face and tusk-like canine teeth...
s, an endemic American group, were very diverse during this time. In Asia during the Oligocene, a group of running rhinos gave rise to the indricotheres, like Indricotherium, which were the largest land mammals ever to walk the Earth.
The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. 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 and toothed cetaceans (whales) just appeared, and their ancestors, the archaeocete
Archaeoceti
Archaeocetes, or "ancient whales", are a paraphyletic group of cetaceans that gave rise to the modern cetaceans.The archaeocetes were once thought to have evolved from the mesonychids, based on dental characteristics...
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 carcharhinid shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, which also appeared in this epoch. Early desmostylia
Desmostylia
Desmostylia is an extinct order of marine mammals which existed from the Arikareean age of the late Oligocene epoch to the Tortonian age of the late Miocene epoch . Their dental and skeletal form suggests desmostylians were amphibious herbivores dependent on littoral habitats...
ns, like Behemotops
Behemotops
Behemotops is an extinct genus of herbivorous marine mammal of the family Desmostylidae living from the Rupelian stage of the Early Oligocene subepoch through the Late Oligocene subepoch and in existence for approximately ....
, are known from the Oligocene. Pinnipeds probably appeared near the end of the epoch from a bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...
-like or otter
Otter
The Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....
-like ancestor.
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 (or lack thereof) is still a matter of some dispute.Effects of Oceanic Gateways on Circulation
The opening and closing of ocean gateways (the Drake Passage, the Tasmanian Gateway, the Tethys seaway, and the Greenland-Iceland-Faroes sill) played a vital part in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation.Drake Passage
The Drake Passage is located between South America and Antarctica. Once the Tasmanian Gateway (between Australia and Antarctica) opened, all that kept the Southern Ocean from being completely isolated was the Drake Passage. The opening of the Drake Passage enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept cold, Antarctic waters circulating about the 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 (Oi-1) 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 formation of a preliminary ACC.Stemming from the DOP timing issue is dispute over the extent of the DPO’s influence on 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 δO18 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 ka, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.
Late Oligocene Drake Passage Opening
The latest-hypothesized time for the Drake Passage Opening (DPO) 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 ACC. If the DPO occurred as late as hypothesized, then the ACC could not have had much of an effect on early Oligocene cooling, as it wouldn't have existed.
Early-Oligocene Drake Passage Opening
The earliest-hypothesized time for the DPO is around 30 Ma. One of the possible issues with this timing was the continental debris, as it were, 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 Ma. The aforementioned study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would facilitate an earlier onset of the ACC, but the rapidity of the
Currently, an early Oligocene DPO is favored.
Tasman Gateway Opening
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 ACC strengthened.Tethys Seaway Closing
Though the Tethys was not a gateway, but rather a sea in its own right, its closing during the Oligocene had significant impacts on both ocean circulation and climate. The collision of the African plate with the European plate and of the Indian subcontinent with the Asian plate all conspired to cut off the Tethys seaway that had provided a zonal low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more CO2 from the atmosphere, contributing to global cooling.Greenland-Iceland-Faroes
The gradual separation of the clump of continental crust and the deepening of tectonic sill 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. Extinction patterns and species migration can also be studied to gain insight into ocean conditions. For a while, it was thought that the Oi-1 event 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 and the Southern Ocean. As the Greenland-Iceland-Faroe (GIR) sill deepened, connecting the Norwegian-Greenland sea with the Atlantic Ocean, North Atlantic Deep Water began to come into play as well. Model runs suggest that once this occurred, a more modern-looking thermohaline circulation initialized.North Atlantic Deep Water
Evidence for the early Oligocene onset of North Atlantic Deep Water (NADW) lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe driftsSouth Ocean Deep Water
South Ocean Deep Water (SODW) began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. Regardless of the time at which the DOP occurred, the effect of cooling the Southern Ocean and leading to increased deep-water formation would have been the same.Impact events
Recorded extraterrestrial impacts:- Nunavut, Canada (23 Ma, crater 24 km (14.9 mi) diameter,)