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97-524: No Precambrian fossils are known from South Dakota, so the state's fossil record does not begin until the Paleozoic Era . At the start of the Paleozoic, South Dakota was submerged by a sea. The state's Cambrian life left behind a rich trace fossil record. Paleozoic marine life of South Dakota included brachiopods , cephalopods , and corals . The sea temporarily withdrew from South Dakota during

194-708: A supereon , but this is also an informal term, not defined by the ICS in its chronostratigraphic guide. Eozoic (from eo- "earliest") was a synonym for pre-Cambrian , or more specifically Archean . A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia . Probable fossils 100 million years older have been found in

291-667: 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. The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving Nothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions. Angiosperms continued their expansion throughout

388-681: A broader trend of global cooling lasting from the Bartonian 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 5 to 6 °C (9 to 10 °F) of global cooling . The transition likely took place in three closely spaced steps over

485-494: A critical threshold value. Brachiopod oxygen isotope ratios from New Zealand suggest that a proto-Subtropical Convergence developed during the Early Oligocene, with northern New Zealand being subtropical and southern and eastern New Zealand being cooled by cold, subantarctic water. Oligocene climate following the Eocene-Oligocene event is poorly known. There were several pulses of glaciation in middle Oligocene, about

582-592: A major mountain chain in the Oligocene, as subduction became more direct into the coastline. 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. The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere, being part of

679-735: A possible 2450 Ma red alga from the Kola Peninsula , 1650 Ma carbonaceous biosignatures in north China, the 1600 Ma Rafatazmia , and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic. The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period. A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota . Hard-shelled creatures appeared toward

776-460: A semiarid prairie home and included entelodonts like Archaeotherium , camelids (such as Poebrotherium ), running rhinoceratoids , three-toed equids (such as Mesohippus ), nimravids , protoceratids , and early canids like Hesperocyon . 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

873-642: Is discernible in pollen counts from the Tibetan Plateau, which also show that the South Asian Monsoon had already developed by the late Oligocene. Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau. A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound and King George Island . The early Eocene climate

970-612: Is known to occur during the RNA replication of extant coronaviruses . Evidence of the details of plate motions and other tectonic activity in the Precambrian is difficult to interpret. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia , broke up around 750 Ma. A number of glacial periods have been identified going as far back as

1067-419: Is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene. The lower boundary of the Oligocene (its Global Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of the foraminiferan genus Hantkenina in a quarry at Massignano , Italy . However, this GSSP has been criticized as excluding

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1164-668: Is the earliest part of Earth's history , set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian , the first period of the Phanerozoic Eon , which is named after Cambria , the Latinized name for Wales , where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons ( Hadean , Archean , Proterozoic ) of

1261-521: Is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet called Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis ). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma. The term "Precambrian"

1358-486: Is thought to have formed about 1300-900 Ma, to have included most or all of Earth's continents and to have broken up into eight continents around 750–600 million years ago. Oligocene The Oligocene ( IPA : / ˈ ɒ l ɪ ɡ ə s iː n , - ɡ oʊ -/ OL -ə-gə-seen, -⁠goh- ) is a geologic epoch of the Paleogene Period that extends from about 33.9 million to 23 million years before

1455-731: Is used by geologists and paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey and the International Commission on Stratigraphy regard the term as informal. Because the span of time falling under the Precambrian consists of three eons (the Hadean , the Archean , and the Proterozoic ), it is sometimes described as

1552-535: The Blackfeet and Cheyenne have a tradition of using Baculites fossils to summon bison herds. When used this way, the fossils are called "buffalo-calling stones" or Iniskim. This practice derives from the complex shapes of the fossil's internal structure, which can sometimes bear shapes resembling buffalo. Iniskim have been discovered in South Dakota archeological sites. Archaeological evidence exists for

1649-621: The Cliff Dweller and Navajo peoples of the southwest, which may explain how the fossil ended up so far from its place of origin. On September 10, 1804, four members of the Lewis and Clark Expedition recorded in their journals a fossil discovery along the banks of the Missouri River in what is now Gregory County in south-central South Dakota. The find was a 45-foot-long (14 m) articulated vertebral column, ribs, and teeth at

1746-534: The Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a " Snowball Earth ". The atmosphere of the early Earth is not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and

1843-555: The Ordovician period. But, in the middle or late Ordovician , ostracoderms swam over South Dakota. Similar ostracoderms were preserved near Canon City , Colorado . Later, during the Carboniferous period, sea levels again began to rise and fall. Marine life from this time included brachiopods and corals, but the rock record preserves evidence for local brackish and freshwater environments as well. The sea withdrew from

1940-426: The archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the requiem sharks , which also appeared in this epoch. Early desmostylians , like Behemotops , are known from the Oligocene. Pinnipeds appeared near

2037-666: The creodonts that had dominated the Paleocene in the Old World. Rodents and rabbits underwent tremendous diversification due to the increase in suitable habitats for ground-dwelling seed eaters, as habitats for squirrel-like nut- and fruit-eaters diminished. The primates, once present in Eurasia, were reduced in range to Africa and South America. Many groups, such as equids , entelodonts , rhinos , merycoidodonts , and camelids , became more able to run during this time, adapting to

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2134-522: The geologic time scale . It spans from the formation of Earth about 4.6 billion years ago ( Ga ) to the beginning of the Cambrian Period, about 538.8 million years ago ( Ma ), when hard-shelled creatures first appeared in abundance. Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history , and what is known has largely been discovered from

2231-482: The oxygen catastrophe . At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering

2328-401: The 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic , and fossils from the Precambrian (e.g. stromatolites ) are of limited biostratigraphic use. This is because many Precambrian rocks have been heavily metamorphosed , obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata. It

2425-601: The Antarctic Circumpolar Current. There is some evidence that it occurred much earlier, during the early Eocene. 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

2522-717: The Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute. The opening and closing of ocean gateways: the opening of the Drake Passage ; the opening of the Tasmanian Gateway and the closing of the Tethys seaway; along with the final formation of the Greenland – Iceland – Faroes Ridge; played vital parts in reshaping oceanic currents during

2619-536: The Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is now Vietnam was very similar to its present flora. Kelps make their first appearance in the fossil record during the earliest Oligocene. Most extant mammal families had appeared by the end of the Oligocene. These included primitive three-toed horses, rhinoceroses, camels, deer, and peccaries. Carnivores such as dogs , nimravids , bears, weasels, and raccoons began to replace

2716-402: 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

2813-524: 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

2910-973: The East African large igneous province , which also initiated rifting along the Red Sea and Gulf of Aden . The Alps were rapidly rising in Europe as the African Plate continued to push north into the Eurasian Plate , isolating the remnants of the Tethys Sea . 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

3007-563: 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

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3104-677: 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 and ruminants , 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

3201-504: 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 between South America and Antarctica is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene), but oceanic circulation through

3298-745: The Murray and Mendocino Fracture Zones at the North American subduction zone in the Oligocene. This initiated strike-slip movement along the San Andreas Fault and extensional tectonics in the Basin and Range province , 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

3395-506: The New World and Old World monkeys. The South American continent was home to animals such as pyrotheres and astrapotheres , as well as litopterns and notoungulates . Sebecosuchians , terror birds , and carnivorous metatheres , like the borhyaenids remained the dominant predators. Africa was also relatively isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms. Egypt in

3492-735: The Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about 5 °C (9.0 °F) 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 7 to 11 °C (13 to 20 °F) in

3589-611: 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 and Kerguelen Plateau , in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene. The causes of

3686-522: The Oligocene White River Formation. Among the creatures discovered were rhinoceroses, saber-toothed cats, giant pig-like animals, Protoceros , tapirs, horses and more. In 1990, Sue Hendrickson discovered a new specimen of Tyrannosaurus rex that would later be nicknamed in her honor. The specimen made headlines when a dispute over ownership rights raged for more than five years. "Sue" was determined to rightfully belong to

3783-720: The Oligocene camel Poebrotherium , which was discovered in the same general region as Prout's titanothere jaw. The government responded to these discoveries by dispatching an expedition into the area. In 1850 the Smithsonian sent its own collectors into the area. The Tertiary deposits of the White River Badlands was active for decades and still ongoing in 1920 when the South Dakota School of Mines published its "Bulletin No. 13". This publication summarized

3880-401: 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. Isotopic evidence suggests that during the early Oligocene,

3977-532: The Oligocene included a global expansion of grasslands , and a regression of tropical broad leaf forests to the equatorial belt . The start of the Oligocene is marked by a notable extinction event called the Grande Coupure ; it featured the replacement of European fauna with Asian fauna , except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary

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4074-518: The Oligocene was an environment of lush forested deltas. Nevertheless, the Early Oligocene saw a major reduction in the diversity of many Afro-Arabian mammal clades, including hyaenodonts, primates, and hystricognath and anomaluroid rodents. During the Oligocene, the Tethyan marine biodiversity hotspot collapsed as the Tethys Ocean contracted. The seas around Southeast Asia and Australia became

4171-529: The Oligocene, creating a persistent land connection between the continents. The Paratethys Sea stretched from what is now the Balkan Peninsula across Central Asia to the Tian Shan region of what is now Xinjiang . 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

4268-497: 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 may have reached nearly its present elevation by the late Oligocene. The Andes first became

4365-529: The Oligocene. The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into the Indo-Pacific. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen whales and toothed whales had just appeared, and their ancestors,

4462-601: The Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation. The Drake Passage is located between South America and Antarctica . Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the Southern Ocean was its connection to South America. As the South American continent moved north,

4559-425: The atmosphere, contributing to global cooling. The gradual separation of the clump of continental crust and the deepening of the tectonic ridge in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area. More information about the evolution of North Atlantic Deep Water will be given a few sections down. Evidence for ocean-wide cooling during

4656-406: The buffalo-calling stone tradition that is at least 1,000 years old. One interesting South Dakota fossil was found not far from the Gobernador ruins in New Mexico during the 1980s. It was the jawbone of an Oligocene mammal endemic to South Dakota. This means someone would have had to transport the bone for 800 miles from the place it was discovered. The Blackfeet engaged in regular trade with

4753-469: The camel Poebrotherium , Protoceras , rhinoceroses , rodents , saber-toothed cats , tapirs , and titanotheres . Contemporary birds also left behind bones and even an egg. These are significant because bird fossils are very rare. Many streams carried even more sediment into the region from the young Rocky Mountains and Black Hills. At the time, South Dakota consisted of plains dotted with marshes and shallow lakes and split by wide streams. Some of

4850-413: The coast, reaching from central Mexico through the Mogollon-Datil volcanic field to the San Juan volcanic field , then through Utah and Nevada to the ancestral Northern Cascades. Huge ash deposits from these volcanoes created the White River and Arikaree Groups of the High Plains, with their excellent fossil beds. Between 31 and 26 mya, the Ethiopia-Yemen Continental Flood Basalts were emplaced by

4947-401: The current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs . The Precambrian could be divided into five "natural" eons, characterized as follows: The movement of Earth's plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent containing most or all of

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5044-446: 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. 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

5141-417: The earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene paleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this. 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

5238-428: 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

5335-496: 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 levels ( pCO2 ) 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 place only when pCO2 dropped below

5432-433: The early part of the Cenozoic , central and eastern South Dakota was still covered by the sea. The uplift responsible for the Black Hills continued to elevate their topography. As the Cenozoic continued, the sea shrank away from the state. In its place, grasslands formed and were roamed by herds of grazing mammals. Later, during the Oligocene , at least part of South Dakota was covered in seawater. The White River Formation

5529-453: The early years of the Earth's existence, as radiometric dating has allowed absolute dates to be assigned to specific formations and features. The Precambrian is divided into three eons: the Hadean (4567.3–4031 Ma), Archean (4031-2500 Ma) and Proterozoic (2500-538.8 Ma). See Timetable of the Precambrian . It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than

5626-578: The end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the Burgess Shale , including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas. Tracks from an animal with leg-like appendages have been found in what

5723-432: The end of the epoch from an otter -like ancestor. 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

5820-468: The foraminiferan Paragloborotalia kugleri and with the base of magnetic polarity chronozone C6Cn.2n. Oligocene faunal stages from youngest to oldest are: During the Oligocene Epoch, the continents continued to drift toward their present positions. Antarctica became more isolated as deep ocean channels were established between Antarctica and Australia and South America . Australia had been very slowly rifting away from West Antarctica since

5917-414: The fossils were so precariously located that the excavators had to use block and tackle to lower the fossils down from the tops of "slender pinnacles". The fossils were preserved in channel sandstones that had received little scientific attention prior to the expedition. In June 1947, South Dakota School of Mines sent another expedition into the Badlands. They uncovered a wide variety of fossils preserved in

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6014-536: The gateway widened, the Antarctic Circumpolar Current strengthened. The Tethys Seaway was not a gateway, but rather a sea in its own right. Its closing during the Oligocene had significant impact on both ocean circulation and climate. The collisions of the African plate with the European plate and of the Indian subcontinent with the Asian plate, cut off the Tethys Seaway that had provided a low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from

6111-479: The giant indricotheres , that grew up to 6 meters (20 ft) at the shoulder and weighed up to 20 tons. Paraceratherium 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 , Proailurus , originated in Asia during

6208-547: The group. During the Late Cretaceous , the region now occupied by the Black Hills of South Dakota may have attracted long-necked plesiosaurs from hundreds of miles away as a source of gastroliths . Short-necked plesiosaurs like Dolichorhynchops also lived in the Western Interior Seaway of South Dakota during the Campanian . They were fast swimmers who fed on contemporary small fish and cephalopods. Most short-necked plesiosaurs were relatively small, with body lengths of less than ten feet. However, one South Dakotan individual

6305-541: The landmass. The earliest known supercontinent was Vaalbara . It formed from proto-continents and was a supercontinent 3.636 billion years ago. Vaalbara broke up c. 2.845–2.803 Ga ago. The supercontinent Kenorland was formed c. 2.72 Ga ago and then broke sometime after 2.45–2.1 Ga into the proto-continent cratons called Laurentia , Baltica , Yilgarn craton and Kalahari . The supercontinent Columbia , or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago. The supercontinent Rodinia

6402-456: 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 of central North America inhabited

6499-400: The legends of local people. The Sioux believed that in the first creatures in creation were the insects and reptiles , who were ruled by the water monster Unhcegila . Reptiles were very diverse and came in all shapes and sizes, but they became violent and bloodthirsty until they were petrified by lightning sent by the thunderbirds . The physical bodies of the Thunder Beings killed by

6596-440: The lightning, including Unhcegila, also ended up being buried. The Sioux believe that earth has a history of four distinct ages. These events occurred during the Age of Rock. This portrayal of the thunderbirds may have been influenced by associations of fossils of the Cretaceous pterosaur Pteranodon with marine reptiles of the same age in the western United States. Local people also employed fossils in ritual. Plains Indians like

6693-478: The local Oligocene wildlife left behind footprints that would later fossilize. The Brule Formation preserves one of only seven Oligocene fossil tracksites in the western United States. Volcanic activity sporadically showered the state with ash. During the Ice Age , glaciers scoured the state. As they melted, they deposited sediments that would preserve the fossil remains of creatures like bison , horses, mammoths , and mastodons . Fossils feature in some of

6790-467: The main source of deep water was the North Pacific and the Southern Ocean . As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of the North Atlantic began to come into play as well. Computer models suggest that once this occurred, a more modern in appearance thermo-haline circulation started. Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in

6887-419: The mammal discoveries were the remains of rhinoceroses , tapirs , three-toed horses , pig-like animals, and rodents . The team also uncovered some bird fossils, which are very rare. One of these was a fossil egg , which author Marian Murray has called "[t]he best find" of the entire expedition. Only a few plant specimens were discovered, but these included fossil hackberry seeds and petrified wood . Some of

6984-508: The new dominant hotspot of marine biodiversity. 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

7081-576: 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

7178-468: The opening of the Drake Passage is during the early Miocene. Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true Antarctic Circumpolar Current. If the opening occurred as late as hypothesized, then the Antarctic Circumpolar Current could not have had much of an effect on early Oligocene cooling, as it would not have existed. The earliest hypothesized time for

7275-772: The owner of the property it was found on. The rancher put the specimen up for auction and was purchased by the Chicago Field Museum for $ 8.36 million. In 1996, Bell and others reported the discovery of a mosasaur of the genus Plioplatecarpus in South Dakota's Pierre Shale. The specimen was important because it preserved several juvenile skeletons inside its pelvic region. Similar fossils have been put forward as evidence of live birth in other types of marine reptiles . Precambrian The Precambrian ( / p r i ˈ k æ m b r i . ə n , - ˈ k eɪ m -/ pree- KAM -bree-ən, -⁠KAYM- ; or Pre-Cambrian , sometimes abbreviated pC , or Cryptozoic )

7372-517: The period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by 105 meters (344 ft), 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

7469-568: The period, grass was not quite common enough for modern savannas . In North America, much of the dense forest was replaced by patchy scrubland with riparian forests. Subtropical species dominated with cashews and lychee trees present, and temperate woody plants such as roses , beeches , and pines were common. The legumes spread, while sedges and ferns continued their ascent. In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity. The Ha Long megafossil flora from

7566-642: The plains that were spreading as the Eocene rainforests receded. Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates , an ancient lineage of primitive mammals that originated back in the Jurassic , also became extinct in the Oligocene, aside from the gondwanatheres . The Eocene-Oligocene transition in Europe and Asia has been characterized as

7663-594: The present ( 33.9 ± 0.1 to 23.03 ± 0.05  Ma ). 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 from his studies of marine beds in Belgium and Germany. The name comes from Ancient Greek ὀλίγος ( olígos ) 'few' and καινός ( kainós ) 'new', and refers to

7760-491: The results of all of the paleontological fieldwork done in the White River Badlands. In 1877, the United States Geological Survey published a report on the ancient plants and invertebrates of South Dakota. In 1895, George Wieland discovered YPM 3000, the nearly complete and articulated type specimen of the giant sea turtle now known as Archelon ischyros , which had been preserved in the Pierre Shale . The discovery

7857-526: The same area. However, there is evidence that life could have evolved over 4.280 billion years ago. There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian. Complex multicellular organisms may have appeared as early as 2100 Ma. However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers." Other possible early complex multicellular organisms include

7954-574: The sea retreated, South Dakota became a terrestrial environment dotted with lakes , streams , and swamps . The state was covered again by the sea during the Cretaceous period. This sea, the Western Interior Seaway , was home to many invertebrates, aquatic birds, and marine reptiles . The Cretaceous life of South Dakota was similar to that found in what is now Wisconsin . Some of South Dakota's ammonites were very unusual for

8051-642: The sparsity of extant forms of molluscs . The Oligocene is preceded by 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 ecosystems of the Miocene. Major changes during

8148-481: 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

8245-514: The state altogether during the Permian and local sediments began being eroded rather than deposited. During the Triassic period, sedimentation resumed. The geologic record reveals that South Dakota was a moist coastal plain at that time. Seawater once more covered South Dakota during the Jurassic period. This sea was home to creatures like ammonites , clams , crinoids , and starfish . As

8342-537: The time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about 75 meters (246 ft). 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 and other climate drivers. Other evidence indicates significant cooling at high latitudes. Part of

8439-522: The top of a high ridge. The men interpreted the remains as originating from a giant fish, but today scientists think the specimen was probably a mosasaur or plesiosaur . The expedition sent back some of the fossils, but these were later lost. In 1847, Hiram A. Prout published a description of a fragmentary titanothere jaw discovered in the White River Badlands in the American Journal of Science . Not long afterward, Joseph Leidy described

8536-399: 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 , Italy , which coincides with the first appearance of

8633-401: The world as tropical and sub- tropical forests were replaced by temperate deciduous forests . Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts. The decline in pCO2 favored C4 photosynthesis , which is found only in angiosperms and is particularly characteristic of grasses. However, even at the end of

8730-414: The δ O signature is too strong for glaciation to be the main trigger for cooling. Through study of Pacific Ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300,000 years, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling. The latest hypothesized time for

8827-558: Was 3.5 meters (11 feet) long. Archelon is the largest known turtle in history. Its size is comparable to that of a small car. Also during the Cretaceous, geologic uplift was forming the Black Hills in the western part of the state. Local dinosaurs included the armored Edmontonia , duck-billed Edmontosaurus , the ostrich dinosaur Ornithomimus , Pachycephalosaurus , Triceratops , and Tyrannosaurus . During

8924-556: Was 6–7 meters (20–23 ft) long. More shark species are known from the Cretaceous Western Interior Seaway deposits of South Dakota than other states, with rocks from the same environment like those of Kansas . Otherwise, these two states had similar shark communities. During the Late Campanian , South Dakota was home to the colossal sea turtle Archelon ischyros . The first specimen

9021-497: Was being deposited in the White River badlands as the sea gradually receded. The Oligocene flora left behind few fossils, but among them were hackberry seeds and petrified wood . Although plant fossils are scarce, these deposits preserve one of the best Tertiary mammal faunas in the world. More than 175 different kinds of animals were preserved from this time. The local mammals included three-toed horses , pig-like animals,

9118-482: Was lacking in free oxygen . There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism . This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis , sometimes called

9215-852: Was mud 551 million years ago. The RNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes. During the Hadean Eon (4,567–4,031 Ma) abundant geothermal microenvironments were present that may have had the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form. It was shown that porous rock systems comprising heated air-water interfaces could allow ribozyme - catalyzed RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes. This primitive RNA replicative system also may have been able to undergo template strand switching during replication ( genetic recombination ) as

9312-552: Was the likely instigator for Wieland's subsequent research into Late Cretaceous sea turtles that began the next year. In 1940, the South Dakota School of Mines and Technology collaborated with National Geographic on an expedition into the badlands. They uncovered tons of fossils from at least 175 different species of Oligocene life. The fossils were taken to the South Dakota School of Mines in Rapid City . Among

9409-596: Was very warm, with crocodilians and temperate plants thriving north of the Arctic Circle . The cooling trend that began in the middle Eocene continued into the Oligocene, bringing both poles well below freezing for the first time in the Phanerozoic . The cooling climate, together with the opening of some land bridges and the closing of others, led to a profound reorganization of the biosphere and loss of taxonomic diversity. Land animals and marine organisms reached

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