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Paleontology in South Dakota
Paleontology in South Dakota refers to paleontological research occurring within or conducted by people from the U.S. state of South Dakota. South Dakota is an excellent source of fossils as finds have been widespread throughout the state.〔 During the early Paleozoic era South Dakota was submerged by a shallow sea that would come to be home to creatures like brachiopods, cephalopods, corals, and ostracoderms. Local sea levels rose and fall during the Carboniferous and the sea left completely during the Permian. During the Triassic, the state became a coastal plain, but by the Jurassic it was under a sea where ammonites lived. Cretaceous South Dakota was also covered by a sea that was home to mosasaurs. The sea remained in place after the start of the Cenozoic before giving way to a terrestrial mammal fauna including the camel ''Poebrotherium'', three-toed horses, rhinoceroses, saber teeth, and titanotheres. During the Ice Age glaciers entered the state, which was home to mammoths and mastodons. Local Native Americans interpreted fossils as the remains of the water monster Unktehi and used bits of ''Baculites'' shells in magic rituals to summon buffalo herds. Local fossils came to the attention of formally trained scientists with the Lewis and Clark expedition. The Cretaceous horned dinosaur ''Triceratops horridus'' is the South Dakota state fossil.
==Prehistory==
No Precambrian fossils are known from South Dakota, so the state's fossil record does not begin until the Paleozoic. 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 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 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 moist coastal plain at that time. Seawater once more covered South Dakota during the Jurassic. This sea was home to creatures like ammonites, clams, crinoids, and starfish. As 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 was called the Western Interior Seaway.〔 This sea was home to many invertebrates, aquatic birds, and marine reptiles.〔 The Cretaceous life of South Dakota was similar to that of Wisconsin.〔 Some of South Dakota's ammonites were very unusual for 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 was 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 was 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 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 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 the three-toed horses, pig-like animals, the camel ''Poebrotherium'', ''Protoceras'', rhinoceroses, rodents, saber teeth, 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 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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