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Paleontology in Iowa refers to paleontological research occurring within or conducted by people from the U.S. state of Iowa. The paleozoic fossil record of Iowa spans from the Cambrian to Mississippian.〔 During the early Paleozoic Iowa was covered by a shallow sea that would later be home to creatures like brachiopods, bryozoans, cephalopods, corals, fishes, and trilobites. Later in the Paleozoic, this sea left the state, but a new one covered Iowa during the early Mesozoic. As this sea began to withdraw a new subtropical coastal plain environment which was home to duck-billed dinosaurs spread across the state. Later this plain was submerged by the rise of the Western Interior Seaway, where plesiosaurs lived. The early Cenozoic is missing from the local rock record, but during the Ice Age evidence indicates that glaciers entered the state, which was home to mammoths and mastodons. ==Prehistory== No Precambrian fossils are known from Iowa, so the fossil record does not begin until the Paleozoic. During the early Paleozoic, most of Iowa was submerged by an ancient sea. The bottom of this sea was home to creatures like brachiopods, bryozoans, cephalopods, corals, molluscs, and trilobites.〔 The life of Mississippian Iowa included blastoids, brachiopods, coral, crinoids, and starfish.〔 Blastoids from this time period left behind remains in what is now the southeastern part of the state.〔 Brachiopods from this time period left behind remains in what is now the southeastern Benton County and the southeastern part of the state.〔 The Benton County brachiopods included ''Atrypa'' and ''Spirifer''.〔 Corals of Mississippian Iowa included the colonial ''Hexagonaria'' of southeastern Benton County.〔 Crinoids left behind remains in what are now the Le Grand area and the southeastern part of the state.〔 Some of the Le Grand crinoids are rare in other places.〔 Some of the Le Grand starfishes are rare in other places.〔 Vertebrate life included fishes. Fish teeth of this time period fossilized in the southeastern part of the state.〔 As the Paleozoic drew to a close, Iowa's sea's dried up. For a significant length of time sediments were being eroded away from the state rather than deposited.〔 During the early Mesozoic, a new sea covered Iowa, and sediment deposition resumed. As this sea withdrew later in the Mesozoic Iowa became a subtropical coastal plain with rivers interspersed throughout.〔 Later, dinosaurs lived and left behind fossil remains in Iowa, although the record of their presence is scant. Further, dinosaur fossils have been found in neighboring states like Minnesota, Missouri, Nebraska, and South Dakota. There were no barriers that would have prevented the dinosaurs of this region from crossing what are now the state boundaries.〔 No Jurassic dinosaur fossils are known from Iowa, although the Iowan Fort Dodge Formation is the same age as the dinosaur-bearing deposits of the western states.〔 Cretaceous deposits cover a wider area of Iowa than those of Jurassic age.〔 Between 95 and 100 million years ago, areas of Iowa were covered by a complex of rivers flowing toward the west through floodplains and coastal lowlands. These deposited the sediments that are now called the Dakota Formation, the oldest Cretaceous rocks in the state. Global temperatures were elevated and Iowa was richly vegetated by a subtropical flora.〔 Unlike the Jurassic deposits, dinosaur fossils were preserved in these rocks.〔 At least one kind of primitive hadrosaur that left behind remains in Iowa was more than 30 feet long.〔 Gradually over time the rivers depositing the Dakota Formation sediments were submerged by the northward expansion of the Western Interior Seaway.〔〔 This body of water deposited shale and chalk.〔 Plesiosaurs lived in this sea and left behind fossils in several regions of Iowa.〔 Northwestern Iowa's quartzite rocks may have attracted long-necked elasmosaurs as a source of gastroliths.〔 Outside of Iowa the same deposits have preserved animals like mosasaurs and pterosaurs.〔 During the mid-Campanian, about 75 million years ago, an asteroid roughly 1.5 miles in diameter struck the earth from the southeast near the east coast of the Western Interior Seaway. The impact site is located near the modern site of a town called Manson on the western half of the state. This event would have had a catastrophic impact on life on both land and sea for hundreds of miles around the crater, which had a diameter of roughly 22 miles. It scattered rock debris all the way to South Dakota and triggered huge tsunamis. However, despite the devastation South Dakotan fossils revealed that life recovered from the Iowan impact until the Cretaceous-Paleogene extinction event, itself likely caused by an extraterrestrial impact down in the Yucatan Peninsula of Mexico.〔 During the early part of the Cenozoic era, sediments were being eroded away from Iowa rather than deposited.〔 Late in the Cenozoic glaciers intruded southward into Iowa. As they melted they left behind new sedimentary deposits. At this time Iowa was home to mammoths and mastodons, whose remains were preserved in a wide variety of locations in the state.〔 During the glaciations of the Ice Age over the past 2.5 million years the glaciers transported and deposited fossils eroded from Cretaceous sediments. Some of the Cretaceous fossils to be redeposited in this manner included plesiosaur bones, shark teeth, and two dinosaur bones. The dinosaur bones reworked by glacial activity are the best in the state.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Paleontology in Iowa」の詳細全文を読む スポンサード リンク
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