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Prehistory of the United States
The prehistory of the United States comprises the occurrences within regions now part of the United States of America during the interval of time spanning from the formation of the Earth to the documentation of local history in written form. At the start of the Paleozoic era, what is now "North" America was actually in the southern hemisphere. Marine life flourished in the country's many seas, although terrestrial life had not yet evolved. During the latter part of the Paleozoic, seas were largely replaced by swamps home to amphibians and early reptiles. When the continents had assembled into Pangaea drier conditions prevailed. The evolutionary precursors to mammals dominated the country until a mass extinction event ended their reign.
The Triassic, first period of the Mesozoic era followed. Dinosaurs evolved and began their rise to dominance, quickly spreading into the United States. Soon Pangaea began to split up and North America began drifting north and westward. During the latter Jurassic, the floodplains of the western states were home to dinosaurs like ''Allosaurus'', ''Apatosaurus'', and ''Stegosaurus''. During the Cretaceous, the Gulf of Mexico expanded until it split North America in half. Plesiosaurs and mosasaurs swam in its waters. Later into the period it began to withdraw and the coastal plains of the western states were home to dinosaurs like ''Edmontosaurus'', ''Triceratops'', and ''Tyrannosaurus''. Another mass extinction ended the reign of the dinosaurs.
The Cenozoic era began afterward. The inland sea of the Cretaceous gradually vanished and mammals were beginning to dominate the land. During the Eocene the western states were home to small primitive camels and horses as well as the carnivorous creodonts. Soon mammals had entered the oceans and the early whale ''Basilosaurus'' swam the coastal waters of the southeast. Rhino-like titanotheres dominated Oligocene South Dakota. From this point on the climate in the United States cooled until the Pleistocene, when glaciers spread. Saber-toothed cats, woolly mammoths, mastodons, and dire wolves roamed the land. Humans arrived across a land bridge between Siberia and Alaska and may have played a role in hunting these animals into extinction.
==Precambrian==
The oldest known eukaryotes lived around 1.4 billion years ago during the Precambrian near Beck Spring, California. These were photosynthetic organisms and produced oxygen as a byproduct of their physiology. The Beck Spring eukaryotes and other Precambrian photosynthesizers released the oxygen that now makes the planet's atmosphere breathable.〔 Precambrian Blue-green algae remains from this age were preserved between Copper Harbor and Eagle harbor on the shoreline of Lake Superior in the Upper Peninsula of Michigan.〔 The appearance of the first eukaryotic cells in the fossil record were relatively followed by evidence of complex multicellular life. Rocks in Nevada dating back to a billion years ago preserve trace fossils left behind by worms as they burrowed through the sediment.〔 Other complex Precambrian lifeforms were preserved in North Carolina〔 and Arizona.〔
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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