Paleontology in the United States

Paleontologists have found that during the Precambrian, the areas now composing the United States were home to the first known eukaryotes, whose remains were preserved in California. Other simple Precambrian life is known from Michigan,[1] while more complex forms in Arizona[2] and North Carolina.[3]

During the Cambrian, the land masses now composing the United States were separate and located in the southern hemisphere.[4] Trilobites are the most characteristic animal of the time,[5] and are especially abundant in Utah.[6] Ordovician America was still home to a wide variety of marine invertebrates. An important fauna was preserved in the states of Indiana, Kentucky, and Ohio in the Cincinnati region.[7] Life in Silurian America was especially diverse around the coral reefs of Indiana.[8] It was also the time of New York's famous sea scorpion, Eurypterus.[9] Fishes diversified greatly during the Devonian.[10] On land the first known seed plants appeared in Pennsylvania and some of the world's first forests appeared in New York.[11] Mississippian America was once more covered in seas,[12] now notably home to abundant crinoids.[13] During the Pennsylvanian America was largely terrestrial[14] and vast swamps expanded across the country[15] which were home to amphibians.[16] Reptiles were appearing around this time.[17] Into the Permian the continents had collided uniting into a single supercontinent called Pangaea. Much of the country was dry.[18] Precursors to mammals like Dimetrodon and Edaphosaurus lived in Texas.[19] At the end of the Permian the largest mass extinction in earth's history occurred, killing some 96% of species on the planet.[18]

During the Triassic, the Mesozoic era began.[20] Ichthyosaurs entered the sea and achieved large sizes in Nevada.[21] The Petrified Forest appeared in the southwest.[22] The dinosaurs appeared and achieved dominance on land[23] while pterosaurs flew overhead.[24] Into the Jurassic, plesiosaurs began usurping marine dominance from the ichthyosaurs.[25] Dinosaurs continued to diversify on land and increase in size.[26] A vast complex of floodplains covered the western United States[27] that was home to iconic creatures like Allosaurus,[28] Apatosaurus,[29] Ceratosaurus,[30] and Stegosaurus.[31] During the Early Cretaceous dinosaur faunas began to change.[32] Around the same time sea levels began to gradually rise and the Gulf of Mexico extended up into Alaska, dividing North America in two.[33] This formed the Western Interior Seaway,[34] where ichthyosaurs and plesiosaurs gradually gave way to mosasaurs.[35] Overhead pterosaurs like Pteranodon of Kansas achieved vast wingspans.[36] Before the end of the Cretaceous the seaway began to retreat and the coastal plains of Montana and the Dakotas were home to iconic dinosaurs like Edmontosaurus, Triceratops, and Tyrannosaurus rex.[37] Soon however, the marine reptiles, dinosaurs, and pterosaurs that dominated the planet went extinct, likely because a meteor impact destabilized the planet's ecosystems.[38]

After the extinction of the dominant reptile groups, the Cenozoic[39] ushered in the Age of Mammals.[40] Sea levels continued to decline[41] until the vast Western Interior Seaway was reduced to a small inland body in North Dakota.[42] Mammals were beginning to diversify and dominate the land.[40] The creodonts, forerunners of modern carnivores, arose to prominence during the Paleocene.[43] During the Eocene the western states were home to small primitive camels[44] and horses.[45] Over time both groups got larger and lost several toes.[44][45] Even by the late Eocene mammals had entered the oceans and the great primitive whale Basilosaurus swam in the coastal waters of the southeastern states like Alabama[46] and Mississippi.[47] Rhino-like titanotheres dominated the South Dakota badlands of the Oligocene.[48] From this point on the climate in the United States cooled[41] until the iconic Ice Age mammals of the Pleistocene like saber-toothed cats, woolly mammoths, mastodons and dire wolves[49] spread across the continent in advance of glaciers.[50] Humans arrived across a land bridge between Siberia and Alaska[51] and may have played a role in hunting these animal into extinction.[52]

Native Americans have been familiar with fossils for thousands of years.[53] They both told myths[54] about them and applied them to practical purposes.[55] The idea of powerful natural forces contending for balance is a common motif in fossil-influenced Native American folklore.[54] Practical applications for fossils in Native American cultures include ingestion as medicine[56] and use in food preparation.[55] African slaves also contributed their knowledge to the early study of paleontology in the United States. The first reasonably accurate recorded identification of vertebrate fossils in the new world was made by slaves on a South Carolina plantation who recognized the elephant affinities of mammoth molars uncovered there in 1725.[57] However, the first major fossil discovery to attract the attention of formally trained scientists were the Ice Age fossils of Kentucky's Big Bone Lick.[58] These fossils were studied by eminent intellectuals like France's George Cuvier and local statesmen and frontiersman like Daniel Boone,[59] Benjamin Franklin,[60] William Henry Harrison,[61] Thomas Jefferson,[62] and George Washington.[63] They triggered controversy regarding the idea that entire species of animals could become extinct.[64] By the end of the 18th century possible dinosaur fossils might already have been found in New Jersey.[65]

By the beginning of the 19th century, their fossil footprints definitely had been found in Massachusetts[66] and later, Connecticut.[67] These tracks were researched by the Reverend Edward Hitchcock and instrumental to the establishment of ichnology, the study of trace fossils, as a science.[66] During the late 1850s, the world's first reasonably complete dinosaur skeleton was discovered in New Jersey.[68] Joseph Leidy would name it Hadrosaurus. This was the first known dinosaur, the first dinosaur to be interpreted as two-legged, and the first to be mounted for exhibition in a museum.[69] Later in the century Edward Drinker Cope described a new carnivorous dinosaur from New Jersey he called Laelaps. When he gave fellow paleontologist Othniel Charles Marsh a tour of the quarry the fossils were taken from Marsh secretly convinced Cope's workers to send fossils to himself instead. Marsh also learned that the name Laelaps had already been used for a different animal and renamed Cope's dinosaur Dryptosaurus out from under him. This instigated a bitter rivalry to collect the most fossils and name the most new prehistoric species that took the two paleontologists and their crews deep into the west looking for fossils and attempting to undermine each other's fieldwork.[70]

Early in the 20th century major finds continued, like the Ice Age mammals of the La Brea Tar Pits, which preserved hundreds of thousands fossils in central Los Angeles.[71] The South Dakota School of Mines and Technology uncovered vast Oligocene bonebeds in the South Dakota badlands.[72] Seven years later, another bonebed was discovered in New Mexico, that preserved hundreds of the dinosaur Coelophysis.[73] During the 1960s, John Ostrom led an expedition into Montana that uncovered the remains of the nimble, bird-like carnivorous dinosaur Deinonychus. Based on this find, Ostrom and his student, Robert Bakker began arguing that dinosaurs had sophisticated physiologies and were even the ancestors of modern birds. These ideas revolutionized dinosaur paleontology through a period called the Dinosaur Renaissance.[74] Another notable 20th-century find occurred in Montana during the 1970s. Jack Horner and Bob Makela's new duck-billed dinosaur genus Maiasaura provided both the first known dinosaur nests in North America as well as early evidence that dinosaurs provided parental care for their young, unlike most reptiles.[75] During the early 1990s, the new genus "Seismosaurus" was put forward as a candidate for the largest known dinosaur.[76]

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