page_title: Prehistory of the United States
section_title: Paleozoic
hierarchical_section_title: Prehistory of the United States / Paleozoic
caption_attribution_description: Reconstruction of Eurypterus in life.
context_page_description: 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.
context_section_description: The Phanerozoic eon began following the Precambrian. The first major unit of time it contained was the Cambrian period of the Paleozoic era. At the time, earth's continents were in a very different arrangement and were generally smaller than they are today. The southeastern part of the US was connected to South America and Africa and located in the polar latitudes of the southern hemisphere. The western states were located near the equator.
  All known life during the Cambrian was aquatic so areas of the United States above sea level would have been sterile wastelands. Trilobites are the most common kind of animal in the Cambrian fossil record. Especially notable are the Elrathia of the Antelope Springs region of Utah. Sponge-like archaeocyathids were common in Nevada. Brachiopods, gastropods, and sponges were also important Cambrian animals. Sea levels around North America dropped at the end of the Cambrian. Ecosystems in shallow water probably devastated. The Cambrian ended with a mass extinction. Globally, brachiopods and gastropods lost much of their biodiversity, sponges lost about half of their families and almost three quarters of contemporary trilobite families vanished at the same time.
  Much of the continent would be submerged when the sea finally rose again. The most common animal in the Ordovician fossil record seems to have been brachiopods. Large numbers flourished and were preserved in great detail near Cincinnati during the Middle Ordovician. Members of this fauna were preserved in Indiana, Kentucky, and Ohio. Most known Ordovician trilobites are distinct from their Cambrian forebears as the few taxa surviving the mass extinction diversified once again. During the Middle Ordovician, early armored jawless fish called ostracoderms left behind fragmentary shards of bone in what is now the Rocky Mountains region. Late in the Ordovician sea levels dropped. Another mass extinction marked the end of the Ordovician. Globally, 25% of families disappeared. Major losses were experienced by brachiopods, fishes, echinoids, sponges and trilobites. This mass extinction was also most damaging to trilobites, which lost 50% of their families.
  During the Silurian, warm, shallow seas covered most of North America. Illinois, Indiana, and Michigan were home to vast coral reefs. The reefs of Indiana in particular are among the most diverse of the period on the entire continent. The southeastern United States were still part of Gondwanaland during the Silurian. Graptolites still inhabited the waters near the eastern coast of the United States but were not as big a component of the Silurian fauna as they used to be during the Ordovician. As the Silurian progressed the seas covering most of the country would retreat. Only the Michigan and New York areas (then near the equator) were still inundated. However, these landlocked seas were not being replenished by freshwater and so gradually evaporated, leaving concentrated salt deposits in those regions.
  Devonian North America once again experience home to seas that teemed with life. In fact Devonian marine life may have been more abundant and diverse than at any other point in the Paleozoic. Glass sponges became abundant in western New York during the Devonian. Especially notable are the marine fossils of Cuyahoga County, Ohio, which was home to more than 120 kinds of marine life. The local fishes alone left behind more than 50,000 fossils. During the Late Devonian the oldest known seed-bearing plants grew in Pennsylvania. The plants responsible for leaving behind the local fossil seeds may have been seed ferns, plants whose fronds resemble ferns but who reproduce through seeds instead of spores. Gilboa Forest, among the first in the world, formed in New York around this time. The Devonian ended with another mass extinction. Globally, 25% of families were lost. Nearly every family of ammonoids, fishes, and amphibians became extinct. Most known families of coral and trilobite became extinct. Other taxa to suffer declines in diversity include brachiopods, bryozoans, crinoids, and ostracode
caption: Eurypterus
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