The geological history of North America comprises the history of geological occurrences and emergence of life in North America during the interval of time spanning from the formation of the Earth through to the emergence of humanity and the start of prehistory. 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, the 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. Rhinoceros-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 in 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 cyanobacteria 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 below the sediment. Other complex Precambrian life forms were preserved in North Carolina and Arizona.
Paleozoic
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 was 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 ostracodes.
The ensuing Mississippian has been nicknamed the Age of Crinoids because this group left behind such abundant fossils. Other common lifeforms include their relatives the blastoids. The pinhead-sized foraminiferan Endothyra fed on seafloor detritus and was extremely abundant during the Mississippian. Some limestones in Illinois and Missouri that formed at this time are almost entirely composed of its remains. Few fossils of terrestrial life are known from the Mississippian of North America.
During the ensuing Pennsylvanian, the northward drift of Gondwanaland finally joined the southeastern United States to North America as Pangaea began to form. Mountain building raised the Ancestral Rockies in Colorado, Utah, and Wyoming. Seawaters left the interior of the country. Densely vegetated swamps were widespread. The largest insects in geologic history lived during the Pennsylvanian. Giant salamander-like amphibians left behind footprints near Lawrence, Kansas that would later fossilize. Fossil footprints from this time period were also preserved in eastern states like Alabama, Georgia, West Virginia, Ohio, and Pennsylvania where Carboniferous fossil footprints are known.
The world's continents were joined as Pangaea throughout all of the Permian. Volcanic activity occurred on the west coast. Seas were present in the southwest and west coast. The seas of Texas were home to massive reefs made of spined brachiopods jumbled together. Seas that had previously covered Kansas, New Mexico and Texas began drying up and left behind salt deposits. The inner portions of the western United States had a hot dry climate. Sand dunes were common throughout the country. The insects of the Permian were common and diverse, but smaller than those of the Pennsylvanian. An example of this bounty were the twenty different orders preserved at a site called Insect Hill near Elmo, Kansas. Massive amphibians were not rare in the southwestern United States but were unable to stray too far from bodies of water. Seymouria was a twenty inch long transitional form documenting the origin of reptiles from their amphibian-like forebears from Texas. Reptiles were becoming common during the Permian. Texas was also home to the pelycosaurs Dimetrodon and Edaphosaurus. Late in the Permian therapsids became the dominant large-bodied terrestrial vertebrates. These were the precursors of mammals.
The Permian ended with the most destructive mass extinction in all of Earth's history. Globally up to 96% of all species may have disappeared. Rugose and tabulate corals became extinct. The cryptostome and trepostome bryozoans also became extinct at this time despite their long history of diversity and abundance earlier in the Paleozoic. Brachiopods suffered greatly and never regained their previous numbers or variety. Ammonoids lost all but one family. Eurypterids and trilobites became extinct. Blastoids became extinct. Crinoids lost all but one family. The echinoids just squeaked past; only one genus is known to have survived. Typical survivors were small detritivores and sediments feeders. The worst losses were among filter feeders and carnivores.
Mesozoic
The eastern United States was part of Pangaea's interior for most of the Triassic Period of the Mesozoic era. At the time, the area lay close to the equator and was connected to western Europe and Africa. The union of all of Earth's continents into a single land mass changed the way the atmosphere and oceans circulated. This left what is now the eastern US with a hot climate and pronounced seasons. Much of the country was located at about 30 latitudinal degrees North, where there tends to be high atmospheric pressures and little precipitation. The western United States were largely covered in seawater during the Late Triassic. Mountain building and volcanic activity were ongoing in the same region.
The Triassic saw the Earth's biosphere recovering from the end-Permian mass extinction. After the Permian mass extinction, ammonoids were one of the first groups of marine invertebrates to become abundant and diverse again. During the Middle Triassic the scleractinian corals typical of modern oceans appeared. During the Triassic, corals were rare in North America. Nevertheless, some were present on the west coast, although these corals did not congregate into reefs. Ichthyosaurs were one of the most important groups of marine reptiles during the Triassic. Important ichthyosaur fossils of this age were preserved in Nevada.