Prehistory of the United States
The prehistory of the United States
comprises the happenings 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 is actually in the southern
Marine life flourishes in the country's many
seas, although terrestrial life has not yet evolved.
During the latter part of the Paleozoic, seas are largely
replaced by swamps home to amphibians and early reptiles.
When the continents have assembled into Pangaea drier
conditions prevailed. The evolutionary precursors to mammals
dominate the country until a mass extinction event ends their
The Triassic, first period of the Mesozoic
era follows. Dinosaurs evolve and beg in their rise to
dominance, quickly spreading into the United States.
Soon Pangaea begin to split up and North America began
drifting north and westward. During the latter Jurassic,
the floodplains of the western states are home to dinosaurs
like Allosaurus, Apatosaurus, and Stegosaurus. During
the Cretaceous, the Gulf of Mexico expands until it splits
North America in half. Plesiosaurs and mosasaurs swim in
its waters. Later into the period it begins to withdraw
and the coastal plains of the western states are home to
dinosaurs like Edmontosaurus, Triceratops, and Tyrannosaurus.
Another mass extinction ends the reign of the dinosaurs.
The Cenozoic era begins afterward.
The inland sea of the Cretaceous gradually vanishes and
mammals are beginning to dominate the land. During the
Eocene the western states are home to small primitive camels
and horses as well as the carnivorous creodonts. Soon mammals
have entered the oceans and the early whale Basilosaurus swim
the coastal waters of the southeast. Rhino-like titanotheres
dominate Oligocene South Dakota. From this point on the
climate in the United States cools until the Pleistocene, when
glaciers spread. Saber-toothed cats, wooly mammoths,
mastodons, and dire wolves roam the land. Humans arrive across
a land bridge between Siberia and Alaska and may play a role
in hunting these animals into extinction.