page_title: Prehistory of the United States
section_title: Cenozoic
hierarchical_section_title: Prehistory of the United States / Cenozoic
caption_attribution_description: Patriofelis ferox
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: After the Cretaceous, a new era of geologic time began; the Cenozoic era, which means the era of "recent life". Traditionally, the first period of the Cenozoic was called the Tertiary, however recent recommendations of the International Commission on Stratigraphy discourage its use, with the correspondent time span divided between the earlier Paleogene and the more recent Neogene periods. The geologic turmoil on the west coast was maintained as the Pacific Plate continued to slide under the North American Plate. During the early part of the Cenozoic period climates were much warmer than they are today. Latitudes as high as South Dakota had a subtropical climate until as recently as the end of the Oligocene.
  
  Sea level fell throughout the Cenozoic. Areas of Cenozoic North America that were covered by seawater tended to be areas near the modern coasts. The Cannonball Sea near Minot, North Dakota was the last of the North American interior. Cenozoic marine invertebrates are best known from deposits near the coasts and tend to resemble modern forms. Solitary corals became common, but coral reefs formed only around the Gulf of Mexico. Sharks were common during the Cenozoic. From Eocene times onward they began increasing in size.
  The beginning of the Cenozoic was also the beginning of the Age of Mammals. The small shrew-like generalist insectivores that survived the Late Cretaceous extinction event began diversifying into the mammals that dominate the modern world's terrestrial ecosystems. The creodonts, which first appeared in the Paleocene, were among the first mammals to specialize in carnivory. The coastal region of the southeastern states, like Alabama and Mississippi were covered in seawater and home to the primitive whale Basilosaurus during the Eocene. The diversity of mammalian carnivores on land increased from the Eocene to the Miocene.
  Around the same time, camels were becoming common in North America, although they were just barely bigger than modern rabbits during the Eocene. Early camels had four toes. During the early part of the Eocene the first primitive horses also began appearing. Among the earliest was Eohippus, a small animal with four toes on the front feet and three on the rear. A group of mammals called oreodonts also appeared during the Eocene. By the Oligocene, camels were the size of sheep and had only two toes. Gradually over time horses also lost toes, but unlike camels, horses' were reduced to a single digit. By the Oligocene their teeth had adapted to endure abrasion from silica in their increasingly grassy diets. Horses gradually became common throughout the country. Large numbers of oreodonts grazed in the badlands of South Dakota by the middle of the Cenozoic.  Oreodonts were vaguely pig like and about the size of modern goats. Their numbers peaked during the Oligocene. The largest mammals of Oligocene North America were the rhinoceros-like titanotheres. One spectacular example was the abundant Brontotherium of South Dakota, which could be up to 8 feet tall at the shoulder. Despite their early success, by the end of the epoch the entire group became extinct.
  Following the Oligocene temperatures began to decline, and with it warm-weather vegetation was forced southward into lower latitudes. By the Miocene some sharks, which had been increasing in size since the Eocene, were over 60 feet long. Mastodons arrived in North America by crossing the Bering land bridge from the old world during the Miocene as well. The oreodonts became extinct during the Pliocene. By the time the Pliocene ended more modern carnivores like wolves and cats appeared. Notable among the latter group were the saber-toothed cats. Woolly mammoths became abundant across the US during the late Cenozoic. During the late Pleistocene, the large volumes of water were held frozen as part of glaciers. This caused a drop in sea level, which exposed a land bridge between Asia and Alaska. Humans crossed over this bridge and started becoming abundant in North America between 11,000 and 12,000 years ago. Despite withstanding the fluctuatin
caption: The creodont Patriofelis.
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