The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek ὀλίγος and καινός, and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.
Proailurus is an extinct felid genus that lived in Europe and Asia approximately 25-30.8 million years ago in the Late Oligocene and Miocene. Fossils have been found in Mongolia, Germany, and Spain.
Pseudaelurus is a prehistoric cat that lived in Europe, Asia and North America in the Miocene between approximately twenty and eight million years ago. It is considered to be a paraphyletic grade ancestral to living felines and pantherines as well as the extinct machairodonts (saber-tooths), and is a successor to Proailurus. It originated from Eurasia and was the first cat to reach North America, when it entered the continent at about 18.5 Ma ending a 'cat-gap' of 7 million years. The slender proportions of the animal, together with its short, viverrid-like legs, suggest that it may have been an agile climber of trees.
Paraceratherium is an extinct genus of hornless rhinocerotoids belonging to the family Paraceratheriidae. It is one of the largest terrestrial mammals that has ever existed and lived from the early to late Oligocene epoch. The first fossils were discovered in what is now Pakistan, and remains have been found across Eurasia between China and the Balkans. Paraceratherium means "near the hornless beast", in reference to Aceratherium, the genus in which the type species P. bugtiense was originally placed.
Sparassodonta is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now thought to be a separate side branch that split before the last common ancestor of all modern marsupials. A number of these mammalian predators closely resemble placental predators that evolved separately on other continents, and are cited frequently as examples of convergent evolution. They were first described by Florentino Ameghino, from fossils found in the Santa Cruz beds of Patagonia. Sparassodonts were present throughout South America's long period of "splendid isolation" during the Cenozoic; during this time, they shared the niches for large warm-blooded predators with the flightless terror birds. Previously, it was thought that these mammals died out in the face of competition from "more competitive" placental carnivorans during the Pliocene Great American Interchange, but more recent research has showed that sparassodonts died out long before eutherian carnivores arrived in South America. Sparassodonts have been referred to as borhyaenoids by some authors, but currently the term Borhyaenoidea refers to a restricted subgroup of sparassodonts comprising borhyaenids and their close relatives.
Hyaenodon ("hyena-tooth") is an extinct genus of carnivorous placental mammals from extinct tribe Hyaenodontini within extinct subfamily Hyaenodontinae, that lived in Eurasia and North America from the middle Eocene, throughout the Oligocene, to the early Miocene.
Palaeotherium is an extinct genus of perissodactyl ungulate known from the Mid Eocene to earliest Oligocene of Europe. First described by French naturalist Georges Cuvier in 1804, Palaeotherium was among the first Paleogene mammals to be described.
Brachyhyops is an extinct genus of entelodont artiodactyl mammal that lived during the Eocene Epoch of western North America and southeastern Asia. The first fossil remains of Brachyhyops are recorded from the late Eocene deposits of Beaver Divide in central Wyoming and discovered by paleontology crews from the Carnegie Museum of Natural History during the early 20th century. The type species, Brachyhyops wyomingensis, is based on a single skull and was named by E.H. Colbert in 1937, but was not officially described until 1938. During the latter half of the 20th century, additional specimens from North America have been recorded from Saskatchewan and as far south as Texas, indicating that Brachyhyops had a broad distribution and was well-dispersed throughout western North America.
Asiavorator is an extinct genus of civet-like carnivoran belonging in the family Stenoplesictidae. It was endemic to Asia and lived during the Eocene and Oligocene epochs.
The Chipola Formation is a Late Oligocene to Early Miocene geologic formation in the Florida Panhandle and member of the Alum Bluff Group.
The Cypress Hills Formation is a stratigraphic unit of middle Eocene to early Miocene age in the Western Canada Sedimentary Basin. It is named for the Cypress Hills of southeastern Alberta and southwestern Saskatchewan and was first described from outcrops on the slopes of the Cypress Hills in 1930. It is known for preserving a wealth of vertebrate fossils.
The Bouldnor Formation is a geological formation in the Hampshire Basin of southern England. It is the youngest formation of the Solent Group and was deposited during the uppermost Eocene and lower Oligocene.
The Belgrade Formation is a limestone geologic formation in North Carolina characterized by limestone coquina mixed with sand, and thinly laminated clays. It preserves fossils dating back to the Paleogene period.
The Alum Bluff Group is a geologic group in the states of Georgia, Florida, and Alabama. It preserves fossils dating back to the Neogene period.
Ronzotherium is an extinct genus of perissodactyl mammal from the family Rhinocerotidae. The name derives from the hill of 'Ronzon', the French locality near Le Puy-en-Velay at which it was first discovered, and the Greek suffix 'therium' meaning 'beast'. At present 5 species have been identified from several localities in Europe and Asia, spanning the Late Eocene to Upper Oligocene.
The geology of South Dakota began to form more than 2.5 billion years ago in the Archean eon of the Precambrian. Igneous crystalline basement rock continued to emplace through the Proterozoic, interspersed with sediments and volcanic materials. Large limestone and shale deposits formed during the Paleozoic, during prevalent shallow marine conditions, followed by red beds during terrestrial conditions in the Triassic. The Western Interior Seaway flooded the region, creating vast shale, chalk and coal beds in the Cretaceous as the Laramide orogeny began to form the Rocky Mountains. The Black Hills were uplifted in the early Cenozoic, followed by long-running periods of erosion, sediment deposition and volcanic ash fall, forming the Badlands and storing marine and mammal fossils. Much of the state's landscape was reworked during several phases of glaciation in the Pleistocene. South Dakota has extensive mineral resources in the Black Hills and some oil and gas extraction in the Williston Basin. The Homestake Mine, active until 2002, was a major gold mine that reached up to 8000 feet underground and is now used for dark matter and neutrino research.
Diegoaelurus is an extinct genus of placental mammals from extinct the subfamily Machaeroidinae within extinct family Oxyaenidae. This genus contains only one species Diegoaelurus vanvalkenburghae, which was found in the Santiago Formation in California. This mammal lived during the Uintan stage of the Middle Eocene Epoch around 46.2 to 39.7 million years ago.
Amphicticeps is an extinct genus of small, weasel-like carnivoran mammal. It lived in Mongolia during the Oligocene. The genus was erected in 1924 for the species A. shackelfordi on the basis of a well-preserved skull. Historically, the systematic position of this genus has been problematic until more specimens were described decades later.
The Ergilin Dzo Formation, is a geologic formation in southeastern Mongolia. It preserves fossils of a wide variety of animals, which date back to the late Eocene epoch. The type fauna of the Ergilian Asian land mammal age is defined by the mammalian fossils of this formation.
Ergilictis is an extinct genus of mammal that lived during the late Eocene and early Oligocene epochs. It was endemic to Asia, and its fossils have been found in the Ergilin Dzo and Hsanda Gol formations of Mongolia. It belongs to the family Didymoconidae, which has uncertain taxonomic affinities.
Mongolia 
