The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene.

These two groups first appeared during the late Paleocene and early Eocene ( about 54 million years ago ), rapidly spreading to a wide variety of species on numerous continents, and have developed in parallel since that time.

At the time, two of the identified extinction events ( Cretaceous-Paleogene and Late Eocene ) could be shown to coincide with large impact events.

Furthermore, it is not constant over time, having been globally much shallower in the Cretaceous through to Eocene.

The oldest known leporid species date from the late Eocene, by which time the family was already present in both North America and Asia.

By that time, however, the group was already present in the Americas and Europe, and it is hypothesized that they actually evolved much earlier, maybe as early as the Early Eocene ( 50 mya ).

Going further back in time, the Late Eocene or Early Oligocene ( some 34 million years ago ) Filholornis from France has also been considered " proof " of a link between the Hoatzin and the gamebirds.

During the Paleocene and Eocene, extensive forests of Metasequoia occurred as far north as Strathcona Fiord on Ellesmere Island and sites on Axel Heiberg Island ( northern Canada ) at around 80 ° N latitude .< ref > Metasequoia was likely deciduous by this time.

* A living taxon that lived through a large portion of geologic time. Elephant Shrews resemble the extinct Leptictidium of Eocene Europe.

Composed primarily of Eocene sandstone and shale, much of the island also has marine terrace deposits of Pleistocene age, indicating that it was probably completely submerged at that time.

One of the first branches to split from basal eutherians, they appeared in the Lower Eocene, a time of warm temperatures and high humidity, roughly fifty million years ago.

While no direct association exists between Ornitholithus and Gastornis fossils, no other birds of sufficient size are known from that time and place: while the large Diogenornis and Eremopezus are known from the Eocene, the former lived in South America ( still separated from North America by the Tethys Ocean then ) and the latter is only known from the Late Eocene of North Africa, which also was separated by an ( albeit less wide ) stretch of the Tethys Ocean from Europe.

This would nicely match the remains of eggs a bit smaller than those of the living Ostrich which have also been found in Paleogene deposits of Provence, were it not for the fact that these eggshell fossils also date from the Eocene but no Remiornis bones are known from that time yet.

At one time, the highly folded layers of sand, silt, mud, clay, and brown coal containing these coalified tree fossils were once thought to have accumulated during either the Miocene or Eocene Epoch.

Though most of the other soil orders known today existed by the time of the Carboniferous Ice Age 280 million years ago, Mollisols are not known from the paleopedological record any earlier than the Eocene.

It centers on an extraordinary assemblage of Eocene Epoch ( 56 to 34 million years ago ) animal and plant fossils associated with the smallest lake — Fossil Lake — of the three great lakes which were present at that time in what are now Wyoming, Utah, and Colorado.

The earliest gharial may have been related to the modern types: some died out at the same time as the dinosaurs ( at the end of the Cretaceous ), others survived until the early Eocene.

These " wolves on hooves " were probably one of the more important predator groups ( although they may have been scavengers ) in the late Paleocene and Eocene ecosystems of Europe ( which was an archipelago at the time ), Asia ( which was an island continent ), and North America.

It appears, however, to have been partly derived from older Eocene deposits and it occurs also as a derivative phase in later formations, such as glacial drift.

The most generalized type was Coryphodon, representing the family Coryphodontidae, from the lower Eocene of Europe and North America, in which there were 44 teeth and no horn-like excrescences on the long skull, while the femur had a third trochanter.

The transition from Miacidae to Carnivora was a general trend in the middle and late Eocene, with taxa from both North America and Eurasia involved.

The divergence of carnivorans from other miacids, as well as the divergence of the two clades within Carnivora, Caniformia and Feliformia, is now inferred to have happened in the middle Eocene, about 42 million years ago ( mya ).

The Cayman Rise extends from southeastern Cuba along the northern margin of the Cayman Trough toward Costa Rica and resulted from Paleocene to Eocene island arc formation with associated volcanism along an extinct subduction zone.

Ciconiiformes are known from the Late Eocene.

The Eocene ( symbol E < sub > O </ sub >) epoch, lasting from about 56 to 34 million years ago ( to ), is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era.

The name Eocene comes from the Greek ( eos, dawn ) and ( kainos, new ) and refers to the " dawn " of modern (' new ') mammalian fauna that appeared during the epoch.

The oldest known fossils assigned to Equidae date from the early Eocene, 54 million years ago.

The oldest known primate-like mammal species, the Plesiadapis, came from North America, but they were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.

Eocene fossil fish Priscacara liops from Green River Formation of Wyoming

Eocene fossil flower, collected August 2010 from Clare family fossil quarry, Florissant, Colorado

Fossil kingfishers have been described from Lower Eocene rocks in Wyoming and Middle Eocene rocks in Germany, around 30-40 million years ago.

Several fossil birds have been erroneously ascribed to the kingfishers, including Halcyornis, from the Lower Eocene rocks in Kent, which has also been considered a gull, but is now thought to have been a member of an extinct family.

There are several species from the Eocene, such as Petermanniopsis anglesaensis or Smilax, but their identification is not definite.

The name Oligocene comes from the Greek ( oligos, few ) and ( kainos, new ), and refers to the sparsity of additional modern mammalian species of fauna after a burst of evolution during the Eocene.

The earliest fossil of Ostrich-like birds is the Palaeotis living near the Asiatic steppes, from the Middle Eocene, a middle-sized flightless bird that was originally believed to be a bustard.

Horses and tapirs both evolved in North America ; rhinoceroses appear to have developed in Asia from tapir-like animals and then recolonised the Americas during the middle Eocene ( about 45 Mya ).

The global climate during the Paleogene departed from the hot and humid conditions of the late Mesozoic era and began a cooling and drying trend which, although having been periodically disrupted by warm periods such as the Paleocene – Eocene Thermal Maximum, persists today.

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.

Deeper alluvial strata, overlying Precambrian basement rock, Eocene alluvial deposits from a shallow Laramide depression, the Blanco Basin Formation ; and Oligocene ash and lava flows associated with the Conejos Formation, is the Santa Fe formation, mixed alluvium and lava outflows from the San Juan volcanic field to the west ; later strata comprise the Alamosa formation, alternating layers of sand, gravel, and clay.

The end is set at a major extinction event called the Grande Coupure ( the " Great Break " in continuity ) or the Eocene – Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay.

The bay's geology, its present form, and its very location were created by a bolide impact event at the end of the Eocene ( about 35. 5 million years ago ), forming the Chesapeake Bay impact crater and the Susquehanna River valley much later.

The conflicting molecular data regarding their relationships is not much resolved by the fossil record ; though they seem to have originated at the end of the Late Cretaceous like their presumed relatives, modern loons are only known with certainty since the Eocene.

Republic is also the site of the Stonerose Interpretive Center and Fossil Site, famous for the Eocene fossils found in a 49 mya lake bed at the north end of Republic.

At the end of the Eocene the climate seems to have become unsuitable.

The Apodiformes diversified during the Eocene, at the end of which the extant families were present ; fossil genera are known from all over temperate Europe, between today's Denmark and France, such as the primitive Scaniacypselus ( Early-Middle Eocene ) and the more modern Procypseloides ( Late Eocene / Early Oligocene-Early Miocene )

There were tertiary fossil snakes that became extinct by the end of the Eocene period.

The Iridaceae originated in Antarctica-Australasia in the late Cretaceous, about 82 million years ago, although the family's subsequent radiation occurred elsewhere, notably in southern Africa and temperate and highland South America, at the end of the Eocene or later.

The Iridaceae originated in Antarctica-Australasia, although its subsequent radiation occurred elsewhere, notably in southern Africa and temperate and highland South America at the end of the Eocene or later.

A crude molecular clock in combination with the evidence given by Pygaeus allows to place the initial split between the two main lineages to the mid-late Eocene, and together with the few other fossils it allows to deduce that most living genera were probably distinct by the end of the Paleogene 23 mya.

Saber-toothed cat, sabre-toothed cat ( see spelling differences ), often referred to by the misnomer Saber-toothed tiger refers to the extinct subfamilies of Machairodontinae ( Felidae ), Barbourofelidae ( Feliformia ), and Nimravidae ( Feliformia ) as well as two families related to marsupials that were found worldwide from the Eocene Epoch to the end of the Pleistocene Epoch ( 42 mya — 11, 000 years ago ), existing for approximately.

Toward the end of the Eocene very large mammals ( such as the brontotheres ) had evolved in the region of Central Asia.

Pteridosperms declined during the Mesozoic Era and had mostly disappeared by the end of the Cretaceous Period, though some fossil pteridosperm-like plants seem to have survived into Eocene times in Tasmania.

In 1978 Egypt began building the Sadat Canal NW from Lake Nasser through Wadi Toshka to allow water levels higher than to be drained off into a depression at the south end of the Eocene limestone plateau.

They first appeared in the Early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes, became extinct in the Early Oligocene.