[11][n 1] British geologist John Phillips proposed the Cenozoic in 1840 in place of the Tertiary,[12] and Austrian paleontologist Moritz Hörnes introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853.

[22] Lasting for about 400,000 years, the MECO was responsible for a globally uniform 4° to 6°C warming of both the surface and deep oceans, as inferred from foraminiferal stable oxygen isotope records.

[citation needed] The Western North American floras of the Eocene were divided into four floral "stages" by Jack Wolfe (1968) based on work with the Puget Group fossils of King County, Washington.

At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents may have mixed with colder Antarctic waters, distributing the heat around the planet and keeping global temperatures high.

[30] In western North America, the Laramide Orogeny came to an end in the Eocene, and compression was replaced with crustal extension that ultimately gave rise to the Basin and Range Province.

[39] Planktonic foraminifera in the northwestern Peri-Tethys are very similar to those of the Tethys in the middle Lutetian but become completely disparate in the Bartonian, indicating biogeographic separation.

[53] This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, and it is hypothesized that the sequestration was mainly due to organic carbon burial and weathering of silicates.

As a result of the warmer climate and the sea level rise associated with the early Eocene, more wetlands, more forests, and more coal deposits would have been available for methane release.

[65] These hyperthermals led to increased perturbations in planktonic and benthic foraminifera,[66][67] with a higher rate of fluvial sedimentation as a consequence of the warmer temperatures.

Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and trap outgoing longwave radiation.

[citation needed] Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene.

In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand.

[citation needed] The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked the decline into an icehouse climate and the rapid expansion of the Antarctic ice sheet.

[83] The isolation of the Arctic Ocean, evidenced by euxinia that occurred at this time,[84] led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments on the seabed and effectively sequestered the carbon by locking it out of the atmosphere for good.

One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the Himalayas; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data.

[54] The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that was a major step into the icehouse climate.

[106] Multiple proxies, such as oxygen isotopes and alkenones, indicate that at the Eocene–Oligocene transition, the atmospheric carbon dioxide concentration had decreased to around 750–800 ppm, approximately twice that of present levels.

[100][106] Along with the decrease of atmospheric carbon dioxide reducing the global temperature, orbital factors in ice creation can be seen with 100,000-year and 400,000-year fluctuations in benthic oxygen isotope records.

Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from the Eocene have been found on Ellesmere Island in the Arctic.

[citation needed] Palm trees were growing as far north as Alaska and northern Europe during the early Eocene, although they became less abundant as the climate cooled.

[114] The earliest definitive Eucalyptus fossils were dated from 51.9 Ma, and were found in the Laguna del Hunco deposit in Chubut province in Argentina.

The newly evolved grasses began to expand during the climatic cooling and drying following the extreme warmth of the EECO, with subhumid savannas being known from South America since the Middle Eocene.

[118] It became much colder as the period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra.

These modern mammals, like artiodactyls, perissodactyls, and primates, had features like long, thin legs, feet, and hands capable of grasping, as well as differentiated teeth adapted for chewing.

Early forms of many other modern mammalian orders appeared, including horses (most notably the Eohippus), bats, proboscidians (elephants), primates, and rodents.

Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt, and southeast Asia.

[citation needed] After the Paleocene–Eocene Thermal Maximum, members of the Equoidea arose in North America and Europe, giving rise to some of the earliest equids such as Sifrhippus and basal European equoids such as the palaeothere Hyracotherium.

Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like Hyaenodon and Daphoenus (the earliest lineage of a once-successful predatory family known as bear dogs).

Bird taxa of the Eocene include carnivorous psittaciforms, such as Messelasturidae, Halcyornithidae, large flightless forms such as Gastornis and Eleutherornis,[126] long legged falcon Masillaraptor, ancient galliforms such as Gallinuloides, putative rail relatives of the family Songziidae, various pseudotooth birds such as Gigantornis, the ibis relative Rhynchaeites, primitive swifts of the genus Aegialornis, and primitive penguins such as Archaeospheniscus and Inkayacu.

For instance the bibionid genus Plecia is common in fossil faunas from presently temperate areas, but only lives in the tropics and subtropics today.