The Eocene epoch (56-34 million years ago) is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch.
The start of the Eocene is marked by the emergence of the first modern mammals. Within a geologically brief period of time in the early Eocene, most of the modern mammal orders appeared. Studies of fossils of early Eocene mammals and those of the Eocene-Oligocene showed that species tend to remain stable (in stasis) for millions of years, with speciation occurring over relatively short periods of time. Such findings accord well with the theory of punctuated equilibrium, as well as with expectations from theistic views of creation. (See Eocene fauna below.)
The end of the Eocene is set at a major extinction event called Grande Coupure (the "Great Break" in continuity), which may be related to the impact of one or more large bolides (meteoroids) in Siberia and in what is now Chesapeake Bay.
As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.
The name Eocene comes from the Greek eos (dawn) and ceno (new) and refers to the "dawn" of modern ('new') mammalian fauna that appeared during the epoch.
|Cenozoic era (65-0 mya)|
|Paleocene epoch||Eocene epoch||Oligocene epoch|
|Danian | Selandian
|Ypresian | Lutetian
Bartonian | Priabonian
|Rupelian | Chattian|
|Priabonian||(37.2 ± 0.1 – 33.9 ± 0.1 million years ago (mya))|
|Bartonian||(40.4 ± 0.2 – 37.2 ± 0.1 mya)|
|Lutetian||(48.6 ± 0.2 – 40.4 ± 0.2 mya)|
|Ypresian||(55.8 ± 0.2 – 48.6 ± 0.2 mya)|
Marking the start of the Eocene, the planet heated up in one of the most rapid (in geologic terms) and extreme global warming events recorded in geologic history, called the Paleocene-Eocene Thermal Maximum or Initial Eocene Thermal Maximum (PETM or IETM). (See Paleoclimatology.) This was an episode of rapid and intense warming (up to seven °C at high latitudes) that lasted less than 100,000 years. Sea surface temperatures rose between five and eight °C over a period of a few thousand years. The Thermal Maximum provoked a sharp extinction event that distinguishes Eocene fauna from the ecosystems of the Paleocene.
The Eocene global climate was perhaps the most homogeneous of the Cenozoic; the temperature gradient from equator to pole is considered to be only half that of today's, and deep ocean currents were exceptionally warm. The polar regions were much warmer than today, perhaps as mild as the modern-day Pacific Northwest. Warm temperate forests are considered to have extended right to the poles, while rainy tropical climates extended as far north as 45 degrees latitude away from the Equator. The difference was greatest in the temperate latitudes; the climate in the tropics however, was probably similar to today's (Stanley 1999).
Climates remained warm through the rest of the Eocene, although slow global cooling, which eventually led to the Pleistocene glaciations, started around the end of the epoch as ocean currents around Antarctica cooled.
During the Eocene, the continents continued to drift toward their present positions.
At the beginning of the period, Australia and Antarctica remained connected, and it is considered that warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the world and keeping global temperatures high. But when Australia split from the southern continent around 45 mya, the warm equatorial currents were deflected away from Antarctica, and an isolated cold water channel developed between the two continents. The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north, reinforcing the cooling.
In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts.
Europe saw the Tethys Sea finally vanish, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe, as the faunas of the two regions are very similar.
It is hypothesized that the Eocene hothouse world was due to runaway global warming from released methane clathrates deep in the oceans. The clathrates were buried beneath mud that was disturbed as the oceans warmed. Methane (CH4) has ten to twenty times the greenhouse gas effect of carbon dioxide (CO2).
At the beginning of the Eocene, the high temperatures and warm oceans assumably created a moist, balmy environment, with forests spreading throughout the earth from pole to pole. Apart from the driest deserts, Earth must have been entirely covered in forests.
Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from the Eocene have been found in Ellesmere Island in the Canadian Arctic. The preserved remains found in the Canadian Arctic are not fossils, but actual pieces preserved in oxygen-poor water in the swampy forests of the time, and then buried before they had the chance to decompose. Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene have also been found in Greenland and Alaska. Tropical rainforests grew as far north as the Pacific Northwest and Europe.
Palm trees were growing as far north as Alaska and northern Europe during the early Eocene, although they became less and less abundant as the climate cooled. Dawn redwoods were far more extensive as well.
Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry out, with forests thinning out considerably in some areas. The newly-evolved grasses were still confined to river banks and lake edges, and had not yet expanded into plains and savannas.
The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By the end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia, and the Arctic, and rainforests held on only in equatorial South America, Africa, India, and Australia.
Antarctica, which began the Eocene fringed with a warm temperate to sub-tropical rainforest, 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.
The oldest known fossils of most of the modern mammal orders appear within a geologically brief period during the early Eocene. At the beginning of the Eocene, several new mammal groups arrived in North America. 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. Dwarf forms reigned. All the members of the new mammal orders were small, under ten kg; based on comparisons of tooth size, Eocene mammals were only 60 percent of the size of the primitive Paleocene mammals that had preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage heat.
Both groups of modern ungulates (hoofed animals) became prevalent due to a major radiation between Europe and North America; along with carnivourous ungulates like Mesonyx. Early forms of many other modern mammalian orders appeared, including bats, proboscidians, primates, rodents, and marsupials. Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt, and South-East Asia. Marine fauna are best known from South Asia and the southeast United States.
As noted above, most mammal orders appeared in the early Eocene over a relatively brief period of time. Prothero (1992) reported that an examination of the entire mammalian fauna from the early Eocene (about 50-55 mya) of the Bighorn Basin of northwestern Wyoming showed that most species did change gradually through time. He also found that mammals from the Ecocene-Oligocene (30-35 mya) beds of the Big Badlands of South Dakota and related areas were static through millions of years and if speciated, did so abruptly. They were remarkably stable even over an interval of well-documented climatic change. Such findings accord well with the view of punctuated equilibrium, as advocated by Stephen Jay Gould and Niles Eldredge, as well as theistic views of creation.
Reptile fossils are also known from the Eocene, such as the enormous crocodile Deinosuchus, which lived as far north as Wyoming during the Eocene and grew much larger than the modern-day saltwater crocodile. Python fossils and turtle fossils are also known from North America.
During the Eocene plants and marine faunas became quite modern. Many modern bird orders first appear in the Eocene.
The Eocene oceans were warm and teeming with fish and other sea life. The first Carcharinid sharks appeared, as did early marine mammals, including Basilosaurus, an early species of whale that is thought to be descended from land animals, the hoofed predators called mesonychids, of which Mesonyx was a member. The first sirenians, relatives of the elephants, also appeared at this time.
The Grande Coupure, or "great break" in continuity, with a major European turnover in mammalian fauna about 33.5 mya, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian species. The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations (called "dispersal-generated origination" in Hooker et al. 2004).
The Grande Coupure was given its name in 1910 by the Swiss palaeontologist Hans Georg Stehlin to characterize the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling."
The Grande Coupure marks a break between endemic European faunas before the break and mixed faunas with a strong Asian component afterwards. J.J. Hooker and his team (2004) summarized the break:
They further noted that the only families to cross the faunal divide undiminished were the marsupial family Herpetotheriidae, the rodent families Theridomyidae and Gliridae (dormise) and the artiodactyl family Cainotheriidae.
Whether this abrupt change was caused by climate change associated with the earliest polar glaciations and a major fall in sea levels, or by competition with taxa dispersing from Asia, few would argue for an isolated single cause. More spectacular causes are related to the impact of one or more large bolides (in present day Siberia and Chesapeake Bay). Improved correlation of northwest European successions to global events (Hooker et al. 2004) confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350 thousand years (ka) prior to the first record of post-Grande Coupure Asian immigrant taxa.
An element of the paradigm of the Grande Coupure was the apparent extinction of all European primates at the Coupure: the recent discovery (Kohler and Moya-Sola 1999) of a mouse-sized early Oligocene omomyid, reflecting the better survival chances of small mammals, further undercut the Grand Coupure paradigm.
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