Eocene
   HOME

TheInfoList



The Eocene ( ) Epoch is a geological
epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-E ...
that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
Period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Period, a descriptor for a historical or period drama ...
in the modern
Cenozoic The Cenozoic Era ( ) meaning "new life" is the current and most recent of the three geological eras of the Phanerozoic Eon. The Cretaceous–Paleogene extinction event near Drumheller, Alberta, where erosion has exposed the K–Pg boundary ...
Era An era is a span of time defined for the purposes of chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to ...
. The name ''Eocene'' comes from the
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages () ...
(''ēṓs'', "
dawn Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, vi ...
") and (''kainós'', "new") and refers to the "dawn" of modern ('new')
fauna Fauna is all of the animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular r ...

fauna
that appeared during the epoch. The Eocene spans the time from the end of the
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name is a combinatio ...
Epoch to the beginning of the
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define ...
Epoch. The start of the Eocene is marked by a brief period in which the concentration of the
carbon isotope Carbon (6C) has 15 known isotopes, from 8C to 22C, of which Carbon-12, 12C and Carbon-13, 13C are Stable nuclide, stable. The longest-lived radioisotope is Carbon-14, 14C, with a half-life of 5,730 years. This is also the only carbon radioisotope ...
13C
<sup>13</sup>C
in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The end is set at a major
extinction event An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It o ...
called the ''Grande Coupure'' (the "Great Break" in continuity) or the
Eocene–Oligocene extinction event The Eocene–Oligocene extinction event, the transition between the end of the Eocene The Eocene ( ) Epoch is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronolog ...
, which may be related to the impact of one or more large
bolide A bolide (; Italian via Latin from the Greek language, Greek , 'missile') is an extremely bright meteor, especially one that explodes in the atmosphere. In astronomy, it refers to a fireball (meteor), fireball about as bright as the full moon, and ...

bolide
s in
Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spanning much of Northern Asia. Siberia has been Russian conquest of Siberia, part of modern Russia since the latter half of th ...
and in what is now
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the E ...
. As with other
geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe t ...
s, the
strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is ...
that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.


Etymology

The term "Eocene" is derived from
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages () ...
eo—''eos'' meaning "dawn", and—cene ''kainos'' meaning "new" or "recent", as the epoch saw the dawn of recent, or modern, life. Scottish geologist
Charles Lyell Sir Charles Lyell, 1st Baronet, (14 November 1797 – 22 February 1875) was a Scottish geologist who demonstrated the power of known natural causes in explaining the earth Earth is the third planet from the Sun and the only astrono ...

Charles Lyell
(ignoring the Quaternary) had divided the Tertiary Epoch into the Eocene,
Miocene The Miocene ( ) is the first Epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish author Charles Lyell; its name comes from the Greek words (', "less") and (', "new") and means "le ...
,
Pliocene The Pliocene ( ; also Pleiocene) epoch (geology), Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. It began approximately 11,650 radiocarbon calibration, cal years Before Present, before present, after the last glacial period, which concluded with the Holocene glacial re ...
) Periods in 1833. British geologist
John Phillips
John Phillips
had proposed the
Cenozoic The Cenozoic Era ( ) meaning "new life" is the current and most recent of the three geological eras of the Phanerozoic Eon. The Cretaceous–Paleogene extinction event near Drumheller, Alberta, where erosion has exposed the K–Pg boundary ...
in 1840 in place of the Tertiary, and Austrian paleontologist Moritz Hörnes had introduced the
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
for the Eocene and
Neogene The Neogene ( ) (informally Upper Tertiary or Late Tertiary) is a geologic period and system that spans 20.45 million years from the end of the Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or E ...

Neogene
for the Miocene and Pliocene in 1853. After decades of inconsistent usage, the newly formed
International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigrap ...
(ICS), in 1969, standardized stratigraphy based on the prevailing opinions in Europe: the Cenozoic Era subdivided into the Tertiary and Quaternary sub-eras, and the Tertiary subdivided into the Paleogene and Neogene Periods. In 1978, the Paleogene was officially defined as the Paleocene, Eocene, and Oligocene Epochs; and the Neogene as the Miocene and Pliocene Epochs. In 1989, Tertiary and Quaternary were removed from the time scale due to the arbitrary nature of their boundary, but Quaternary was reinstated in 2009, which may lead to the reinstatement of the Tertiary in the future.


Geology


Boundaries

The beginning of the Eocene is marked by the
Paleocene–Eocene Thermal Maximum The Paleocene–Eocene Thermal Maximum (PETM), alternatively (ETM1), and formerly known as the "Initial Eocene" or "", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event oc ...
, a short period of intense warming and
ocean acidification File:Acidifiedupwelledwater.jpg, upright=1.5, NOAA provides evidence for the upwelling of "acidified" water onto the Continental Shelf. In the figure above, note the vertical sections of (A) temperature, (B) aragonite saturation, (C) pH, (D) DIC ...
brought about by the release of carbon en masse into the atmosphere and ocean systems, which led to a mass extinction of 30–50% of benthic
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
–single-celled species which are used as
bioindicator A bioindicator is any species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of orga ...
s of the health of a marine ecosystem—one of the largest in the Cenozoic. This event happened around 55.8 mya, and was one of the most significant periods of global change during the Cenozoic. The end of the Eocene was marked by the
Eocene–Oligocene extinction event The Eocene–Oligocene extinction event, the transition between the end of the Eocene The Eocene ( ) Epoch is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronolog ...
, also known as the ''Grande Coupure''.


Stratigraphy

The Eocene is conventionally divided into early (56–47.8 million years ago), middle (47.8–38m), and late (38–33.9m) subdivisions. The corresponding
rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in which it is formed. Rocks form the Earth's oute ...

rocks
are referred to as lower, middle, and upper Eocene. The
Ypresian In the geologic timescale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Ea ...
Stage constitutes the lower, the
Priabonian The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoc ...
Stage the upper; and the
Lutetian The Lutetian is, in the geologic timescale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other ...
and
Bartonian The Bartonian is, in the ICS's geologic time scale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, ...
Stages are united as the middle Eocene.


Palaeogeography and tectonics

During the Eocene, the
continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to small ...
s continued to drift toward their present positions. At the beginning of the period,
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, smal ...

Australia
and
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oce ...

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. When Australia split from the southern continent around 45 Ma, the warm equatorial currents were routed away from Antarctica. An isolated cold water channel developed between the two continents. However, modeling results call into question the thermal isolation model for late Eocene cooling, and decreasing carbon dioxide levels in the atmosphere may have been more important. Once the Antarctic region began to cool down, the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north and reinforcing the cooling. The northern
supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...
of
Laurasia Laurasia (), a portmanteau A portmanteau (, ) or portmanteau word (from "Portmanteau (luggage), portmanteau") is a Blend word, blend of words
began to fragment, as
Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered from largest ...

Europe
,
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an Autonomous administrative division, autonomous territory* * * within the Danish Realm and the List of islands by area, world's largest island, located between the Arctic Ocean, Arcti ...

Greenland
and
North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to ...

North America
drifted apart. In western North America, the
Laramide Orogeny The Laramide orogeny was a time period of mountain building in western North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the nort ...
came to an end in the Eocene, and compression was replaced with crustal extension that ultimately gave rise to the
Basin and Range Province The Basin and Range Province is a vast physiographic region Physiographic regions of the world are a means of defining Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth ...
. Huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the
Green River Formation The Green River Formation is an Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic ...
lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' ''wikt:Stätte, Stätte'' 'place'; plural ''Lagerstätten'') is a Sedimentation, sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preser ...
. At about 35 Ma, an asteroid impact on the eastern coast of North America formed the Chesapeake Bay impact crater. In Europe, the
Tethys Sea 250px, First phase of the Tethys Ocean's forming: the (first) Tethys Sea starts dividing Pangaea into two supercontinents, Laurasia and Gondwana">Laurasia.html" ;"title="Pangaea into two supercontinents, Laurasia">Pangaea into two supercontinents, ...
finally disappeared, while the uplift of the
Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across eight Alpine countries (from west to east): France, Switzerl ...

Alps
isolated its final remnant, the
Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin In biogeography, the Mediterranean Basin (also known as the Mediterranean region or sometimes Mediterranea) is the region of lands aroun ...

Mediterranean
, and created another shallow sea with island
archipelago An archipelago ( ), sometimes called an island group or island chain, is a chain, cluster or collection of islands, or sometimes a sea containing a small number of scattered islands. Examples of archipelagos include: the Indonesian Archipel ...
s to the north. Though the North was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar.
India India (Hindi: ), officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by population, second-most populous country, the List of countries and dependencies by area, seventh-largest ...

India
began its collision with
Asia Asia () is a landmass variously described as part of Eurasia Eurasia () is the largest continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict crite ...
, folding to initiate formation of the
Himalaya The Himalayas, or Himalaya (); Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the In ...

Himalaya
s.


Climate

The Eocene Epoch contained a wide variety of different climate conditions that includes the warmest climate in the
Cenozoic Era The Cenozoic Era ( ) meaning "new life" is the current and most recent of the three geological eras of the Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologic time scale (GTS) is a system of ch ...
and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the
Paleocene–Eocene Thermal Maximum The Paleocene–Eocene Thermal Maximum (PETM), alternatively (ETM1), and formerly known as the "Initial Eocene" or "", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event oc ...
(PETM) at 56 million years ago to a maximum during the Eocene Optimum at around 49 million years ago. During this period of time, little to no ice was present on Earth with a smaller difference in temperature from the equator to the poles. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. During this decrease, ice began to reappear at the poles, and the Eocene-Oligocene transition is the period of time where the
Antarctic ice sheet , showing glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier A glacier ( or ) is a persistent body of dense ice ...
began to rapidly expand.


Atmospheric greenhouse gas evolution

Greenhouse gases, in particular
carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

carbon dioxide
and
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics an ...
, played a significant role during the Eocene in controlling the surface temperature. The end of the PETM was met with very large sequestration of carbon dioxide into the forms of
methane clathrate . Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...

methane clathrate
,
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half o ...

coal
, and
crude oil Petroleum (), also known as crude oil and oil, is a naturally occurring, yellowish-black liquid found in geological formations beneath the Earth's surface. It is commonly refined into various types of fuels. Components of petroleum are sep ...
at the bottom of the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is also known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocea ...

Arctic Ocean
, that reduced the atmospheric carbon dioxide. 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 Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in wh ...
of silicates. For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at the maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm while other proxies such as pedogenic (soil building) carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years. Sources for this large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from the PETM event in the sea floor or wetland environments. For contrast, today the
carbon dioxide levels Carbon dioxide () is an important trace gas in atmosphere of Earth, Earth's atmosphere. It is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Pla ...
are at 400 ppm or 0.04%. At about the beginning of the Eocene Epoch (55.8–33.9 million years ago) the amount of oxygen in the earth's atmosphere more or less doubled. During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. In comparison to carbon dioxide, methane has much greater effect on temperature as methane is around 34 times more effective per molecule than carbon dioxide on a 100-year scale (it has a higher
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energ ...

global warming potential
). Most of the methane released to the atmosphere during this period of time would have been from wetlands, swamps, and forests. The
atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year global w ...
concentration today is 0.000179% or 1.79
ppmv In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantity, dimensionless quantities, e.g. mole fraction or mass fraction (chemistry), mass fraction. Since these fr ...
. 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. If we compare the early Eocene production of methane to current levels of atmospheric methane, the early Eocene would have produced triple the amount of methane. The warm temperatures during the early Eocene could have increased methane production rates, and methane that is released into the atmosphere would in turn warm the troposphere, cool the stratosphere, and produce water vapor and carbon dioxide through oxidation. Biogenic production of methane produces carbon dioxide and water vapor along with the methane, as well as yielding infrared radiation. The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide. The middle to late Eocene marks not only the switch from warming to cooling, but also the change in carbon dioxide from increasing to decreasing. At the end of the Eocene Optimum, carbon dioxide began decreasing due to increased siliceous plankton productivity and marine carbon burial. At the beginning of the middle Eocene an event that may have triggered or helped with the draw down of carbon dioxide was the Azolla event at around 49 million years ago. With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of
azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that under ...
, which is a floating aquatic fern, on the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is also known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocea ...

Arctic Ocean
. Compared to current carbon dioxide levels, these azolla grew rapidly in the enhanced carbon dioxide levels found in the early Eocene. As these
azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that under ...
sank into the Arctic Ocean, they became buried and sequestered their carbon into the seabed. This event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Assuming the carbon dioxide concentrations were at 900 ppmv prior to the Azolla Event they would have dropped to 430 ppmv, or 30 ppmv more than they are today, after the Azolla Event. Another event during the middle Eocene that was a sudden and temporary reversal of the cooling conditions was the Middle Eocene Climatic Optimum. At around 41.5 million years ago, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene. The main hypothesis for such a radical transition was due to the continental drift and collision of the India continent with the Asia continent and the resulting formation of the
Himalayas The Himalayas, or Himalaya (; Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Ind ...

Himalayas
. Another hypothesis involves extensive sea floor rifting and metamorphic decarbonation reactions releasing considerable amounts of carbon dioxide to the atmosphere. At the end of the Middle Eocene Climatic Optimum, cooling and the carbon dioxide drawdown continued through the late Eocene and into the Eocene–Oligocene transition around 34 million years ago. Multiple proxies, such as oxygen isotopes and
alkenone α,β-Unsaturated carbonyl compounds refers to organic compounds with the general structure (O=CR)−Cα=Cβ-R. Examples would be enones and enals. In these compounds the carbonyl group is conjugated system, conjugated with an alkene (hence the adje ...
s, 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.


Early Eocene and the equable climate problem

One of the unique features of the Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of proxies support the presence of a warmer equable climate being present during this period of time. A few of these proxies include the presence of fossils native to warm climates, such as
crocodile Crocodiles (family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subj ...
s, located in the higher latitudes, the presence in the high latitudes of frost-intolerant flora such as which cannot survive during sustained freezes, and fossils of
snakes Snakes are elongated, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Many species of snakes ...

snakes
found in the tropics that would require much higher average temperatures to sustain them. Using isotope proxies to determine ocean temperatures indicates sea surface temperatures in the tropics as high as and, relative to present-day values, bottom water temperatures that are higher. With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures. An issue arises, however, when trying to model the Eocene and reproduce the results that are found with the proxy data. Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to colder than the actual determined temperature at the poles. This error has been classified as the “equable climate problem”. To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below.


Large lakes

Due to the nature of water as opposed to land, less temperature variability would be present if a large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes. To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide, the addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data.


Ocean heat transport

The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles. With the increased sea surface temperatures and the increased temperature of the deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport.


Orbital parameters

While typically seen as a control on ice growth and seasonality, the orbital parameters were theorized as a possible control on continental temperatures and seasonality. Simulating the Eocene by using an ice free planet,
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off- center, in geometry * Eccentricity (graph theory) of a ...
,
obliquity In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mat ...
, and
precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other ...

precession
were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in the North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered.


Polar stratospheric clouds

Another method considered for producing the warm polar temperatures were
polar stratospheric cloud Polar stratospheric clouds (PSCs) are cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteo ...
s. Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and traps outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II). Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized, a significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions. To test the polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., greenhouse gases) in a planet's at ...

greenhouse effect
of the polar stratospheric clouds, the stratosphere would cool and would potentially increase the amount of polar stratospheric clouds. While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds. It was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. 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.


Hyperthermals through the early Eocene

During the warming in the early Eocene between 52 and 55 million years ago, there were a series of short-term changes of
carbon isotope Carbon (6C) has 15 known isotopes, from 8C to 22C, of which Carbon-12, 12C and Carbon-13, 13C are Stable nuclide, stable. The longest-lived radioisotope is Carbon-14, 14C, with a half-life of 5,730 years. This is also the only carbon radioisotope ...
composition in the ocean. These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of at the surface of the ocean. These hyperthermals led to increased perturbations in planktonic and benthic
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
, with a higher rate of sedimentation as a consequence of the warmer temperatures. Recent analysis of and research into these hyperthermals in the early Eocene has led to hypotheses that the hyperthermals are based on orbital parameters, in particular eccentricity and obliquity. The hyperthermals in the early Eocene, notably the Palaeocene–Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2), and the Eocene Thermal Maximum 3 (ETM3), were analyzed and found that orbital control may have had a role in triggering the ETM2 and ETM3.


Greenhouse to icehouse climate

The Eocene is not only known for containing the warmest period during the Cenozoic, but it also marked the decline into an icehouse climate and the rapid expansion of the
Antarctic ice sheet , showing glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier A glacier ( or ) is a persistent body of dense ice ...
. The transition from a warming climate into a cooling climate began at around 49 million years ago. Isotopes of carbon and oxygen indicate a shift to a global cooling climate. The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. One proposed cause of the reduction in carbon dioxide during the warming to cooling transition was the azolla event. The increased warmth at the poles, the isolated Arctic basin during the early Eocene, and the significantly high amounts of carbon dioxide possibly led to
azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that under ...
blooms across the Arctic Ocean. The isolation of the Arctic Ocean led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments and effectively sequestered the carbon. The ability for the azolla to sequester carbon is exceptional, and the enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate. Cooling after this event continued due to continual decrease in atmospheric carbon dioxide from organic productivity and
weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in wh ...
from
mountain building A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at lea ...
. Global cooling continued until there was a major reversal from cooling to warming indicated in the
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean or the Austral Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. As such, i ...

Southern Ocean
at around 42–41 million years ago.
Oxygen isotope There are three known stable isotopes of oxygen (8O): oxygen-16, 16O, Oxygen-17, 17O, and Oxygen-18, 18O. Radioactive isotopes ranging from 11O to 26O have also been characterized, all short-lived. The longest-lived radioisotope is 15O with a hal ...
analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. This warming event is known as the Middle Eocene Climatic Optimum. The warming is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. The increase in atmospheric carbon dioxide is considered to be due to increased
seafloor spreading Seafloor spreading is a process that occurs at mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth sho ...
rates between Australia and Antarctica and increased amounts of
volcanism Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the Earth#Surface, surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called ...
in the region. Another possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release during the Himalayan orogeny; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 million years ago. Cooling continued throughout the rest of the late Eocene into the Eocene-Oligocene transition. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. 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. 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. Another major contribution to the expansion of the ice sheet was the creation of the
Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, ca ...

Antarctic Circumpolar Current
. The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic along with creating
ocean gyre In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction de ...
s that result in the
upwelling Upwelling is an oceanographic phenomenon that involves wind Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. Winds are commonly classified by their scale (spatial), spatial ...

upwelling
of colder bottom waters. The issue with this hypothesis of the consideration of this being a factor for the Eocene-Oligocene transition is the timing of the creation of the circulation is uncertain. For
Drake Passage Image:Drake-Passage profile hg.png, Depth profile with salinity and temperature for surface The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spain and other Spanish speaking countries) is the body of water between South Americ ...
, sediments indicate the opening occurred ~41 million years ago while tectonics indicate that this occurred ~32 million years ago.


Flora

During the early-middle Eocene, forests covered most of the Earth including the poles. Tropical forests extended across much of modern Africa, South America, Central America, India, South-east Asia and China.  Paratropical forests grew over North America, Europe and Russia, with broad-leafed evergreen and broad-leafed deciduous forests at higher latitudes. Polar forests were quite extensive.
Fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the a ...

Fossil
s and even preserved remains of trees such as swamp cypress and
dawn redwood ''Metasequoia glyptostroboides'', the dawn redwood, is a fast-growing, endangered deciduous In the fields of horticulture and botany, the term ''deciduous'' (; ) means "falling off at maturity" and "tending to fall off", in reference to trees ...

dawn redwood
from the Eocene have been found on
Ellesmere Island Ellesmere Island (Inuktitut: ''Umingmak Nuna'', meaning "land of muskoxen"; french: Île d'Ellesmere) is Canada's northernmost and third largest island, and the tenth largest in the world. It comprises an area of , slightly smaller than Great ...
in the
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska (United States), Canada, Finland, Greenland (Danish Realm, Den ...

Arctic
. Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of
subtropical The subtropical zones or subtropics are geographical and climate zones located to the north and south of the Torrid Zone ''Torrid Zone'' is a 1940 adventure film Adventure films are a genre of film whose plots feature elements of travel. ...

subtropical
and even tropical trees and plants from the Eocene also have been found in Greenland and
Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik The Yup'ik or Yupiaq (sg & pl) and Yupiit or Yupiat (pl), also Central Alaskan Yup'ik, Central Yup'ik, Alaskan Yup'ik ( own name ''Yup'ik'' sg ''Yupiik'' dual ''Yupiit'' pl; rus ...
.
Tropical rainforest File:Koppen-Geiger Map Af present.svg, upright=1.8, Tropical rainforest climate zones (Af). Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average p ...

Tropical rainforest
s grew as far north as northern
North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to ...

North America
and
Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered from largest ...

Europe
.
Palm tree The Arecaceae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain th ...

Palm tree
s were growing as far north as Alaska and
northern Europe Northern Europe is a loosely defined Geography, geographical and cultural region in Europe. Narrower definitions may describe Northern Europe as being roughly north of the southern coast of the Baltic Sea, which is about 54°N, or may be based o ...
during the early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well. The earliest definitive ''
Eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of Flowering plant, flowering trees, shrubs or Mallee (habit), mallees in the Myrtaceae, myrtle Family (biology), family, Myrtaceae. Along with several other genera in the Tribe (biology), ...

Eucalyptus
'' fossils were dated from 51.9 Mya, and were found in the Laguna del Hunco deposit in
Chubut province Chubut ( cy, Talaith Chubut; es, Provincia del Chubut ) is a province A province is almost always an administrative division within a country or state. The term derives from the ancient Roman '' provincia'', which was the major territorial an ...
in
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is also bordered b ...
. 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 Poaceae () or Gramineae () is a large and nearly ubiquitous Family (biology), family of monocotyledonous flowering plants known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in l ...

grasses
were still confined to
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water ...

river
banks and
lake A lake is an area filled with water, localized in a Depression (geology), basin, surrounded by land, and set apart from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the World Ocean, oc ...

lake
shores, and had not yet expanded into
plain In geography, a plain is a flat expanse of land that generally does not change much in elevation. Plains occur as lowlands along valleys or on the doorsteps of mountains, as coastal plains, and as plateaus or uplands. In a valley, a plain i ...

plain
s and
savanna A savanna or savannah is a mixed woodland A woodland () is, in the broad sense, land covered with trees, or in a narrow sense, synonymous with wood (or in the U.S., the '' plurale tantum'' woods), a low-density forest A forest is an ...

savanna
s. The cooling also brought
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In ...

season
al changes.
Deciduous In the fields of horticulture Horticulture is the art of cultivating plants in gardens to produce food and medicinal ingredients, or for comfort and ornamental purposes. Horticulturists grow flowers, fruits and nuts, vegetables and herbs, as we ...
trees, better able to cope with large temperature changes, began to overtake
evergreen In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anci ...

evergreen
tropical species. By the end of the period, deciduous forests covered large parts of the northern continents, including North America,
Eurasia Eurasia () is the largest continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as contin ...

Eurasia
and the Arctic, and rainforests held on only in equatorial
South America South America is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered fr ...

South America
,
Africa Africa is the world's second-largest and second-most populous continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are ...

Africa
,
India India (Hindi: ), officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by population, second-most populous country, the List of countries and dependencies by area, seventh-largest ...

India
and
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, smal ...

Australia
.
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oce ...

Antarctica
began the Eocene fringed with a warm temperate to sub-tropical
rainforest Rainforests are characterized by a closed and continuous tree canopy Canopy may refer to: Plants * Canopy (biology), aboveground portion of plant community or crop (including forests) * Canopy (grape), aboveground portion of grapevine Religi ...

rainforest
. Pollen found in
Prydz Bay Prydz Bay is a deep embayment of Antarctica between the Lars Christensen Coast and Ingrid Christensen Coast. The Bay is at the downstream end of a giant glacial drainage systems that originates in the East Antarctic interior. The Lambert Glacier ...
from the Eocene suggest
taiga Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemis ...

taiga
forest existed there. It became much colder as the period progressed; the heat-loving tropical
flora Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plants. The corresponding term for animal life is ''fauna''. Flora, fauna, and other forms of kingdom ...
was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of
tundra In physical geography Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the ...

tundra
.


Fauna

During the Eocene, plants and marine faunas became quite modern. Many modern
bird Birds are a group of warm-blooded vertebrates constituting the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of indiv ...
orders first appeared in the Eocene. The Eocene oceans were warm and teeming with
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
and other sea life.


Mammals

The oldest known
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the a ...

fossil
s of most of the modern mammal orders appear within a brief period during the early Eocene. At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like
artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulate Ungulates (pronounced ) are members of a diverse clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms tha ...
s,
perissodactyls Odd-toed ungulates, mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammali ...
, and primates, had features like long, thin legs, feet, and hands capable of grasping, as well as differentiated teeth adapted for chewing. Insular dwarfism, Dwarf forms reigned. All the members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that 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 the heat. Both groups of modern ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like ''Mesonyx''. Early forms of many other modern mammalian orders appeared, including bats, Proboscidea, proboscidians (elephants), 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 southeast Asia. Marine fauna are best known from South Asia and the southeast United States. ''Basilosaurus'' is a very well-known Eocene whale, but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in cetaceans occurred. The first sirenians were evolving at this time, and would eventually evolve into the extant manatees and dugongs.


Birds

Eocene birds include some enigmatic groups with resemblances to modern forms, some of which continued from the Paleocene. Bird taxa of the Eocene include carnivorous psittaciforms, such as Messelasturidae, Halcyornithidae, large flightless forms such as ''Gastornis'' and ''Eleutherornis'', long legged falcon ''Masillaraptor'', ancient galliformes, galliforms such as Gallinuloides, Rail (bird), putative rail relatives of the family Songzia, Songziidae, Pelagornithidae, various pseudotooth birds such as ''Gigantornis'', the ibis relative ''Rhynchaeites'', primitive swifts of the genus ''Aegialornis'', and primitive penguins such as ''Archaeospheniscus'' and ''Inkayacu''.


Reptiles

Reptile fossils from this time, such as fossils of pythons and turtles, are abundant.


Insects and arachnids

Several rich fossil insect faunas are known from the Eocene, notably the Baltic amber found mainly along the south coast of the Baltic Sea, amber from the Paris Basin, France, the Fur Formation, Denmark, and the Bembridge Marls from the Isle of Wight, England. Insects found in Eocene deposits mostly belong to genera that exist today, though their range has often shifted since the Eocene. For instance the Bibionidae, bibionid genus ''Plecia'' is common in fossil faunas from presently temperate areas, but only lives in the tropics and subtropics today.


Gallery

File:Moeritherium lyonsi (fossil mammal) (Eocene) (32167459460).jpg, ''Moeritherium'' File:Hyracotherium vasacciense (fossil horse) (Huerfano Formation, Lower Eocene; Huerfano Basin, Colorado, USA) 3 (32380466091).jpg, ''Hyracotherium'' File:Brontotherium skull IMG 4441.jpg, ''Brontotherium'' File:Basilosaurus isis fossil, Nantes History Museum 03.jpg, ''Basilosaurus'' File:Andrewsarchus mongoliensis.jpg, ''Andrewsarchus'' File:Borealosuchus wilsoni (15529256785).jpg, ''Borealosuchus'' File:Diatrymaskeleton.JPG, ''Gastornis'' File:Pakicetus Canada.jpg, ''Pakicetus'' File:Hyracodon nebraskensis.jpg, ''Hyracodon'' File:Eocene Turtle Fossil.jpg, Eocene turtle fossil File:Leptictidium auderiense skeleton.JPG, ''Leptictidium'' File:Peratherium skull.jpg, ''Peratherium'' File:Hesperocyon skull Smithsonian.jpg, ''Hesperocyon'' File:Tritemnodon skull.jpg, ''Tritemnodon'' File:Coryphodon skull.jpg, ''Coryphodon'' File:Pseudocrypturus Smithsonian fossil.jpg, ''Pseudocrypturus''


See also

* Bolca in Italy * List of fossil sites ''(with link directory)'' * London Clay * Messel pit in Germany * Wadi El Hitan in Egypt


Notes


References


Further reading

* Ogg, Jim; June, 2004, ''Overview of Global Boundary Stratotype Sections and Points (GSSP's)'
Global Stratotype Sections and Points
Accessed April 30, 2006. *Stanley, Steven M. ''Earth System History.'' New York City, New York: W.H. Freeman and Company, 1999.


External links


PaleoMap ProjectEocene and Oligocene FossilsThe UPenn Fossil Forest Project, focusing on the Eocene polar forests in Ellesmere Island, CanadaEocene Microfossils: 60+ images of Foraminifera
*Eocene Epoch. (2011). In Encyclopædia Britannica. Retrieved fro
Eocene Epoch , geochronology
{{Authority control Eocene, Geological epochs Paleogene geochronology