Eocene

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Eocene_table_infobox_0

EoceneEocene_header_cell_0_0_0
ChronologyEocene_header_cell_0_1_0
EtymologyEocene_header_cell_0_2_0
Name formalityEocene_header_cell_0_3_0 FormalEocene_cell_0_3_1
Usage InformationEocene_header_cell_0_4_0
Celestial bodyEocene_header_cell_0_5_0 EarthEocene_cell_0_5_1
UsageEocene_header_cell_0_6_0 Global (ICS)Eocene_cell_0_6_1
Time scale(s) usedEocene_header_cell_0_7_0 ICS Time ScaleEocene_cell_0_7_1
DefinitionEocene_header_cell_0_8_0
Chronological unitEocene_header_cell_0_9_0 EpochEocene_cell_0_9_1
Stratigraphic unitEocene_header_cell_0_10_0 SeriesEocene_cell_0_10_1
Time span formalityEocene_header_cell_0_11_0 FormalEocene_cell_0_11_1
Lower boundary definitionEocene_header_cell_0_12_0 Base of negative Carbon Isotope Excursion (CIE).Eocene_cell_0_12_1
Lower boundary GSSPEocene_header_cell_0_13_0 Dababiya section, Luxor, EgyptEocene_cell_0_13_1
GSSP ratifiedEocene_header_cell_0_14_0 2003Eocene_cell_0_14_1
Upper boundary definitionEocene_header_cell_0_15_0 LAD of Planktonic Foraminifers Hantkenina and CribrohantkeninaEocene_cell_0_15_1
Upper boundary GSSPEocene_header_cell_0_16_0 Massignano quarry section, Massignano, Ancona, ItalyEocene_cell_0_16_1
GSSP ratifiedEocene_header_cell_0_17_0 1992Eocene_cell_0_17_1

The Eocene ( /ˈiː.əˌsiːn, ˈiː.oʊ-/ EE-ə-seen, EE-oh-) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). Eocene_sentence_0

It is the second epoch of the Paleogene Period in the modern Cenozoic Era. Eocene_sentence_1

The name Eocene comes from the Ancient Greek ἠώς (ēṓs, "dawn") and καινός (kainós, "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch. Eocene_sentence_2

The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. Eocene_sentence_3

The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C. Eocene_sentence_4

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. Eocene_sentence_5

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. Eocene_sentence_6

Etymology Eocene_section_0

The term "Eocene" is derived from Ancient Greek eo—eos ἠώς meaning "dawn", and—cene kainos καινός meaning "new" or "recent", as the epoch saw the dawn of recent, or modern, life. Eocene_sentence_7

Scottish geologist Charles Lyell (ignoring the Quaternary) had divided the Tertiary epoch into the Eocene, Miocene, Pliocene, and New Pliocene (Holocene) periods in 1833. Eocene_sentence_8

British geologist John Phillips had proposed the Cenozoic in 1840 in place of the Tertiary, and Austrian paleontologist Moritz Hörnes had introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853. Eocene_sentence_9

After decades of inconsistent usage, the newly formed International Commission on Stratigraphy (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. Eocene_sentence_10

In 1978, the Paleogene was officially defined as the Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs. Eocene_sentence_11

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. Eocene_sentence_12

Geology Eocene_section_1

Boundaries Eocene_section_2

The beginning of the Eocene is marked by the Paleocene–Eocene Thermal Maximum, a short period of intense warming and ocean acidification 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–single-celled species which are used as bioindicators of the health of a marine ecosystem—one of the largest in the Cenozoic. Eocene_sentence_13

This event happened around 55.8 mya, and was one of the most significant periods of global change during the Cenozoic. Eocene_sentence_14

The end of the Eocene was marked by the Eocene–Oligocene extinction event, also known as the Grande Coupure. Eocene_sentence_15

Stratigraphy Eocene_section_3

The Eocene is conventionally divided into early (56–47.8 million years ago), middle (47.8–38m), and late (38–33.9m) subdivisions. Eocene_sentence_16

The corresponding rocks are referred to as lower, middle, and upper Eocene. Eocene_sentence_17

The Ypresian stage constitutes the lower, the Priabonian stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene. Eocene_sentence_18

Palaeogeography Eocene_section_4

During the Eocene, the continents continued to drift toward their present positions. Eocene_sentence_19

At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the planet and keeping global temperatures high, but when Australia split from the southern continent around 45 Ma, the warm equatorial currents were routed away from Antarctica. Eocene_sentence_20

An isolated cold water channel developed between the two continents. Eocene_sentence_21

The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north, reinforcing the cooling. Eocene_sentence_22

The northern supercontinent of Laurasia began to fragment, as Europe, Greenland and North America drifted apart. Eocene_sentence_23

In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte. Eocene_sentence_24

At about 35 Ma, an asteroid impact on the eastern coast of North America formed the Chesapeake Bay impact crater. Eocene_sentence_25

In Europe, the Tethys Sea finally disappeared, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Eocene_sentence_26

Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. Eocene_sentence_27

India began its collision with Asia, folding to initiate formation of the Himalayas. Eocene_sentence_28

It is hypothesized that the Eocene hothouse world was caused by runaway global warming from released methane clathrates deep in the oceans. Eocene_sentence_29

The clathrates were buried beneath mud that was disturbed as the oceans warmed. Eocene_sentence_30

Methane (CH4) has ten to twenty times the greenhouse gas effect of carbon dioxide (CO2). Eocene_sentence_31

Climate Eocene_section_5

The Eocene Epoch contained a wide variety of different climate conditions that includes the warmest climate in the Cenozoic Era and ends in an icehouse climate. Eocene_sentence_32

The evolution of the Eocene climate began with warming after the end of the Paleocene–Eocene Thermal Maximum (PETM) at 56 million years ago to a maximum during the Eocene Optimum at around 49 million years ago. Eocene_sentence_33

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. Eocene_sentence_34

Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. Eocene_sentence_35

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 began to rapidly expand. Eocene_sentence_36

Atmospheric greenhouse gas evolution Eocene_section_6

Greenhouse gases, in particular carbon dioxide and methane, played a significant role during the Eocene in controlling the surface temperature. Eocene_sentence_37

The end of the PETM was met with very large sequestration of carbon dioxide into the forms of methane clathrate, coal, and crude oil at the bottom of the Arctic Ocean, that reduced the atmospheric carbon dioxide. Eocene_sentence_38

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. Eocene_sentence_39

For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. Eocene_sentence_40

This is due to numerous proxies representing different atmospheric carbon dioxide content. Eocene_sentence_41

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. Eocene_sentence_42

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. Eocene_sentence_43

For contrast, today the carbon dioxide levels are at 400 ppm or 0.04%. Eocene_sentence_44

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. Eocene_sentence_45

During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. Eocene_sentence_46

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). Eocene_sentence_47

Most of the methane released to the atmosphere during this period of time would have been from wetlands, swamps, and forests. Eocene_sentence_48

The atmospheric methane concentration today is 0.000179% or 1.79 ppmv. Eocene_sentence_49

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. Eocene_sentence_50

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. Eocene_sentence_51

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. Eocene_sentence_52

Biogenic production of methane produces carbon dioxide and water vapor along with the methane, as well as yielding infrared radiation. Eocene_sentence_53

The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. Eocene_sentence_54

The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Eocene_sentence_55

Water vapor traps more infrared than does carbon dioxide. Eocene_sentence_56

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. Eocene_sentence_57

At the end of the Eocene Optimum, carbon dioxide began decreasing due to increased siliceous plankton productivity and marine carbon burial. Eocene_sentence_58

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. Eocene_sentence_59

With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of azolla, which is a floating aquatic fern, on the Arctic Ocean. Eocene_sentence_60

Compared to current carbon dioxide levels, these azolla grew rapidly in the enhanced carbon dioxide levels found in the early Eocene. Eocene_sentence_61

As these azolla sank into the Arctic Ocean, they became buried and sequestered their carbon into the seabed. Eocene_sentence_62

This event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Eocene_sentence_63

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. Eocene_sentence_64

Another event during the middle Eocene that was a sudden and temporary reversal of the cooling conditions was the Middle Eocene Climatic Optimum. Eocene_sentence_65

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. Eocene_sentence_66

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. Eocene_sentence_67

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. Eocene_sentence_68

Another hypothesis involves extensive sea floor rifting and metamorphic decarbonation reactions releasing considerable amounts of carbon dioxide to the atmosphere. Eocene_sentence_69

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. Eocene_sentence_70

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. Eocene_sentence_71

Early Eocene and the equable climate problem Eocene_section_7

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. Eocene_sentence_72

A multitude of proxies support the presence of a warmer equable climate being present during this period of time. Eocene_sentence_73

A few of these proxies include the presence of fossils native to warm climates, such as crocodiles, located in the higher latitudes, the presence in the high latitudes of frost-intolerant flora such as palm trees which cannot survive during sustained freezes, and fossils of snakes found in the tropics that would require much higher average temperatures to sustain them. Eocene_sentence_74

Using isotope proxies to determine ocean temperatures indicates sea surface temperatures in the tropics as high as 35 °C (95 °F) and, relative to present-day values, bottom water temperatures that are 10 °C (18 °F) higher. Eocene_sentence_75

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. Eocene_sentence_76

An issue arises, however, when trying to model the Eocene and reproduce the results that are found with the proxy data. Eocene_sentence_77

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. Eocene_sentence_78

The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to 20 °C (36 °F) colder than the actual determined temperature at the poles. Eocene_sentence_79

This error has been classified as the “equable climate problem”. Eocene_sentence_80

To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Eocene_sentence_81

Some hypotheses and tests which attempt to find the process are listed below. Eocene_sentence_82

Large lakes Eocene_section_8

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. Eocene_sentence_83

In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes. Eocene_sentence_84

To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. Eocene_sentence_85

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. Eocene_sentence_86

Ocean heat transport Eocene_section_9

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. Eocene_sentence_87

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. Eocene_sentence_88

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. Eocene_sentence_89

Orbital parameters Eocene_section_10

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. Eocene_sentence_90

Simulating the Eocene by using an ice free planet, eccentricity, obliquity, and precession were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. Eocene_sentence_91

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%. Eocene_sentence_92

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. Eocene_sentence_93

Polar stratospheric clouds Eocene_section_11

Another method considered for producing the warm polar temperatures were polar stratospheric clouds. Eocene_sentence_94

Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Eocene_sentence_95

Polar stratospheric clouds have a great impact on radiative forcing. Eocene_sentence_96

Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and traps outgoing longwave radiation. Eocene_sentence_97

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). Eocene_sentence_98

Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Eocene_sentence_99

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. Eocene_sentence_100

When methane is oxidized, a significant amount of water vapor is released. Eocene_sentence_101

Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Eocene_sentence_102

Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. Eocene_sentence_103

With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions. Eocene_sentence_104

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. Eocene_sentence_105

The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. Eocene_sentence_106

A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Eocene_sentence_107

Any ice growth was slowed immensely and would lead to any present ice melting. Eocene_sentence_108

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. Eocene_sentence_109

Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase the amount of polar stratospheric clouds. Eocene_sentence_110

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. Eocene_sentence_111

Separate model runs were used to determine the sustainability of the polar stratospheric clouds. Eocene_sentence_112

It was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. Eocene_sentence_113

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. Eocene_sentence_114

Hyperthermals through the early Eocene Eocene_section_12

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 composition in the ocean. Eocene_sentence_115

These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of 4–8 °C (7.2–14.4 °F) at the surface of the ocean. Eocene_sentence_116

These hyperthermals led to increased perturbations in planktonic and benthic foraminifera, with a higher rate of sedimentation as a consequence of the warmer temperatures. Eocene_sentence_117

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. Eocene_sentence_118

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. Eocene_sentence_119

Greenhouse to icehouse climate Eocene_section_13

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. Eocene_sentence_120

The transition from a warming climate into a cooling climate began at around 49 million years ago. Eocene_sentence_121

Isotopes of carbon and oxygen indicate a shift to a global cooling climate. Eocene_sentence_122

The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. Eocene_sentence_123

One proposed cause of the reduction in carbon dioxide during the warming to cooling transition was the azolla event. Eocene_sentence_124

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 blooms across the Arctic Ocean. Eocene_sentence_125

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. Eocene_sentence_126

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. Eocene_sentence_127

Cooling after this event continued due to continual decrease in atmospheric carbon dioxide from organic productivity and weathering from mountain building. Eocene_sentence_128

Global cooling continued until there was a major reversal from cooling to warming indicated in the Southern Ocean at around 42–41 million years ago. Eocene_sentence_129

Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. Eocene_sentence_130

This warming event is known as the Middle Eocene Climatic Optimum. Eocene_sentence_131

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. Eocene_sentence_132

The increase in atmospheric carbon dioxide is considered to be due to increased seafloor spreading rates between Australia and Antarctica and increased amounts of volcanism in the region. Eocene_sentence_133

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. Eocene_sentence_134

Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. Eocene_sentence_135

This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 million years ago. Eocene_sentence_136

Cooling continued throughout the rest of the late Eocene into the Eocene-Oligocene transition. Eocene_sentence_137

During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. Eocene_sentence_138

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. Eocene_sentence_139

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. Eocene_sentence_140

Another major contribution to the expansion of the ice sheet was the creation of the Antarctic Circumpolar Current. Eocene_sentence_141

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 gyres that result in the upwelling of colder bottom waters. Eocene_sentence_142

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. Eocene_sentence_143

For Drake Passage, sediments indicate the opening occurred ~41 million years ago while tectonics indicate that this occurred ~32 million years ago. Eocene_sentence_144

Flora Eocene_section_14

At the beginning of the Eocene, the high temperatures and warm oceans created a moist, balmy environment, with forests spreading throughout the Earth from pole to pole. Eocene_sentence_145

Apart from the driest deserts, Earth must have been entirely covered in forests. Eocene_sentence_146

Polar forests were quite extensive. Eocene_sentence_147

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. Eocene_sentence_148

Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Eocene_sentence_149

Fossils of subtropical and even tropical trees and plants from the Eocene also have been found in Greenland and Alaska. Eocene_sentence_150

Tropical rainforests grew as far north as northern North America and Europe. Eocene_sentence_151

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. Eocene_sentence_152

Dawn redwoods were far more extensive as well. Eocene_sentence_153

The earliest definitive Eucalyptus fossils were dated from 51.9 Mya, and were found in the Laguna del Hunco deposit in Chubut province in Argentina. Eocene_sentence_154

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. Eocene_sentence_155

The newly evolved grasses were still confined to river banks and lake shores, and had not yet expanded into plains and savannas. Eocene_sentence_156

The cooling also brought seasonal changes. Eocene_sentence_157

Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. Eocene_sentence_158

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. Eocene_sentence_159

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. Eocene_sentence_160

Fauna Eocene_section_15

During the Eocene, plants and marine faunas became quite modern. Eocene_sentence_161

Many modern bird orders first appeared in the Eocene. Eocene_sentence_162

The Eocene oceans were warm and teeming with fish and other sea life. Eocene_sentence_163

Mammals Eocene_section_16

The oldest known fossils of most of the modern mammal orders appear within a brief period during the early Eocene. Eocene_sentence_164

At the beginning of the Eocene, several new mammal groups arrived in North America. Eocene_sentence_165

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. Eocene_sentence_166

Dwarf forms reigned. Eocene_sentence_167

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. Eocene_sentence_168

They were also smaller than the mammals that followed them. Eocene_sentence_169

It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat. Eocene_sentence_170

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. Eocene_sentence_171

Early forms of many other modern mammalian orders appeared, including bats, proboscidians (elephants), primates, rodents, and marsupials. Eocene_sentence_172

Older primitive forms of mammals declined in variety and importance. Eocene_sentence_173

Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt, and southeast Asia. Eocene_sentence_174

Marine fauna are best known from South Asia and the southeast United States. Eocene_sentence_175

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. Eocene_sentence_176

The first sirenians were evolving at this time, and would eventually evolve into the extant manatees and dugongs. Eocene_sentence_177

Birds Eocene_section_17

Reptiles Eocene_section_18

Reptile fossils from this time, such as fossils of pythons and turtles, are abundant. Eocene_sentence_178

The remains of Titanoboa, a snake recorded as attaining up to 12.8 m (42 ft) in length, was discovered in South America along with other large reptilian megafauna. Eocene_sentence_179

Insects and arachnids Eocene_section_19

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. Eocene_sentence_180

Insects found in Eocene deposits mostly belong to genera that exist today, though their range has often shifted since the Eocene. Eocene_sentence_181

For instance the bibionid genus Plecia is common in fossil faunas from presently temperate areas, but only lives in the tropics and subtropics today. Eocene_sentence_182


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Eocene.