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Eocene

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Eocene epoch (56-34 Ma) is a major division of the geologic
   timescale and the second epoch of the Palaeogene period in the Cenozoic
   era. The Eocene spans the time from the end of the Paleocene epoch to
   the beginning of the Oligocene epoch. The start of the Eocene is marked
   by the emergence of the first modern mammals. The end is set at a major
   extinction event called Grande Coupure (the "Great Break" in
   continuity), which may be related to the impact of one or more large
   bolides in Siberia and in what is now Chesapeake Bay. As with other
   geologic periods, the strata that define the start and end of the epoch
   are well identified, though their exact dates are slightly uncertain.

   The name Eocene comes from the Greek eos (dawn) and ceno (new) and
   refers to the "dawn" of modern ('new') mammalian fauna that appeared
   during the epoch.
                        Paleogene period
    Paleocene epoch       Eocene epoch        Oligocene epoch
   Danian | Selandian
   Thanetian         Ypresian | Lutetian
                     Bartonian | Priabonian Rupelian | Chattian

Eocene subdivisions

   The Eocene is usually broken into lower and upper subdivisions. The
   Faunal stages from youngest to oldest are:
   Priabonian (37.2 ± 0.1 – 33.9 ± 0.1 Ma)
   Bartonian  (40.4 ± 0.2 – 37.2 ± 0.1 Ma)
   Lutetian   (48.6 ± 0.2 – 40.4 ± 0.2 Ma)
   Ypresian   (55.8 ± 0.2 – 48.6 ± 0.2 Ma)

Eocene climate

   Marking the start of the Eocene, the planet heated up in one of the
   most rapid (in geologic terms) and extreme global warming events
   recorded in geologic history, called the Paleocene-Eocene Thermal
   Maximum or Initial Eocene Thermal Maximum (PETM or IETM). This was an
   episode of rapid and intense warming (up to 7°C at high latitudes) that
   lasted less than 100,000 years . The Thermal Maximum provoked a sharp
   extinction event that distinguishes Eocene fauna from the ecosystems of
   the Paleocene.

   The Eocene global climate was perhaps the most homogeneous of the
   Cenozoic; the temperature gradient from equator to pole was only half
   that of today's, and deep ocean currents were exceptionally warm. The
   polar regions were much warmer than today, perhaps as mild as the
   modern-day Pacific Northwest; that warm temperate forests extended
   right to the poles, while rainy tropical climates extended as far north
   as 45 degrees latitude away from the Equator. The difference was
   greatest in the temperate latitudes; the climate in the tropics
   however, was probably similar to today's.(Stanley, 508)

   Climates remained warm through the rest of the Eocene, although slow
   global cooling, which eventually led to the Pleistocene glaciations,
   started around the end of the epoch as ocean currents around Antarctica
   cooled.

Eocene paleogeography

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

   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 world and keeping global
   temperatures high. But when Australia split from the southern continent
   around 45 mya, the warm equatorial currents were deflected away from
   Antarctica, and an isolated cold water channel developed between the
   two continents. The Antarctic region cooled down, and the ocean
   surrounding Antarctica began to freeze, sending cold water and icefloes
   north, reinforcing the cooling.

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

   In western North America, mountain building started in the Eocene, and
   huge lakes formed in the high flat basins among uplifts.

   Europe saw the Tethys Sea finally vanish, while the uplift of the Alps
   isolated its final remnant, the Mediterranean, and created another
   shallow sea with island archipelagos to the north. Though the North
   Atlantic was opening, a land connection appears to have remained
   between North America and Europe, as the faunas of the two regions are
   very similar.

   India continued its journey away from Africa, and began its collision
   with Asia, folding the Himalayas into existence.
     * Detailed maps of Tertiary Western North America: Eocene
     * Map of Eocene Earth

   It is hypothesized that the Eocene hothouse world was due to runaway
   global warming from released methane clathrates deep in the oceans. The
   clathrates were buried beneath mud that was disturbed as the oceans
   warmed. Methane (CH[4]) has ten to twenty times the greenhouse gas
   effect of carbon dioxide (CO[2]).

Eocene flora

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

   Polar forests were quite extensive. Fossils and even preserved remains
   of trees such as swamp cypress and dawn redwood from the Eocene have
   been found in Ellesmere Island in the Canadian Arctic. As
   aforementioned, the preserved remains found in the Canadian Arctic are
   not fossils, but actual pieces preserved in oxygen-poor water in the
   swampy forests of the time, and then buried before they had the chance
   to decompose. Even at that time, Ellesmere Island was only a few
   degrees in latitude further south than it is today. Fossils of
   subtropical and even tropical trees and plants from the Eocene have
   also been found in Greenland and Alaska. Tropical rainforests grew as
   far north as the Pacific Northwest and Europe.

   Palm trees were growing as far north as Alaska and northern Europe
   during the early Eocene, although they became less and less abundant as
   the climate cooled. Dawn redwoods were far more extensive as well.

   Cooling began mid-period, and by the end of the Eocene continental
   interiors had begun to dry out, with forests thinning out considerably
   in some areas. The newly-evolved grasses were still confined to river
   banks and lake edges, and had not yet expanded into plains and
   savannas.

   The cooling also brought seasonal changes. Deciduous trees, better able
   to cope with large temperature changes, began to overtake evergreen
   tropical species. By the end of the period, deciduous forests covered
   large parts of the northern continents, including North America,
   Eurasia and the Arctic, and rainforests held on only in equatorial
   South America, Africa, India and Australia.

   Antarctica, which began the Eocene fringed with a warm temperate to
   sub-tropical rainforest, became much colder as the period progressed;
   the heat-loving tropical flora was wiped out, and by the beginning of
   the Oligocene the continent hosted deciduous forests and vast stretches
   of tundra.

Eocene fauna

   Mesonyx, a carnivorous ungulate
   Enlarge
   Mesonyx, a carnivorous ungulate

   The oldest known fossils 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 artiodactyls, perissodactyls and primates, had
   features like long, thin legs, feet and hands capable of grasping, as
   well as differentiated teeth adapted for chewing. Dwarf forms reigned.
   All the members of the new mammal orders were small, under 10 kg; based
   on comparisons of tooth size, Eocene mammals were only 60 per cent of
   the size of the primitive Paleocene mammals that had preceded them.
   They were also smaller than the mammals that followed them. It is
   assumed that the hot Eocene temperatures favored smaller animals that
   were better able to manage heat.

   Both groups of modern ungulates (hoofed animals) became prevalent due
   to a major radiation between Europe and North America; along with
   carnivourous ungulates like Mesonyx. Early forms of many other modern
   mammalian orders appeared, including bats, proboscidians, primates,
   rodents and marsupials. Older primitive forms of mammals declined in
   variety and importance. Important Eocene land fauna fossil remains have
   been found in western North America, Europe, Patagonia, Egypt and
   South-East Asia. Marine fauna are best known from South Asia and the
   southeast United States.

   Reptile fossils are also known from the Eocene, such as the fearsomely
   enormous crocodile Deinosuchus, which lived as far north as Wyoming
   during the Eocene and grew much larger than the modern-day saltwater
   crocodile. Python fossils and turtle fossils are also known from North
   America.

   During the Eocene plants and marine faunas became quite modern. Many
   modern bird orders first appear in the Eocene.

Eocene oceans

   The Eocene oceans were warm and teeming with fish and other sea life.
   The first Carcharinid sharks appeared, as did early marine mammals,
   including Basilosaurus, an early species of whale that is thought to be
   descended from land animals, the hoofed predators called mesonychids,
   of which Mesonyx was a member. The first sirenians, relatives of the
   elephants, also appeared at this time.

The Grande Coupure

   The Grande Coupure, or "great break" in continuity, with a major
   European turnover in mammalian fauna about 33.5 Ma, marks the end of
   the last phase of Eocene assemblages, the Priabonian, and the arrival
   in Europe of Asian immigrants. The Grande Coupure is characterized by
   widespread extinctions and allopatric speciation in small isolated
   relict populations. It was given its name in 1910 by the Swiss
   palaeontologist Hans Georg Stehlin, to characterise the dramatic
   turnover of European mammalian fauna, which he placed at the
   Eocene-Oligocene boundary. A comparable turnover in Asian fauna has
   since been called the "Mongolian Remodelling".

   The Grande Coupure marks a break between endemic European faunas before
   the break and mixed faunas with a strong Asian component afterwards.
   J.J. Hooker and his team summarized the break:

          "Pre-Grande Coupure faunas are dominated by the perissodactyl
          family Palaeotheriidae (distant horse relatives), six families
          of artiodactyls (cloven-hoofed mammals) (Anoplotheriidae,
          Xiphodontidae, Choeropotamidae, Cebochoeridae, Dichobunidae and
          Amphimerycidae), the rodent family Pseudosciuridae, the primate
          families Omomyidae and Adapidae, and the archontan family
          Nyctitheriidae.

          "Post-Grande Coupure faunas include the true rhinos (family
          Rhinocerotidae), three artiodactyl families (Entelodontidae,
          Anthracotheriidae and Gelocidae) related respectively to pigs,
          hippos and ruminants, the rodent families Eomyidae, Cricetidae
          (hamsters) and Castoridae (beavers), and the lipotyphlan family
          Erinaceidae ( hedgehogs). The speciose genus Palaeotherium plus
          Anoplotherium and the families Xiphodontidae and Amphimerycidae
          were observed to disappear completely.

          "Only the marsupial family Herpetotheriidae, the artiodactyl
          family Cainotheriidae, and the rodent families Theridomyidae and
          Gliridae ( dormice) crossed the faunal divide undiminished."
          (Hooker et al. 2004)

   Whether this abrupt change was caused by climate change associated with
   the earliest polar glaciations and a major fall in sea levels, or by
   competition with taxa dispersing from Asia, few would argue for an
   isolated single cause. More spectacular causes are related to the
   impact of one or more large bolides in Siberia and in the Chesapeake
   Bay impact crater. Improved correlation of northwest European
   successions to global events (Hooker et al. 2004) confirms the Grande
   Coupure as occuring in the earliest Oligocene, with a hiatus of about
   350 ka prior to the first record of post-Grande Coupure Asian immigrant
   taxa.

   An element of the paradigm of the Grande Coupure was the apparent
   extinction of all European primates at the Coupure: the recent
   discovery of a mouse-sized early Oligocene omomyid, reflecting the
   better survival chances of small mammals, further undercut the Grand
   Coupure paradigm.

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