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Ordovician

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Ordovician period is the second of the six (seven in North America)
   periods of the Paleozoic era. It follows the Cambrian period and is
   followed by the Silurian period. The Ordovician, named after the Welsh
   tribe of the Ordovices, was defined by Charles Lapworth in 1879, to
   resolve a dispute between followers of Adam Sedgwick and Roderick
   Murchison, who were placing the same rock beds in northern Wales into
   the Cambrian and Silurian periods respectively. Lapworth, recognizing
   that the fossil fauna in the disputed strata were different from those
   of either the Cambrian or the Silurian periods, realized that they
   should be placed in a period of their own.

   While recognition of the distinct Ordovician period was slow in the
   United Kingdom, other areas of the world accepted it quickly. It
   received international sanction in 1906, when it was adopted as an
   official period of the Paleozoic era by the International Geological
   Congress.
                         Paleozoic era
   Cambrian Ordovician Silurian Devonian Carboniferous Permian

Ordovician dating

   The Ordovician period started at a minor extinction event some time
   about 488.3 million years ago ( mya) and lasted for about 44.6 million
   years. It ended with a major extinction event about 443.7 mya (ICS,
   2004) that wiped out 60% of marine genera. A. Melott et al. (ref. 2006)
   have suggested a ten-second gamma ray burst could have been
   responsible, destroying the ozone layer and exposing terrestrial and
   marine surface-dwelling life to radiation; most scientists continue to
   agree that extinction events are complex events with multiple causes.
   The dates given are recent radiometric dates and vary slightly from
   those used in other sources.This is the second period of the Paleozoic
   era.

   Ordovician rocks contain abundant fossils and contain major petroleum
   and gas reservoirs in some regions.

Ordovician subdivisions

   The Ordovician Period is usually broken into Early (Tremadoc and
   Arenig), Middle (Llanvirn [subdivided into Abereiddian and
   Llandeilian]) and Late (Caradoc and Ashgill) epochs. The corresponding
   rocks of the Ordovician System are referred to as coming from the
   Lower, Middle, or Upper part of the column. The Faunal stages
   (subdivisions of epochs) from youngest to oldest are:
     * Hirnantian/Gamach (Late-Ashgill)
     * Rawtheyan/Richmond (Late-Ashgill)
     * Cautleyan/Richmond (Late-Ashgill)
     * Pusgillian/Maysville/Richmond (Late-Ashgill)

     * Trenton (Middle-Caradoc)
     * Onnian/Maysville/Eden (Middle-Caradoc)
     * Actonian/Eden (Middle-Caradoc)
     * Marshbrookian/Sherman (Middle-Caradoc)
     * Longvillian/Sherman (Middle-Caradoc)
     * Soundleyan/Kirkfield (Middle-Caradoc)
     * Harnagian/Rockland (Middle-Caradoc)
     * Costonian/Black River (Middle-Caradoc)
     * Chazy (Middle-Llandeilo)
     * Llandeilo (Middle-Llandeilo)
     * Whiterock (Middle-Llanvirn)
     * Llanvirn (Middle-Llanvirn)

     * Cassinian (Early-Arenig)
     * Arenig/Jefferson/Castleman (Early-Arenig)
     * Tremadoc/Deming/Gaconadian (Early-Tremadoc)

Ordovician paleogeography

   Sea levels were high during the Ordovician; in fact during the
   Tremadocian, marine transgressions worldwide were the greatest for
   which evidence is preserved in the rocks.

   During the Ordovician, the southern continents were collected into a
   single continent called Gondwana. Gondwana started the period in
   equatorial latitudes and, as the period progressed, drifted toward the
   South Pole. The Early Ordovician was thought to be quite warm, at least
   in the tropics. As with North America and Europe, Gondwana was largely
   covered with shallow seas during the Ordovician. Shallow clear waters
   over continental shelves encouraged the growth of organisms that
   deposit calcium carbonates in their shells and hard parts. Panthalassic
   Ocean covered much of the northern hemisphere, and other minor oceans
   included Proto-Tethys, Paleo-Tethys, Khanty Ocean which was closed off
   by the Late Ordovician, Iapetus Ocean, and the new Rheic Ocean.

   Ordovician rocks are chiefly sedimentary. Because of the restricted
   area and low elevation of solid land, which set limits to erosion,
   marine sediments that make up a large part of the Ordovician system
   consist chiefly of limestone. Shale and sandstone are less conspicuous.

   A major mountain-building episode was the Taconic orogeny that was well
   under way in Cambrian times.

   By the end of the period, Gondwana had neared or approached the pole
   and was largely glaciated.

Ordovician Life

Ordovician fauna

   In North America and Europe, the Ordovician was a time of shallow
   continental seas rich in life. Trilobites and brachiopods in particular
   were rich and diverse. The first bryozoa appeared in the Ordovician as
   did the first coral reefs. Solitary corals date back to at least the
   Cambrian. Molluscs, which had also appeared during the Cambrian, became
   common and varied, especially bivalves, gastropods, and nautiloid
   cephalopods. It was long thought that the first true vertebrates (fish
   - Ostracoderms) appeared in the Ordovician, but recent discoveries in
   China reveal that they probably originated in the Early Cambrian. The
   very first jawed fish appeared in the Late Ordovician epoch.
   Now-extinct marine animals called graptolites thrived in the oceans.
   Some cystoids and crinoids appeared.

Ordovician flora

   The first terrestrial plants appeared in the form of tiny plants
   resembling liverworts. There were pollen fossils found in the Latest
   Ordovician. Plants probably evolved from green algae. Green algae were
   common in Ordovician and Late Cambrian (perhaps earlier).

Fungal life

   The very first land fungi probably appeared in the Latest Ordovician,
   following the plants, even there are no fossil spores in this time.
   However, marine fungi were abundant in the Ordovician seas to decompose
   animal carcasses, and other wastes.

End of the Ordovician

   The Ordovician came to a close in a series of extinction events that,
   taken together, comprise the second largest of the five major
   extinction events in Earth's history in terms of percentage of genera
   that went extinct. The only larger one was the Permian-Triassic
   extinction event.

   The extinctions occurred approximately 444-447 million years ago and
   mark the boundary between the Ordovician and the following Silurian
   Period. At that time all complex multicellular organisms lived in the
   sea, and about 49% of genera of fauna disappeared forever; brachiopods
   and bryozoans were decimated, along with many of the trilobite,
   conodont and graptolite families.

   The most commonly accepted theory is that these events were triggered
   by the onset of an ice age, in the Hirnantian faunal stage that ended
   the long, stable greenhouse conditions typical of the Ordovician. The
   ice age was probably not as long-lasting as once thought; study of
   oxygen isotopes in fossil brachiopods shows that it was probably no
   longer than 0.5 to 1.5 million years (Stanley, 358). The event was
   preceded by a fall in atmospheric carbon dioxide (from 7000ppm to
   4400ppm) which selectively affected the shallow seas where most
   organisms lived. As the southern supercontinent Gondwana drifted over
   the South Pole, ice caps formed on it, which have been detected in
   Upper Ordovician rock strata of North Africa and then-adjacent
   northeastern South America, which were south-polar locations at the
   time.

   Glaciation locks up water from the world-ocean, and the interglacials
   free it, causing sea levels repeatedly to drop and rise; the vast
   shallow intra-continental Ordovician seas withdrew, which eliminated
   many ecological niches, then returned carrying diminished founder
   populations lacking many whole families of organisms, then withdrew
   again with the next pulse of glaciation, eliminating biological
   diversity at each change (Emiliani, 1992 p. 491). Species limited to a
   single epicontinental sea on a given landmass were severely affected
   (Stanley, 360). Tropical lifeforms were hit particularly hard in the
   first wave of extinction, while cool-water species were hit worst in
   the second pulse (Stanley, 360).

   Surviving species were those that coped with the changed conditions and
   filled the ecological niches left by the extinctions.

   At the end of the second event, melting glaciers caused the sea level
   to rise and stabilise once more. The rebound of life's diversity with
   the permanent re-flooding of continental shelves at the onset of the
   Silurian saw increased biodiversity within the surviving Orders.
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