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Silurian

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Silurian is a major division of the geologic timescale that extends
   from the end of the Ordovician period, about 443.7 ± 1.5 Ma (million
   years ago), to the beginning of the Devonian period, about 416.0 ± 2.8
   Ma (ICS 2004). As with other geologic periods, the rock beds that
   define the period's start and end are well identified, but the exact
   dates are uncertain by 5-10 million years. The base of the Silurian is
   set at a major extinction event when 60% of marine species were wiped
   out. See Ordovician-Silurian extinction events.
                         Paleozoic era
   Cambrian Ordovician Silurian Devonian Carboniferous Permian

Historiography

   The Silurian system was first identified by Sir Roderick Murchison, who
   was examining fossil-bearing sedimentary rock strata in south Wales in
   the early 1830s. He named the sequences for a Celtic tribe of Wales,
   the Silures, extending the convention his friend Adam Sedgwick had
   established for the Cambrian. In 1835 the two men presented a joint
   paper, under the title On the Silurian and Cambrian Systems, Exhibiting
   the Order in which the Older Sedimentary Strata Succeed each other in
   England and Wales, which was the germ of the modern geological time
   scale. As it was first identified, the "Silurian" series when traced
   farther afield quickly came to overlap Sedgwick's "Cambrian" sequence,
   however, provoking furious disagreements that ended the friendship.
   Charles Lapworth eventually resolved the conflict by defining a new
   Ordovician system including the contended beds.

Silurian subdivisions

   The Silurian Period of time is usually broken into early ( Llandovery
   and Wenlock) and late ( Ludlow and Pridoli) subdivisions ( epochs).
   Nevertheless, some schemes use an early (Llandovery), middle (Wenlock)
   and late (Ludlow and Pridoli) breakdown. These faunal stages are
   characterized by their index fossils, new species of colonial marine
   Graptolites that appeared in each. Epochs of time correspond to series
   of rocks (as periods of time correspond to systems of rocks), which are
   referred to as belonging to the lower, middle, or upper part of the
   rock column, analogous to early, middle, or late Silurian time. The
   epochs and stages from youngest to oldest are:
     * Pridoli Epoch - no stages defined (late Silurian)

     * Ludlow Epoch divided into
          + Ludfordian (late Ludlow - late Silurian)
          + Gorstian (early Ludlow - late Silurian)

     * Wenlock Epoch divided into
          + Homerian (late Wenlock - early or middle Silurian)
          + Sheinwoodian (early Wenlock - early or middle Silurian)

     * Llandovery Epoch divided into
          + Telychian (late Llandovery - early Silurian)
          + Aeronian (mid Llandovery - early Silurian)
          + Rhuddanian (early Llandovery - early Silurian)

   In North America a different suite of regional stages is used:
     * Cayugan (Late Silurian - Ludlow)
     * Lockportian (Middle Silurian - Wenlock)
     * Tonawandan (Middle Silurian - Wenlock)
     * Ontarian (Early Silurian - Llandovery)
     * Alexandrian (Early Silurian - Llandovery)

Silurian paleogeography

   During the Silurian, Gondwana continued a slow southward drift to high
   southern latitudes, but there is evidence that the Silurian icecaps
   were less extensive than those of the late Ordovician glaciation. The
   melting of icecaps and glaciers contributed to a rise in sea level,
   recognizable from the fact that Silurian sediments overlie eroded
   Ordovician sediments, forming an unconformity. Other cratons and
   continent fragments drifted together near the equator, starting the
   formation of a second supercontinent known as Euramerica.

   When the proto-Europe collided with North America, the collision folded
   coastal sediments that had been accumulating since the Cambrian off the
   east coast of North America and the west coast of Europe. This event is
   the Caledonian orogeny, a spate of mountain building that stretched
   from New York State through conjoined Europe and Greenland to Norway.
   At the end of the Silurian, sea levels dropped again, leaving telltale
   basins of evaporites in a basin extending from Michigan to West
   Virginia, and the new mountain ranges were rapidly eroded. The Teays
   River, flowing into the shallow mid-continental sea, eroded Ordovician
   strata, leaving traces in the Silurian strata of northern Ohio and
   Indiana.

   The vast ocean of Panthalassa covered most of the northern hemisphere.
   Other minor oceans include, Proto-Tethys, Paleo-Tethys, Rheic Ocean, a
   seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and
   newly formed Ural Ocean.

   During this period, the Earth entered a long warm greenhouse phase, and
   warm shallow seas covered much of the equatorial land masses. The
   period witnessed a relative stabilization of the Earth's general
   climate, ending the previous pattern of erratic climatic fluctuations.
   Layers of broken shells (called coquina) provide strong evidence of a
   climate dominated by violent storms generated then as now by warm sea
   surfaces.

Silurian biota

   Silurian high sea levels and warm shallow continental seas provided a
   hospitable environment for marine life of all kinds. Silurian beds are
   oil and gas producers in some areas. Extensive beds of Silurian
   hematite -- an iron ore -- in eastern North America were important to
   the early American colonial economy.

   Coral reefs made their first appearance during this time, built by
   extinct tabulate and rugose corals. The first bony fish, the
   Osteichthyes appeared, represented by the Acanthodians covered with
   bony scales; fishes reached considerable diversity and developed
   movable jaws, adapted from the supports of the front two or three gill
   arches. A diverse fauna of Eurypterus (Sea Scorpions) -- some of them
   several meters in length -- prowled the shallow Silurian seas of North
   America; many of their fossils have been found in New York State.
   Brachiopods, bryozoa, molluscs, and trilobites were abundant and
   diverse.

   Myriapods became the first proper terrestrial animals. The terrestrial
   ecosystems included the first multicellular terrestrial animals that
   have been identified, relatives of modern spiders and millipedes whose
   fossils were discovered in the 1990s.

Silurian flora

   The first fossil records of vascular plants, that is, land plants with
   tissues that carry food, appeared in the Silurian period. The earliest
   known representatives of this group are the Cooksonia (mostly from the
   northern hemisphere) and Baragwanathia (from Australia). A primitive
   Silurian land plant with xylem and phloem but no differentiation in
   root, stem or leaf, was much-branched Psylophyton, reproducing by
   spores and breathing through stomata on every surface, and probably
   photosynthesizing in every tissue exposed to light. Rhyniophyta and
   primitive lycopods were other land plants that first appear during this
   period.

Fungal life

   The first land fungi probably appeared by this time.
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