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Devonian

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Disambiguation: "Devonian" is sometimes used to refer to the
   Southwestern Brythonic language, and the people of the county of Devon
   are sometimes referred to as "Devonians"

   The Devonian is a geologic period of the Paleozoic era. It is named
   after Devon, England, where rocks from this period were first studied.

Important events

   During the Devonian Period, which occured in the Paleozoic era, the
   first fish evolved legs and started to walk on land as tetrapods, and
   the first arthropods like insects and spiders also started to colonize
   terrestrial habitats.

   The first seed-bearing plants spread across dry land, forming huge
   forests. In the oceans, fish diversified into the first sharks, and the
   first lobe-finned and bony fish. The first ammonite mollusks appeared,
   and trilobites, the mollusc-like brachiopods, as well as great coral
   reefs were still common. The Late Devonian extinction severely affected
   marine life.

   The paleogeography was dominated by the supercontinent of Gondwana to
   the south, the continent of Siberia to the north, and the early
   formation of the small supercontinent of Euramerica in the middle.
                         Paleozoic era
   Cambrian Ordovician Silurian Devonian Carboniferous Permian

Naming

   The period is named after Devon, a county in southwestern England,
   where Devonian outcrops are common. While the rock beds that define the
   start and end of the period are well identified, the exact dates are
   uncertain. According to the International Commission on Stratigraphy
   (Ogg, 2004), the Devonian extends from the end of the Silurian Period
   416.0 ± 2.8 million years ago (Mya), to the beginning of the
   Carboniferous Period 359.2 ± 2.5 Mya (in North America, the beginning
   of the Mississippian subperiod of the Carboniferous) (ICS 2004).

   The Devonian is also known as the Age of Fishes, but the term is out of
   favour. While fish underwent a major radiation it was only one of
   several major evolutionary landmarks during the period, and other
   lifeforms were more common.

   The Devonian has also erroneously been characterized as a Greenhouse
   Age, due to sampling bias: most of the early Devonian-age discoveries
   came from the strata of western Europe and eastern North America, which
   at the time straddled the Equator as part of the supercontinent of
   Euramerica where fossil signatures of widespread reefs indicate
   tropical climates that were warm and moderately humid.

   In nineteenth-century texts the Devonian has been called the Old Red
   Age, after the red and brown terrestrial deposits known in the United
   Kingdom as the " Old Red Sandstone" in which these early discoveries
   were found.

Devonian subdivisions

   The Devonian is usually broken into Early, Middle, and Late
   subdivisions. The rocks corresponding to these epochs are referred to
   as belonging to the lower, middle, and upper parts of the Devonian
   System. The faunal stages from youngest to oldest are:

Late (most recent)

     * Famennian/Chautauquan/Canadaway/Conneaut/Conneautan/Conewango/Conew
       angan
     * Frasnian/Senecan/Sonyea/Sonyean/West Falls

Middle

     * Cazenovia/Cazenovian
     * Givetian/Erian/Senecan/Tioughniogan/Tioughnioga/Taghanic/Taghanican
       /Genesee/Geneseean
     * Eifelian/Southwood

Early (oldest)

     * Helderberg
     * Emsian/Sawkill/Deer Park
     * Pragian/Siegenian
     * Lochkovian/Gedinnian

   Devonian rocks are oil and gas producers in some areas.

Devonian palaeogeography

   The Devonian period was a time of great tectonic activity, as Laurasia
   and Gondwanaland drew closer together.

   The continent Euramerica (or Laurussia) was created in the early
   Devonian by the collision of Laurentia and Baltica, which rotated into
   the natural dry zone along the Tropic of Capricorn, which is formed as
   much in Paleozoic times as nowadays by the convergence of two great
   airmasses, the Hadley cell and the Ferrel cell. In these near-deserts,
   the Old Red Sandstone sedimentary beds formed, made red by the oxidized
   iron ( hematite) characteristic of drought conditions.

   Near the equator, Pangaea began to consolidate from the plates
   containing North America and Europe, further raising the northern
   Appalachian Mountains and forming the Caledonian Mountains in Great
   Britain and Scandinavia.

   The west coast of Devonian North America, by contrast, was a passive
   margin with deep silty embayments, river deltas and estuaries, in
   today's Idaho and Nevada; an approaching volcanic island arc reached
   the steep slope of the continental shelf in Late Devonian times and
   began to uplift deep water deposits, a collision that was the prelude
   to the mountain-building episode of Mississippian times called the
   Antler orogeny .

   The southern continents remained tied together in the supercontinent of
   Gondwana. The remainder of modern Eurasia lay in the Northern
   Hemisphere. Sea levels were high worldwide, and much of the land lay
   submerged under shallow seas, where tropical reef organisms lived.

   The deep, enormous Panthalassa (the "universal ocean") covered the rest
   of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys,
   Rheic Ocean, and Ural Ocean (which was closed during the collision with
   Siberia and Baltica).

Devonian biota

   Fossil trilobite Ductina vietnamica from the Devonian of China
   Enlarge
   Fossil trilobite Ductina vietnamica from the Devonian of China

Marine biota

   Sea levels in the Devonian were generally high. Marine faunas continued
   to be dominated by bryozoa, diverse and abundant brachiopods and
   corals. Lily-like crinoids were abundant, and trilobites were still
   fairly common, but less diverse than in earlier periods. The
   ostracoderms were joined in the mid-Devonian by the first jawed fishes,
   the great armored placoderms, as well as the first sharks and
   ray-finned fish. The first shark, the Cladoselache, appeared in the
   oceans during the Devonian period. They became abundant and diverse. In
   the late Devonian the lobe-finned fish appeared, giving rise to the
   first tetrapods.

Reefs

   A great barrier reef, now left high and dry in the Kimberley Basin of
   northwest Australia, once extended a thousand kilometers, fringing a
   Devonian continent. Reefs in general are built by various
   carbonate-secreting organisms that have the ability to erect
   wave-resistant frameworks close to sea level. The main contributors of
   the Devonian reefs were unlike modern reefs, which are constructed
   mainly by corals and calcareous algae. They were composed of calcareous
   algae and coral-like stromatoporoids, and tabulate and rugose corals,
   in that order of importance.

Terrestrial biota

   By the Devonian Period, life was well underway in its colonization of
   the land. The bacterial and algal mats were joined early in the period
   by primitive plants that created the first recognizable soils and
   harbored some arthropods like mites, scorpions and myriapods. Early
   Devonian plants did not have roots or leaves like the plants most
   common today, and many had no vascular tissue at all. They probably
   spread largely by vegetative growth, and did not grow much more than a
   few centimeters tall.

   By the Late Devonian, forests of small, primitive plants existed:
   lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most
   of these plants have true roots and leaves, and many were quite tall.
   The tree-like ancestral fern Archaeopteris, grew as a large tree with
   true wood. These are the oldest known trees of the world's first
   forests. By the end of the Devonian, the first seed-forming plants had
   appeared. This rapid appearance of so many plant groups and growth
   forms has been called the "Devonian Explosion". The primitive
   arthropods co-evolved with this diversified terrestrial vegetation
   structure. The evolving co-dependence of insects and seed-plants that
   characterizes a recognizably modern world had its genesis in the late
   Devonian. The development of soils and plant root systems probably led
   to changes in the speed and pattern of erosion and sediment deposition.

   The 'greening' of the continents acted as a carbon dioxide sink, and
   atmospheric levels of this greenhouse gas may have dropped. This may
   have cooled the climate and led to a massive extinction event. see Late
   Devonian extinction.

   Also in the Devonian, both vertebrates and arthropods were solidly
   established on the land.

Fungal life

   While the marine fungi remained in the oceans and diversified into
   Nature's recyclers, land fungi were also recycling the land,
   decomposing both plants' and animals' dead bodies. Also, due to more
   terrestrial plants and animals appearance, fungi flourished.

Late Devonian extinction

   A major extinction occurred at the boundary that marks the beginning of
   the last phase of the Devonian period, the Famennian faunal stage, (the
   Frasnian-Famennian boundary), about 364 million years ago, when all the
   fossil agnathan fishes suddenly disappeared. A second strong pulse
   closed the Devonian period. The Late Devonian extinction was one of
   five major extinction events in the history of the Earth's biota, more
   drastic than the familiar extinction event that closed the Cretaceous.

   The Devonian extinction crisis primarily affected the marine community,
   and selectively affected shallow warm-water organisms rather than
   cool-water organisms. The most important group to be affected by this
   extinction event were the reef-builders of the great Devonian
   reef-systems .

   Amongst the severely affected marine groups were the brachiopods,
   trilobites, ammonites, conodonts, and acritarchs, as well as jawless
   fish, and all placoderms. Freshwater species, including our tetrapod
   ancestors, were less affected.

   Reasons for the late Devonian extinctions are still speculative.
   Retrieved from " http://en.wikipedia.org/wiki/Devonian"
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