temp=17The Silurian is a geologic period and system that extends from the end of the
Ordovicianperiod, about 443.7 ± 1.5 Ma (million years ago), to the beginning of the Devonianperiod, 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 eventwhen 60% of marine species were wiped out. See Ordovician-Silurian extinction events.
The Silurian system was first identified by Sir
Roderick Murchison, who was examining fossil-bearing sedimentary rock strata in south Walesin the early 1830s. He named the sequences for a Celtic tribe of Wales, the Silures, following the convention his friend Adam Sedgwickhad 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 Lapworthresolved the conflict by defining a new Ordovician system including the contended beds.
The French geologist
Joachim Barrande, building on Murchison's work, used the term "Silurian" in a more comprehensive sense than was justified by subsequent knowledge. He divided the Silurian rocks of Bohemiainto eight stages. His interpretation was questioned in 1854 by Edward Forbes, and the later stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the oldest fossils.
The Llandovery epoch lasted from Period span|Llandovery, and is subdivided into three stages: the visanc|Rhuddanian, [Named for the Cefn-Rhuddan Farm in the Llandovery area; confusingly,
Rhuddlanlies on Silurian strata as well..] lasting until Ma|Aeronian, the Anchor|AeronianAeronian, lasting to Ma|Telychian, and the anchor|TelychianTelychian.
The Wenlock, which lasted from period span|Wenlock, is subdivided into the visanc|Sheinwoodian (to Ma|Homerian) and visanc|Homerian ages. It is named after the
Wenlock Edgein Shropshire, England. During the Wenlock, the oldest known tracheophytes of the genus " Cooksonia", appear. The complexity of slightly younger Gondwanaplants like " Baragwanathia" indicates either a much longer history for vascular plants, perhaps extending into the early Silurian or even Ordovician. See Evolutionary history of plants.
The Ludlow, lasting from period span|Ludlow, comprises the visanc|Gorstian age, lasting until Ma|Ludfordian, and the visanc|Ludfordian.
The Pridoli, lasting from period span|Pridoli, is the final and shortest epoch of the Silurian.
In North America a different suite of regional stages is sometimes used:
Cayugan(Late Silurian - Ludlow)
Lockportian(middle Silurian: late Wenlock)
Tonawandan(middle Silurian: early Wenlock)
Ontarian(Early Silurian: late Llandovery)
Alexandrian(earliest Silurian: early Llandovery)
During the Silurian,
Gondwanacontinued 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 southern continents remained united during this period.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 supercontinentknown 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
Panthalassacovered most of the northern hemisphere. Other minor oceans include two phases of the Tethys— the Proto-Tethysand Paleo-Tethys— the Rheic Ocean, a seaway of the Iapetus Ocean(now in between Avaloniaand Laurentia), and the newly formed Ural Ocean.
During this period, the
Earthentered a long warm greenhousephase, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Poleuntil they almost disappeared in the middle of Silurian. 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. Later in the Silurian, the climate cooled slightly, but in the Silurian-Devonian boundary, the climate became warmer.
ilurian aquatic 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 Americawere 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 Osteichthyesappeared, 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 Eurypterids (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. Leechesalso made their appearance during the Silurian Period. Brachiopods, bryozoa, molluscs, and trilobites were abundant and diverse.
First terrestrial biota
The Silurian was the first period to see macrofossils of extensive terrestrial biota, in the form of
mossforests along lakes and streams.
The first fossil records of
vascular plants, that is, land plants with tissues that carry food, appeared in the second half of 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 xylemand phloembut no differentiation in root, stem or leaf, was much-branched "Psilophyton", reproducing by spores and breathing through stomataon every surface, and probably photosynthesizing in every tissue exposed to light. Rhyniophytaand primitive lycopods were other land plants that first appear during this period.
Some evidence suggests the presence of predatory trigonotarbid arachnoids and
myriapods in Late Silurian facies. Predatory invertebrateswould indicate that simple food webs were in place that included non-predatory prey animals. Extrapolating back from Early Devonianbiota, Andrew Jeram "et al." in 1990 [Andrew J. Jeram, Paul A. Selden and Dianne Edwards, "Land Animals in the Silurian: Arachnids and Myriapods from Shropshire, England", "Science" 2 November 1990:658-61.] suggested a food web based on as yet undiscovered detritivores and grazers on microorganisms. [Anna K. Behrensmeyer, John D. Damuth, "et al." "Terrestrial Ecosystems Through Time" "Paleozoic Terrestrial Ecosystems" (University of Chicago Press), 1992:209.]
End Silurian extinction
At the end of Silurian, a series of minor
extinction events, including the Lau event, occurred. They were probably caused by climate changeor impact events.Fact|date=June 2007
*Emiliani, Cesare. (1992). "Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment". Cambridge University Press. (Paperback Edition ISBN 0-521-40949-7)
* Mikulic, DG, DEG Briggs, and J Kluessendorf. 1985. A new exceptionally preserved biota from the Lower Silurian of Wisconsin, USA. Philosophical Transactions of the Royal Society of London, 311B:75-86.
* Moore, RA, DEG Briggs, SJ Braddy, LI Anderson, DG Mikulic, and J Kluessendorf. 2005. A new synziphosurine (Chelicerata: Xiphosura) from the Late Llandovery (Silurian) Waukesha Lagerstatte, Wisconsin, USA. Journal of Paleontology:79(2), pp. 242-250.
* Ogg, Jim; June, 2004, "Overview of Global Boundary Stratotype Sections and Points (GSSP's)" http://www.stratigraphy.org/gssp.htm Accessed
April 30, 2006.
* [http://www.palaeos.com/Paleozoic/Silurian/Silurian.htm#history Paleos:] Silurian
* [http://www.ucmp.berkeley.edu/silurian/silurian.html UCMP Berkeley:] The Silurian
* [http://www.paleoportal.org/time_space/period.php?period_id=14 Paleoportal: Silurian strata in U.S., state by state]
* [http://tapestry.usgs.gov/ages/silurdevon.html USGS:Silurian and Devonian Rocks (U.S.)]
* [http://www.geo-lieven.com/erdzeitalter/silur/silur.htm Examples of Silurian Fossils]
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Silurian — Si*lu ri*an, a. [From L. Silures, a people who anciently inhabited a part of England and Wales.] (Geol.) Of or pertaining to the country of the ancient Silures; a term applied to the earliest of the Paleozoic eras, and also to the strata of the… … The Collaborative International Dictionary of English
silurian — SILURIÁN, Ă, silurieni, e, s.n., adj. 1. s.n. A treia perioadă a erei paleozoice care se caracterizează prin sedimentări de calcar, de gresie, de şisturi argiloase, prin apariţia primelor plante terestre şi prin dezvoltarea nevertebratelor marine … Dicționar Român
Silurian — [si loor′ē ən, sīloor′ē ən] adj. 1. of the Silures 2. [because the rocks were first found in an area in SE Wales: see SILURES] [sometimes s ] designating or of the third geologic period of the Paleozoic Era, characterized by the development of… … English World dictionary
Silurian — Si*lu ri*an, n. The Silurian age. [1913 Webster] … The Collaborative International Dictionary of English
Silurian — 1708, pertaining to the Silures, from L. Silures ancient British tribe inhabiting southeast Wales. Geological sense is from 1835, coined by Sir Roderick Impey Murchison (1792 1871) because rocks of this period are especially frequent in Wales … Etymology dictionary
Silurian — ► ADJECTIVE Geology ▪ referring to the third period of the Palaeozoic era (between the Ordovician and Devonian periods, about 439 to 409 million years ago), a time when the first fish and land plants appeared. ORIGIN from Silures, the Latin name… … English terms dictionary
Silurian — adjective Etymology: Latin Silures Date: 1708 1. of or relating to the Silures or their place of habitation 2. of, relating to, or being a period of the Paleozoic era between the Ordovician and Devonian or the corresponding system of rocks marked … New Collegiate Dictionary
Silurian — a geological period within the Palaeozoic ca. 441 113 million years ago. Most of the major groups of fishes are thought to have originated in the Early Silurian. Abbreviated as S … Dictionary of ichthyology
Silurian — /si loor ee euhn, suy /, adj. 1. of or pertaining to the Silures or their country. 2. Geol. noting or pertaining to a period of the Paleozoic Era, occurring from 425 to 405 million years ago, notable for the advent of air breathing animals and… … Universalium
Silurian — 1. adjective Of a geologic period within the Paleozoic era; comprises the Llandovery, Wenlock, Ludlow and Pridoli epochs from about 439 to 409 million years ago. 2. noun … Wiktionary