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Of or belonging to the geologic time, system of rocks, or sedimentary deposits of the third period of the Paleozoic Era, characterized by the development of jawed fishes, early invertebrate land animals, and land plants.
n.The Silurian Period or its system of deposits.
[From Latin Silures, an ancient people of southwest Wales, where the rocks were first identified.]
The third oldest period of the Paleozoic Era, spanning an interval from about 412 to 438 million years before the present. The Silurian system includes all sedimentary rocks deposited and all igneous and metamorphic rocks formed in the Silurian Period. Both the base and top of the Silurian have been designated by international agreement at the first appearances of certain graptolite species in rock sequences at easily examined and well-studied outcrops. See also Geologic time scale.
The most prominent feature of Silurian paleogeography was the immense Gondwana plate. It included much of present-day South America, Africa, the Middle East, Antarctica, Australia, and the Indian subcontinent. During the Silurian, many plates continued the relative northward motion that had commenced during the mid-Ordovician. Plate positions and plate motions as well as topographic features of the plates controlled depositional environments and lithofacies. These, in turn, significantly influenced organismal development and distributions. Silurian Northern Hemisphere plates, other than a portion of Siberia, are not known north of the Northern Hemisphere tropics. Presumably, nearly all of the Northern Hemisphere north of the tropics was ocean throughout the Silurian. See also Continents, evolution of; Paleogeography.
Absence of plates bearing continental or shallow shelf marine environments north of about 45° north latitude indicates that ocean circulation in most of the Silurian Northern Hemisphere was zonal. Ocean surface currents in the tropics would have been influenced strongly by the prevailing westerlies. The large size of the Gondwana plate and the presence of land over much of it would have led to development of seasonal monsoon conditions. Surface circulation south of 30° south would have hit the western side of Laurentia and flowed generally northward. See also Paleoceanography.
Collision of the Avalonian and Laurentian plates in the latest Ordovician coincides with development of the Southern Hemisphere continental glaciation. Erosion of the land area formed at the Avalon-Laurentian plate collision generated a large volume of coarse to fine-grained siliclastic materials. That part of South America (modern eastern South America) near the South Pole for the early part of the Silurian was the site of as many as four brief glacial episodes. See also Paleoclimatology.
Both nonvascular and vascular plants continued to develop in land environments following their originations in the early mid-Ordovician. Many of these Silurian plants were mosslike and bryophytelike. Psilophytes assigned to the genus Cooksonia were relatively widespread in Late Silurian terrestrial environments. The probable lycopod (club moss) Baragwanathia apparently lived in nearshore settings in modern Australia during the latter part of the Silurian. Silurian land life also included probable arthopods and annelid worms. Fecal pellets of wormlike activity have been found as well as remains of centipede-, millepede-, and spiderlike arthropods. See also Paleobotany; Paleoecology.
Shallow marine environments in the tropics were scenes of rich growths of algae, mat-forming cyanobacteria, spongelike organisms, sponges, brachiopods, bryozoans, corals, crinoids, and ostracodes. Nearshore marine siliclastic strata bear ostracodes, small clams, and snails and trilobites. Certain nearshore strata bear the remains of horseshoe-crab-like eurypterids. See also Algae;
Fish are prominent in a number of Silurian nearshore and some offshore marine environments. Jawless armored fish include many species of thelodonts that had bodies covered with minute bony scales, heterostracans, and galeaspids that had relatively heavily armored head shields, and anaspids that possessed body armor consisting of scales and small plates. Jawed fish were relatively rare in the Silurian. They were primarily spiny sharks or acanthodians. As well, there are remains of true sharklike fish and fish with interior bony skeletons (osteichthyans) in Late Silurian rocks. See also Anaspida; Heterostraci; Osteichthyes.
A period of Palaeozoic time stretching approximately from 430 to 395 million years bp.
The noun has one meaning:
Meaning #1:
from 405 million to 425 million years ago; first air-breathing animals
Synonym: Silurian period
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.
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.
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 Bohemia into 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 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:
In North America a different suite of regional stages is used:
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.
Image:Siluriantimes.jpg
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. Early in the Silurian, glaciers retreated back into the South Pole until 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.
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. leeches also made their appearance during the Silurian Period. 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.
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 Psilophyton, 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.
At the end of Silurian, a series of minor extinction events, including the Lau event, occurred. They were probably caused by climate change or impact events.[citation needed]
| Silurian period | |||
|---|---|---|---|
| Llandovery | Wenlock | Ludlow | Pridoli |
| Rhuddanian |
Aeronian Telychian |
Sheinwoodian | Homerian | Gorstian | Ludfordian | |
| Paleozoic era | |||||
|---|---|---|---|---|---|
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian |
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