Silurian The Silurian System is the third and youngest of the three Early (Lower) Palaeozoic systems; its formations came into existence over a time-span of about 25 million years, beginning at about 443 Ma. It was first mentioned by Sir Roderick Murchison in 1835, after several summers of fieldwork in Wales and the Welsh Borderland. He did not publish a detailed description until 1839, when it appeared in his monumental book The Silurian System, based on his discoveries there and elsewhere in Britain. Murchison was widely travelled and quick to prepare and publish his findings. The Silurian System is a remarkable account of the strata he had encountered and the system he now espoused. It is named after the ancient celtic tribe, the Silures, inhabitants of the Welsh Borderlands in Roman times. Originally it included rocks lower than those now in the basal Silurian series, which were held in dispute by Adam Sedgwick, Professor of Geology at the University of Cambridge, to be part of his Cambrian System. These two able geologists and formidable gentlemen had been good friends, but the dispute very effectively spoiled their relationship.

The controversy was resolved to general satisfaction by Charles Lapworth, who in 1879 allocated the lowest part of Murchison's Silurian and the uppermost part of Sedgwick's Cambrian to the (new) Ordovician System. The name was not officially recognized by the International Geological Congress until 1960, many European geologists having until then preferred Gotlandian. The Swedish island of Gotland was well known for the fossiliferous limestones it displays.

Modern usage divides the system into lower and upper halves, each with two series. The lower three series are divided into stages based upon basal boundary stratotypes that are well-exposed beds in Scotland, South Wales, and the Welsh Borderland. The youngest series (Pr̆ídolí), based upon a section in the Bohemian region of the Czech Republic, was added to the Murchison's Llandovery, Wenlock, and Ludlow Series in 1984. In Britain the Downton Series was by many authors regarded as the uppermost representative of the Silurian System, and has subsequently proved to be in part equivalent to the Pr̆ídolí. The base of the system is now drawn in the stratotype at Dob's Linn in Scotland at the base of the biozone of the graptolite Parakidograptus acuminatus. This section is in the graptolitic facies, and additional reference sections in other characteristic facies are needed. They exist in North America, Western Europe, China, and Australia. Some 32 graptolite biozones have been recognized, while in the other facies conodonts, chitinozoa, and, to a lesser extent, brachiopods also are of value.

Rocks of the Silurian system occur on all continents, except (so far as is known) Antarctica. Major outcrops occur in North America, north-west Europe, North Africa, and central and eastern Asia. They include broad outcrops of carbonate rocks (mainly limestones) and some evaporites in the mid-west part of North America and the Russian platform, and also in China and Australia. Extensive belts of clastic sedimentary rocks and volcanics, some of the very great thickness, occur in the eastern and western parts of North America, western Europe, central Asia, many parts of China, and eastern Australia. The Silurian continents were distributed as a major agglomeration of ancient continental shields in the high southern latitudes—Gondwanaland—and a chain of separate shields (Laurentia, Baltica, Kazakhstania, and China) in the equatorial region, together with the large Siberian land mass (Angaraland) lying in the middle latitudes north of Baltica (Fig. 1). Plate motions were unceasing and significant, with active island arcs and subduction zones along several continental margins. This must have been particularly spectacular with the closure of the Iapetus Ocean that took place at the collision and suturing of Laurentia with Baltica. It was the culmination of much volcanism and the growth of great wedges of deep-water sands, like submarine deltas on the margins of the two facing land masses. Similar active belts with much volcanic and clastic sediment accumulation also existed on the eastern margin of Australia, in China– Mongolia, and in Kazakhstan.

There is good evidence of former glacial activity in Gondwanaland, though it appears to have been less widespread than it was in the Ordovician period. World sea level rose significantly during early Silurian time, its later behaviour being locally obscured by tectonic activity. Nevertheless, a eustatic rise is thought to have persisted throughout Silurian time, and the level was generally high all the time. In the tropical waters that flooded the margins of the continents, carbonate deposition and the vigorous growth of coral– stromatoporoid–algal reefs in mid- and late Silurian ages were widespread. In some associated shallow coastal basins evaporite deposits include important masses of halite (rock salt).

Life in the sea witnessed a growing diversity of bottom-dwelling creatures, mostly brachiopods, bryozoans, trilobites, corals and echinoderms, with minor numbers of gastropods, bivalves and other animals. Depth-related ‘communities’ of Silurian brachiopods and other forms can be found in Britain. Generally these faunas were cosmopolitan throughout the tropical and temperate latitudes. There were, however, two geographically distinct realms of benthos. The southern, cool, Malvinokafric realm includes the distinct shelf communities with the brachiopod Clarkeia, from South America and west and southern Africa. In the Siberian–Mongolian region, a shelf fauna with the distinctive Tuvaella is also thought to represent a cool setting.

The calcareous algae, stromatoporoids, tabulate corals, and rugose corals became conspicuous reef-builders through-out the tropical seas. The plankton and nekton included graptoloids and many kinds of microfossil organisms and vertebrates. The latter included the primitive ostracoderms and the acanthodians, which were marine creatures early in the period, but which spread to fresh waters before it ended.

Land plants had made their appearance in the early Silurian, as the presence of fossil spores indicates, and vascular plant fossils are widespread from the Mid-Silurian onwards. Fossil soil deposits confirm that algae and, no doubt, other primitive plant life, were spread throughout the warmer latitudes. Scorpion-like eurypterids and other arthropods established themselves in fresh waters during late Silurian time, and might even have been capable of passing considerable time out of the water. All in all, the evidence shows that terrestrial life was making progress, with a variety of plants living close to the water or in areas of suitable climate.

D. L. Dineley

Bibliography

Bassett, M. G., Lane, P. D., and Edwards, D. (eds) (1991) The Murchison Symposium: proceedings of an international conference on the Silurian System. Palaeontological Association Special Paper No. 44.
Cocks, L. R. M. and and McKerrow, W. S. (1978) Silurian. In McKerrow, W. S. (ed.) The ecology of fossils, pp. 93–124. Duckworth, London.
Holland, C. H. and Bassett, M. G. (eds) (1989) A global standard for the Silurian System. National Museum of Wales Geological Series 9, Cardiff.
McKerrow, W. S. and Scotese, C. R. (eds) (1990) Palaeozoic palaeogeography and biogeography. Geological Society of London Memoir No. 12.

Silurian

The Silurian period of the Paleozoic era, 440-410 million years ago, follows the Ordovician period of the Paleozoic, "the age of ancient life." The Silurian was named by the R. I. Murchison in 1835 in honor of the Silure

tribe of Celts who had inhabited the Welsh borderlands where he first studied these rocks.

A long, warm, stable period followed the ice age and mass extinctions of the Ordovician, when over half of all previous life forms became extinct. Silurian fauna built upon the evolutionary patterns that had preceded it. No major new groups of invertebrates appeared, although a great radiation in number and form of existing invertebrates occurred. It has been said that the evolution of life cannot be separated from the evolution of the planet. In the Silurian, geologic trends greatly influenced animal development. As the two great supercontinents, Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere, once again drifted toward one another on their tectonic plates, mountains were heaved up to form distinct ecosystems where species could evolve uninfluenced by one another. And as the glaciers began to melt, warm, shallow seas flooded much of Laurasia, providing ideal conditions for a variety of benthic (bottom-dwelling) species. These included a rich variety of sea lilies, lampshells, trilobites, graptolites, and mollusks. The crinoidal sea lilies and graptolites are particularly interesting. They are both echinoderms: small, soft-bodied wormlike creatures that lack a normal head but have a well-developed nervous system located in a rudimentary notochord ("backchord"), the precursor of a backbone. The presence of a notochord makes these once-abundant sea-floor scavengers the ancestors of the chordates, animals with backbones.

A recurrent theme occurs in the origin of new marine species. They first tend to appear in very shallow waters along the shore, then disperse into deeper habitats. The shoreline is a harsh area of constant tides, storms with silt flows, and temperature fluctuations. These conditions favor species that are resilient and adaptive. Gradually the offspring expand into deeper water. The new forms of all the existing invertebrates followed this pattern: the brachiopods , sponges, bryozoans, arthropods , and echinoderms, as well as the vertebrate fishes. In the deeper waters, mobile predators appeared in unprecedented sizes. The free-swimming nautiloids, which grew up to 3 meters (10 feet) long and the eurypterids, sea-scorpion arthropods at 2-meters (6-feet) long, fed on the vast numbers of early jawed fishes that now appeared. The great reefs destroyed by the ice age were rebuilt, coral by individual coral.

The most noteworthy event of the Silurian (from the human point of view) took place on land. The first minuscule plants began to creep across the previously barren land masses, followed by tiny scorpions and millipedes. The whiskery, or pleated, tracks of arthropods appear in the Silurian rocks of western Australia, and for a brief while these arthropods dominated Earth. The formerly rare agnathans (jawless fishes), became plentiful and began to explore up the brackish estuaries and into the freshwater rivers and upstream pools where they flourished.

see also Geological Time Scale.

Nancy Weaver

Bibliography

Asimov, Isaac. Life and Time. Garden City, NY: Doubleday & Company, 1978.

Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.

———. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Viking Press, 1998.

Friday, Adrian and David S. Ingram, eds. The Cambridge Encyclopedia of Life Sciences. London: Cambridge University, 1985.

Gould, Stephen Jay, ed. The Book of Life. New York: W. W. Norton & Company, 1993.

Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.

McLoughlan, John C. Synapsida: A New Look Into the Origin of Mammals. New York: Viking Press, 1980.

Steele, Rodney and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New York: McGraw Hill, 1979.

Wade, Nicholas, ed. The Science Times Book of Fossils and Evolution. New York: The Lyons Press, 1998.

Silurian Period

In geologic time , the Silurian Period, the third period of the Paleozoic Era , covers the time from roughly 440 million years ago (mya) until 410 mya. The name, Silurian, derives from the Silures, an ancient British tribe. The Silurian Period spans two epochs. The Early Silurian Epoch is the most ancient, followed by the Late Silurian Epoch.

The Early Silurian Epoch is divided chronologically (from the most ancient to the most recent) into the Llandoverian and Wenlockian stages. The Late Silurian Epoch is divided chronologically (from the most ancient to the most recent) into the Ludlovian, and Pridolian stages.

In terms of paleogeography (the study of the evolution of the continents from supercontinents and the establishment of geologic features), the Silurian Period featured cleavage of some supercontinent landmass and fusion of plates into Laurussia. Collision with remnants of other continents later formed the supercontinent Laurasia and eventually the super-continent Pangaea.

The fossil record establishes that the preceding Ordovician Period ended with a mass extinction. This mass extinction, approximately 440 mya, marked the end of the Ordovician Period and the start of the Silurian Period. In accord with a mass extinction, many fossils dated to the Ordovician Period are not found in Silurian Period formations. Differentiated by fossil remains and continental movements, the Devonian Period followed the Silurian Period.

The Silurian Period marked a geologically active period for volcanic activity. The accompanying ash deposits and lava flows are clearly evident in Silurian Period strata.

The fossil record indicates that it was during the Silurian Period that marine species made the evolutionary transition to terrestrial (land-based species). The first true insect fossils date to this period, as do fossils of jawed fish. Atmospheric changes, driven by increasingly diverse plant life, allowed the further development of the protective ozone layer, which filters out harmful ultraviolet radiation.

See also Archean; Cambrian Period; Cenozoic Era; Cretaceous Period; Dating methods; Eocene Epoch; Evolution, evidence of; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Paleocene Epoch; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Tertiary Period; Triassic Period

Silurian period [from the Silures, ancient tribe of S Wales, where the period was first studied; named by the British geologist R. I. Murchison], third period of the Paleozoic era of geologic time (see Geologic Timescale , table) lasting from 405 to 435 million years ago. The continents in the Silurian period remained much as they had been in the preceding Ordovician period , with approximately the same areas being subject to flooding by shallow seas. The earth was relatively tectonically inactive during the Silurian. The Appalachian Mountains, which uplifted during the Ordovician, were being eroded. Large coral reefs and algae were abundant, indicating that warm, shallow seas predominated. Major evaporite basins, including the circular Michigan Basin, showed evidence of subsidence. The transition between the Ordovician and Silurian rocks is not clearly marked in the United States. The Medina sandstone extending from New York to Alabama has been assigned to both periods but is generally considered to be Silurian. Three main series, based on the succession of strata in New York state, are usually distinguished—the lower Silurian (Medinan, or Alexandrian, series), the middle Silurian (Niagaran series), and the upper Silurian (Cayugan series). The early Silurian deposits in the East are commonly sandstone, shale, and conglomerate, comprising erosion products from high-standing mountains; in the West, marine limestone predominates. There were also desert conditions, under which the Salinan "red beds" of the Appalachian area and the salt deposits of New York, Michigan, Ontario, and Ohio were formed. Some areas were later reflooded, depositing Cobleskill and Rondout limestone of New York. The Silurian of the Far West is as yet not well established. In North America, the Silurian ended quietly; however, in the British Isles, Scandinavia, and France, as a result of the Caledonian disturbance, great mountains continued to be thrust up. Economic resources of the Silurian strata, besides salt, are iron ore (near Birmingham, Ala.) and quartz sandstone, used in glass manufacture. Dominating the life of the Silurian were marine invertebrates, including crinoids and cystoids, mollusks, and eurypterids, invertebrates related to crabs and insects. Members of the trilobite family were still numerous; primitive fishes increased in number. Also notable in the Silurian fauna were scorpions, possibly the first animals to live on land and take their oxygen from the air.

Silurian A geological period of the Palaeozoic era following the Ordovician period and extending until the beginning of the Devonian period. It began about 438 million years ago and lasted for about 30 million years. The Silurian was named by Roderick Murchison (1792–1871) after an ancient British tribe that inhabited South Wales, where he observed rocks of this period. The majority of Silurian life was marine but during the later part of the period primitive plants began to make their appearance on land. Trilobites and graptolites became less common, brachiopods were numerous and varied, crinoids became common for the first time, and corals also increased. The only known vertebrates during the Silurian were primitive fish; the first jawed fish appeared later in the period. The Caledonian orogeny (mountain-building period) reached its peak towards the end of the Silurian.

Silurian of or belonging to the ancient British tribe of the Silures, or the south-eastern part of Wales inhabited by them.

In the mid 19th century, Silurian was used to denote the third period of the Palaeozoic era, between the Ordovician and Devonian periods, which lasted from about 439 to 409 million years ago. The first true fish and land plants appeared, and the end of the period is marked by the climax of the Caledonian mountain-forming.

Silurian Third oldest period of the Palaeozoic era, 438–408 million years ago. Marine invertebrates resembled those of Ordovician times, and fragmentary remains show that jawless fish began to evolve. The earliest land plants (psilopsids) and first land animals (archaic mites and millipedes) developed. Mountains formed in nw Europe and Greenland.

Silurian The third of six periods of the Palaeozoic Era, approximately 439–408.5 Ma ago. Its end is marked by the climax of the Caledonian orogeny (mountain-building episode) and the filling of several Palaeozoic basins of deposition. It is the period during which land plants first appeared.

Silurian The third of 6 periods of the Palaeozoic Era, approximately 438–408 Ma ago, whose end is marked by the climax of the Caledonian orogeny and the filling of several Palaeozoic basins of deposition. It is the period during which land plants first appeared.

Silurian The third of six periods of the Palaeozoic Era, approximately 443.7–416 Ma ago, whose end is marked by the climax of the Caledonian orogeny (mountain-building episode) and the filling of several Palaeozoic basins of deposition.

Silurian Third (439–408.5 Ma) of six periods of the Palaeozoic Era. The end of the period is marked by the climax of the Caledonian orogeny and the filling of several Palaeozoic basins of deposition.

Silurian •antipodean, Crimean, Judaean, Korean •Albion •Gambian, Zambian •lesbian •Arabian, Bessarabian, Fabian, gabion, Sabian, Swabian •amphibian, Libyan, Namibian •Sorbian •Danubian, Nubian •Colombian • Serbian • Nietzschean •Chadian, Trinidadian •Andean, Kandyan •guardian •Acadian, Akkadian, Arcadian, Barbadian, Canadian, circadian, Grenadian, Hadean, Orcadian, Palladian, radian, steradian •Archimedean, comedian, epicedian, median, tragedian •ascidian, Derridean, Dravidian, enchiridion, Euclidean, Floridian, Gideon, Lydian, meridian, Numidian, obsidian, Pisidian, quotidian, viridian •Amerindian, Indian •accordion, Edwardian •Cambodian, collodion, custodian, melodeon, nickelodeon, Odeon •Freudian • Bermudian • Burundian •Burgundian •Falstaffian, Halafian •Christadelphian, Delphian, Philadelphian •nymphean • ruffian • Brobdingnagian •Carolingian • Swedenborgian •logion, Muskogean •Jungian •magian, Pelagian •collegian •callipygian, Cantabrigian, Phrygian, Stygian •Merovingian • philologian • Fujian •Czechoslovakian • Pickwickian •Algonquian • Chomskian •Kentuckian •battalion, galleon, medallion, rapscallion, scallion •Anglian, ganglion •Heraklion •Dalian, Malian, Somalian •Chellean, Machiavellian, Orwellian, Sabellian, Trevelyan, triskelion •Wesleyan •alien, Australian, bacchanalian, Castalian, Deucalion, episcopalian, Hegelian, madrigalian, mammalian, Pygmalion, Salian, saturnalian, sesquipedalian, tatterdemalion, Thessalian, Westphalian •anthelion, Aristotelian, Aurelian, carnelian, chameleon, Karelian, Mendelian, Mephistophelian, Pelion, Sahelian •Abbevillian, Azilian, Brazilian, caecilian, Castilian, Chilean, Churchillian, civilian, cotillion, crocodilian, epyllion, Gillian, Lilian, Maximilian, Pamphylian, pavilion, postilion, Quintilian, reptilian, Sicilian, Tamilian, vaudevillian, vermilion, Virgilian •Aeolian, Anatolian, Eolian, Jolyon, Mongolian, napoleon, simoleon •Acheulian, Boolean, cerulean, Friulian, Julian, Julien •bullion •mullion, scullion, Tertullian •Liverpudlian •Bahamian, Bamian, Damian, Mesopotamian, Samian •anthemion, Bohemian •Endymion, prosimian, Simeon, simian •isthmian • antinomian •Permian, vermian •Oceanian •Albanian, Azanian, Iranian, Jordanian, Lithuanian, Mauritanian, Mediterranean, Panamanian, Pennsylvanian, Pomeranian, Romanian, Ruritanian, Sassanian, subterranean, Tasmanian, Transylvanian, Tripolitanian, Turanian, Ukrainian, Vulcanian •Armenian, Athenian, Fenian, Magdalenian, Mycenaean (US Mycenean), Slovenian, Tyrrhenian •Argentinian, Arminian, Augustinian, Carthaginian, Darwinian, dominion, Guinean, Justinian, Ninian, Palestinian, Sardinian, Virginian •epilimnion, hypolimnion •Bosnian •Bornean, Californian, Capricornian •Aberdonian, Amazonian, Apollonian, Babylonian, Baconian, Bostonian, Caledonian, Catalonian, Chalcedonian, Ciceronian, Devonian, draconian, Estonian, Etonian, gorgonian, Ionian, Johnsonian, Laconian, Macedonian, Miltonian, Newtonian, Oregonian, Oxonian, Patagonian, Plutonian, Tennysonian, Tobagonian, Washingtonian •Cameroonian, communion, Mancunian, Neptunian, Réunion, union •Hibernian, Saturnian •Campion, champion, Grampian, rampion, tampion •thespian • Mississippian • Olympian •Crispian •Scorpian, scorpion •cornucopian, dystopian, Ethiopian, Salopian, subtopian, Utopian •Guadeloupian •Carian, carrion, clarion, Marian •Calabrian, Cantabrian •Cambrian • Bactrian •Lancastrian, Zoroastrian •Alexandrian • Maharashtrian •equestrian, pedestrian •agrarian, antiquarian, apiarian, Aquarian, Arian, Aryan, authoritarian, barbarian, Bavarian, Bulgarian, Caesarean (US Cesarean), centenarian, communitarian, contrarian, Darien, disciplinarian, egalitarian, equalitarian, establishmentarian, fruitarian, Gibraltarian, grammarian, Hanoverian, humanitarian, Hungarian, latitudinarian, libertarian, librarian, majoritarian, millenarian, necessarian, necessitarian, nonagenarian, octogenarian, ovarian, Parian, parliamentarian, planarian, predestinarian, prelapsarian, proletarian, quadragenarian, quinquagenarian, quodlibetarian, Rastafarian, riparian, rosarian, Rotarian, sabbatarian, Sagittarian, sanitarian, Sauveterrian, sectarian, seminarian, septuagenarian, sexagenarian, topiarian, totalitarian, Trinitarian, ubiquitarian, Unitarian, utilitarian, valetudinarian, vegetarian, veterinarian, vulgarian •Adrian, Hadrian •Assyrian, Illyrian, Syrian, Tyrian •morion • Austrian •Dorian, Ecuadorean, historian, Hyperborean, Nestorian, oratorian, praetorian (US pretorian), salutatorian, Salvadorean, Singaporean, stentorian, Taurean, valedictorian, Victorian •Ugrian • Zarathustrian •Cumbrian, Northumbrian, Umbrian •Algerian, Cancerian, Chaucerian, Cimmerian, criterion, Hesperian, Hitlerian, Hyperion, Iberian, Liberian, Nigerian, Presbyterian, Shakespearean, Siberian, Spenserian, Sumerian, valerian, Wagnerian, Zairean •Arthurian, Ben-Gurion, centurion, durian, holothurian, Khachaturian, Ligurian, Missourian, Silurian, tellurian •Circassian, Parnassian •halcyon • Capsian • Hessian •Albigensian, Waldensian •Dacian • Keatsian •Cilician, Galician, Lycian, Mysian, Odyssean •Leibnizian • Piscean • Ossian •Gaussian • Joycean • Andalusian •Mercian • Appalachian • Decian •Ordovician, Priscian •Lucian •himation, Montserratian •Atlantean, Dantean, Kantian •bastion, Erastian, Sebastian •Mozartian • Brechtian • Thyestean •Fortean • Faustian • protean •Djiboutian •fustian, Procrustean •Gilbertian, Goethean, nemertean •pantheon •Hogarthian, Parthian •Lethean, Promethean •Pythian • Corinthian • Scythian •Lothian, Midlothian •Latvian • Yugoslavian •avian, Batavian, Flavian, Moldavian, Moravian, Octavian, Scandinavian, Shavian •Bolivian, Maldivian, oblivion, Vivian •Chekhovian, Harrovian, Jovian, Pavlovian •alluvion, antediluvian, diluvian, Peruvian •Servian • Malawian • Zimbabwean •Abkhazian • Dickensian •Caucasian, Malaysian, Rabelaisian •Keynesian •Belizean, Cartesian, Indonesian, Milesian, Salesian, Silesian •Elysian, Frisian, Parisian, Tunisian •Holmesian •Carthusian, Malthusian, Venusian