Ordovician The Ordovician System was originated by Charles Lapworth of the University of Birmingham in 1879 to put an end to the uncertainty and confusion regarding the boundary between the Cambrian System of Adam Sedgwick and the overlying Silurian System of R. I. Murchison. Lapworth resolved the long-protracted squabble between the two earlier geologists by grouping the upper formations of Sedgwick's system with the lower of Murchison's into a new convenient unit, based on the succession in the Arenig–Bala area of North Wales. This region was the home of the early British tribe, the Ordovices. Lapworth's creation rapidly won the approval both of British and foreign geologists. They found that fossils distinguished the new system clearly from those above and below.

The Ordovician is thus the second of the three Lower Palaeozoic systems and spans the time interval 510 Ma to 437 Ma. British usage divides the system into five series; North American practice is to recognize only three series. There are 12 named stages with two unnamed intervals. Recent work has advocated drawing the base at the base of the (graptoloid) zone of Rhabdinopora flabelliformis s. l. The standard British subdivisions are rather difficult to recognize elsewhere. Intercontinental correlation can to some extent be achieved by graptolites and by conodonts, with biostratigraphic zonal schemes established not only in the British Isles but also in North America, China, and Australia.

The two fossil groups mentioned above are effective for the black shale and carbonate facies, respectively, while in the fine-grained clastic sequences trilobites, brachiopods, and palynomorphs are used in correlation.

During the Ordovician period the South American and African parts of the great Gondwana supercontinent (Gondwanaland) occupied Antarctic latitudes while Eastern Gondwana, made up of Antarctica and Australia, reached northwards almost to the Equator and the Chinese continent. Laurentia, Baltica, and Kazakhstania lay in the equatorial belt with Siberia (Angaraland) close to the north of Baltica (Fig.1). The Iapetus Ocean between Laurentia and Baltica continued to close towards the north with active continental margins on each side of the now rather narrow band of ocean crust. Vigorous igneous activity made episodic contributions to the continental crust, with island arc volcanicity and intrusions. Other mobile belts occurred in western South America, the Urals area, and parts of north-eastern Russia, eastern Australia, and China.

During the long Cambrian period many of the old land masses had become flat and low-lying from weathering and erosion. Small changes of sea level would have had far-reaching effects on the sizes and shapes of land masses. Ordovician global sea level was initially rather low but a series of transgressive rises spread epeiric seas across much of the interior of Laurentia, and there were similar floodings of other continental regions. Towards the end of the period a marked regression occurred prior to the rise in sea level of the Silurian period. These eustatic movements are thought to be related to the growth and decay of the ice cap present on the Gondwana continent. This glacial mass appears to have been centred upon Saharan Africa; it has left a remarkable array of glacially striated rock surfaces and glaciogenic deposits. The South Pole itself was probably occupied by western Morocco.

The shallow tidal epeiric seas of the tropical regions became the sites of carbonate (mainly limestone) depositions, especially over Laurentia. It is possible that virtually all the Canadian shield was covered by Ordovician calcareous sediments. Organic productivity in these waters was immense. Reefs appeared in several flooded continental areas. The volume of carbonate sediment deposited was prodigious and may well reflect an increase in the number of different organisms using calcium carbonate for skeletal structures. New kinds and patterns of animal communities spread across the sea floor. The articulate brachiopods were especially abundant and diverse on Ordovician sea floors. Echinoderms, especially crinoids, also were abundant. The corals underwent a radiation with both rugose and tabulate forms contributing—with the stromatoporoids—to the building of reefs in many areas within Laurentia, Baltica, Siberia, Kazakhstan, and eastern Australia. The trilobites diversified and multiplied greatly. We know little about the planktonic and pelagic faunas, except for the graptoloids, nautiloids, and conodonts. The graptolites and the conodont animals became abundant, highly diversified, and very widely distributed, making ideal biozone index fossils. The Ordovician nautiloids possessed predominantly straight or slightly curved shells; the so-called Arctic Ordovician Fauna from Canada contains species that may have reached several metres in length.

So plentiful are many of these fossils that we tend to think of the Ordovician seas as teeming with invertebrate life. Vertebrate (fish) remains suggest small primitive armoured forms that might have been benthonic or even burrowing creatures. They have been found in near-shore sandstones in North and South America and in central Australia. Towards the end of the period the active, swimming acanthodian fishes were also present.

The crisis that beset the marine faunas at the end of Ordovician time saw great reductions in the numbers of reef-forming organisms such as the bryozoa, brachiopods, crinoids, the pelagic nautiloids, trilobites, conodonts, and graptolites. Comparatively rapid cooling of the surface waters around the continents is held to be the most probable cause of these extinctions. The ice cap covering the central and western Saharan area was sufficient to bring about the lowering of global ocean temperatures to a point there they caused real harm to the shallow-water communities. Recovery followed the dawn of the Silurian period.

D. L. Dineley

Bibliography

Bassett, M. G. (ed.) (1976) The Ordovician System, Proceedings of the Palaeontological Association Symposium, Birmingham, September 1974. University of Wales Press and National Museum of Wales, Cardiff.
Cocks, L. R. M. and and McKerrow, W. S. (1978) Ordovician. In McKerrow, W. S. (ed.) The ecology of fossils, pp. 2–92. Duckworth, London.
McKerrow, W. S. Dewey,, J. F., and and Scotese, C. R. (1991) The Ordovician and Silurian development of the Iapetus Ocean. Special Papers in Palaeontology 44, 165–76.
McKerrow, W. S. and Scotese, C. R. (eds) (1990) Palaeozoic palaeogeography and biogeography. Geological Society of London Memoir No. 12.

Ordovician

The Ordovician period (500 to 440 million years ago) comes after the Cambrian in the early Paleozoic era. The period is named for a Celtic tribe named the Ordovices who once lived in the area of Wales (in Britain) where the rocks were first studied. Ordovician limestones are over 6.4 kilometers (4 miles) thick in places and are found on all continents except Antarctica. The uniformity and thickness of the bed indicates a long period of warm and stable climate that allows them to develop.

In fact, the Ordovician period was as remarkable for the diversity of its species as the Cambrian period was for the appearance of most major phyla . A burst of evolutionary creativity in shape, size, and function tripled the number of marine species that appeared. Specialization became the dominant theme of life, with new forms filling every possible niche .

The appearance of highly efficient predators such as the nautiloids and the lobster-size sea scorpions forced the marine community to evolve protective strategies or disappear. Various species responded by developing larger size, thicker shells, or more elaborate defenses. A proliferation in the shapes of the shells of bivalve mollusks allowed them to burrow deeply into sand or mud. Other mollusks learned to swim freely by rapidly clapping their valves together. And still others developed intricate teeth-and-socket arrangements that allowed them to close so tightly that they were almost impossible to open.

Exploring the oceans of the Ordovician world would have been quite similar to exploring the oceans of today. Sea urchins, starfish, and sea lilies lived in profusion among the rocks. The first great coral reefs appeared and gave shelter to crustaceans of all kinds. Sea mats, sea snails, and sea cucumbers abounded in the tide pools. A huge diversity of bivalve mollusks made their slow way across the muddy ocean floor, leaving their tracks and burrows in the fossil record .

The very first primitive fishes appeared, slow and heavily armored, without fins or heads with brains. These agnathans (jawless fishes) were the first animals to have a notochord (flexible rod spine), a precursor of a true spinal chord. These chordates were the ancestor of all animals with backbones.

While almost all animals of the Ordovician were marine, another remarkable occurrence is recorded in the rocks of northwest England. There, arthropods (animals with jointed legs) that lived in shallow, freshwater pools left the first tracks in fossilized mud. Scientists speculate that evaporation of their pools forced these centipede-like creatures to adapt to terrestrial conditions. From this point on, the arthropods, a group that includes insects, spiders, and crabs, ruled the land for 40 million years.

The massive Ordivician limestone ends abruptly with a jumble of glacial till, indicating an ice age that so disrupted Earth's climate that more than half of all species became extinct. This first great extinction wiped out huge numbers of trilobites , with their precise and sensitive eyes, brachiopods , crinoids , and other marine invertebrates . The life-forms that survived the cataclysmic end of the Ordovician contributed to the genetic makeup of the animal kingdom to the present.

see also Geological Time Scale.

Nancy Weaver

Bibliography

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

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.

Ordovician period [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale , table) from 505 to 438 million years ago. It was similar to the preceding Cambrian period , with shallow seas spread for most of the time over the British Isles, Scandinavia, the Baltic region, the Mediterranean region, a large part of Siberia, and much of North America. The Ordovician rocks are chiefly sedimentary. Because of the restricted area and low elevation of the solid land, which set limits to erosion, marine sediments that make up a large part of the Ordovician system consist chiefly of limestone; shale and sandstone are less conspicuous. The Ordovician of North America can best be studied in New York state. In the Early, or Lower, Ordovician epoch, also called the Canadian epoch, the waters spread over the Appalachian area and deposited the Beekmantown limestone, then withdrew generally, to return and deposit the Chazy limestone of the lower Middle Ordovician, also known as the Champlainian epoch. In the interval between Beekmantown time and Chazy time, large areas, chiefly outside New York, were apparently covered with wind-blown sand which became the St. Peter sandstone. In the Middle Ordovician the sea spread over North America to a greater extent than in any other period and laid down the Trenton limestone, which in its eastern section is overlaid or intercalated with the Utica mud shale. In the east, increased erosion of the land subsequently led to the deposition of other shales, which became more and more sandy toward the end of the period. The close of the Ordovician was marked by more general earth disturbances than the close of the Cambrian. The Taconian disturbance created a chain of fold mountains extending from Newfoundland to New Jersey and was accompanied by volcanic activity. The later start of the Acadian-Caledonian uplift may have also been the start of the proto-Atlantic Ocean. Among the economic resources of the Ordovician strata are oil, natural gas, the lead and zinc of Wisconsin, Iowa, and Illinois, the "Portland cement rock" of Pennsylvania and New Jersey, Vermont marble, and the calcium phosphate of the Tennessee limestone. The Ordovician seas were rich in animal life. The most characteristic invertebrates were minute graptolites, other numerous forms being brachiopods, bryozoans, and trilobites. Some cystoids and crinoids appeared; there were a few corals and many cephalopods. Especially noteworthy was the appearance of a few primitive, fishlike vertebrates (jawless fishes) and tiny land plants resembling liverworts .

Ordovician Period

In geologic time , the Ordovician Period, the second period of the Paleozoic Era , covers the time roughly 505 million years ago (mya) until 438 mya. The name Ordovician derives from that of the Ordovices, an ancient British tribe.

The Ordovician Period spans three epochs. The Lower Ordovician Epoch is the most ancient, followed in sequence by the Middle Ordovician Epoch, and the Upper Ordovician Epoch. The Ordovician Period is divided chronologically (from the most ancient to the most recent) into the Tremadocian, Arenigian, Llanvirnian, Llandeilian, Caradocian and Ashgillian stages.

Much of the continental crust that exists now had already been formed by the time of the Ordovician Period and the forces driving plate tectonics actively shaped the fusing continental landmasses. Near the margins of the continental landmasses, extensive orogeny (mountain building) allowed the development of mountain chains .

The fossil record provides evidence to support the demarcation of the preceding Cambrian Period from the Ordovician Period. Drastic changes of sea levels resulted in massive extinctions among marine organisms. In accord with a mass extinction, many fossils dated to the Cambrian Period are not found in Ordovician Period formations.

The fossil record establishes that vertebrates existed during the Ordovician Period. As with the Cambrian Period, the Ordovician Period ended with a mass extinction of nearly a third of all species. This mass extinction, approximately 438 mya, marked the end of the Ordovician Period and the start of the Silurian Period .

Although there is no evidence of an occurrence equivalent to the K-T event , it is possible that an impact from a large meteorite may have been responsible for the mass extinction marking the end of the Cambrian Period and start of the Ordovician Period. Impact craters dating to the Ordovician Period have been identified in Australia .

See also Archean; Cenozoic Era; Cretaceous Period; Dating methods; Devonian Period; 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; Proterozoic Era; Quaternary Period; Tertiary Period; Triassic Period

Ordovician The second geological period of the Palaeozoic era, following the Cambrian and preceding the Silurian periods. It began about 505 million years ago and lasted for about 67 million years. The period was named by the British geologist Charles Lapworth (1842–1920) in 1879. Graptolites, in deep-water deposits, are the dominant fossils. Other fossils include trilobites, brachiopods, ectoprocts, gastropods, bivalves, echinoids, crinoids, nautiloid cephalopods, and the first corals.

Ordovician The second of six periods that constitute the Palaeozoic Era, named after an ancient Celtic tribe, the Ordovices. It lasted from about 510 to 439 Ma ago. The Ordovician follows the Cambrian and precedes the Silurian. It is noted for the presence of various rapidly evolving graptolite genera (Graptolithina) and of the earliest jawless fish. Algae were the predominant plants. Some may have been terrestrial, forming thick, moss-like mats on wet ground.

Ordovician The second of six periods that constitute the Palaeozoic Era, named after an ancient Celtic tribe, the Ordovices. It lasted from about 488.3 to 443.7 Ma ago. The Ordovician follows the Cambrian and precedes the Silurian. It is noted for the presence of various, rapidly evolving, graptolite genera (Graptolithina) and of the earliest jawless fish.

Ordovician The second of 6 periods that constitute the Palaeozoic Era, named after an ancient Celtic tribe, the Ordovices. It lasted from about 505 to 438 Ma ago. The Ordovician follows the Cambrian and precedes the Silurian. Algae were the predominant plants. Some may have been terrestrial, forming thick, moss-like mats on wet ground.

Ordovician Second-oldest period of the Palaeozoic era, 505 to 438 million years ago. All animal life was restricted to the sea. Numerous invertebrates flourished and included trilobites, brachiopods, corals, graptolites, molluscs, and echinoderms. Remains of jawless fish from this period are the first record of the vertebrates.

Ordovician The second (510–439 Ma) of six periods that constitute the Palaeozoic Era, named after an ancient Celtic tribe, the Ordovices. The Ordovician follows the Cambrian and precedes the Silurian. It is noted for the presence of various rapidly evolving graptolite genera and of the earliest jawless fish.

Ordovician •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