ORDOVICIAN PERIOD

interval of geologic time from 505 to 438 million years ago. The second oldest period of the Paleozoic Era, it follows the Cambrian and precedes the Silurian. The Ordovician Period is often divided into the Early Ordovician Epoch (505 to 478 million years ago), the Middle Ordovician Epoch (478 to 458 million years ago), and the Late Ordovician Epoch (458 to 438 million years ago). The rocks that originated during the period constitute the Ordovician System. The Ordovician System was proposed in 1879 by the English geologist Charles Lapworth for rocks exposed in the Arenig Mountains, located along the border of England and Wales and eastward to the Bala district of North Wales. This was part of an area inhabited by an ancient Celtic tribe called the Ordovices. Fossiliferous sedimentary rocks of Ordovician age occur on all the modern continents. The rock types and constituent fossils indicate that the configuration of the continents and their geographic distribution during the Ordovician were quite different from those of today. Many of the landmasses, including Laurentia (consisting primarily of present-day North America and Greenland), Siberia, Kazakstan, North China, Australia, and segments of Southeast Asia and Antarctica, were aligned in the tropics during Ordovician time. As the period proceeded, Laurentia and Baltica (composed mainly of present-day northern Europe, including Scandinavia) began to move toward each other, and the Iapetus Ocean that lay between them started to close. A subduction zone (i.e., a narrow region where two of the various lithospheric plates that make up the Earth's surface come together, with one of the plates buckling downward beneath the other) formed along the eastern margin of North America. Mountain building and volcanic activity took place along this region in the Late Ordovician. Also, during the course of the Ordovician, a large continent known as Gondwana (which included what is now South America, Africa, southern Europe, much of the Middle East, India, and Australia) began moving over the South Pole. Continental glaciers developed in the area of central and northern Africa. A sharp lowering of the sea levelabout 100 m (330 feet) or moreoccurred as a result of this south polar glaciation, which lasted roughly 10 to 15 million years. Ordovician life was dominated by marine invertebrates. The close of the Cambrian Period had been marked by a mass mortality among the trilobites. They had been the predominant marine invertebrates, and the sharp reduction in their numbers opened many marine environments to colonization by other types of animals in the Ordovician Period. Graptolites (colonial organisms with a skeleton of tough chitinlike material) evolved rapidly at this time, as did various forms of brachiopods (lamp shells). Other important marine invertebrates, such as the bryozoans, crinoids, and tabulate corals and tetracorals, appeared for the first time during the Ordovician in the tropical shelf seas. Nautiloids also populated the shallow sectors of these warm seas, and bivalve mollusks inhabited various near-shore environments. Ostracoderms (jawless armoured fishes) lived in the near-shore tropical waters as well. It is thought that some forms of land plants may have appeared during the mid-Ordovician. Spores suggestive of a tropical terrestrial environment have been found in rocks of that age. Table 4: Geologic time scale. To see more information about a period, select one from the chart. second oldest period of the Paleozoic Era, thought to have covered the span of time between 505 and 438 million years ago (see Table), although radiometric age determinations may range from as much as 515 to 435 million years ago. The Ordovician Period is often divided into the Early Ordovician Epoch (505 to 478 million years ago), the Middle Ordovician Epoch (478 to 458 million years ago), and the Late Ordovician Epoch (458 to 438 million years ago). The rocks that originated during the period make up the Ordovician System. Additional reading Discussions of the development of knowledge of Ordovician time correlations, climates, sea-level changes, and life are offered in R.J. Ross, Jr., The Ordovician System, Progress and Problems, Annual Review of Earth and Planetary Sciences, 12:307335 (1984); and in a collection of essays by David L. Bruton (ed.), Aspects of the Ordovician System (1984). William B.N. Berry Ordovician environment Paleogeography and paleoclimate Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the During the Ordovician, the major landmass, Gondwana, extended from the South Pole northward into the tropics (see figure). The geologic history of areas included in Gondwana indicates that the continent fragmented during post-Ordovician time. A number of major lithospheric plates developed from those that made up Ordovician Gondwana. Moreover, several large plates already existed within the tropics during the period. Most of them appear to have been centred approximately on the equator. As can be seen from the map, the Northern Hemisphere north of the tropics consisted primarily of open ocean. The plate on which modern Scandinavia and the Baltic area are situated moved from south of the tropics into the tropics during the Ordovician. Continental glaciers formed in the area of modern central and northern Africa. This area was close to the South Pole during the Late Ordovician. Pronounced glacio-eustatic lowering of sea level, perhaps as much as 100 metres, took place as a consequence of the South Polar glaciation. Late Ordovician glaciation persisted for about 8 to 10 million years. Significant worldwide marine regressions occurred near the end of the Tremadocian and Llanvirnian ages. These regressions appear to have resulted from plate motion rather than from glaciation. Ordovician environmental controls on organismal distribution Geochemical and fossil analyses of Ordovician rocks suggest that the carbonate suite formed in shelf-sea environments analogous to those of the modern Florida Keys and Bahama Banks. The fossil content of the dolomites and associated limestones is primarily algae (notably the mat-forming variety) and nautiloids and small snails. The limestones bear the remains of brachiopods, bryozoans, crinoids, tetracorals, tabulate corals, and sponges and spongelike organisms. Trilobite fossils are not as common in the carbonate rocks as they are in the siliclastic sequences. The latter, on the other hand, contain significantly fewer brachiopods, bryozoans, corals, and crinoids than do the limestones. Besides the remains of trilobites, diverse trace fossils are found more commonly in the siliclastics. Most siliclastic successions formed in Southern Hemispheric, nontropical marine shelf environments in which surface waters were cooler than in tropical shelf seas. Source terrains may have been relatively nearby; thus many of the shallow-shelf environments could have been sites of (at least seasonally) turbid waters. Graptolitic sequences appear to have formed in marine shelf, slope, and open ocean environments in which waters beneath the surface had little to no oxygen. Geochemical studies of Ordovician rocks suggest that the oxygen content of the atmosphere and, accordingly, of the surface ocean water was less than it is today. If this were indeed the case, then most marine environments would have had less dissolved oxygen than analogous modern environments. Because there is less dissolved oxygen in warm waters than in cool waters, the tropical shelf seas could have contained very little oxygen close to the watersediment interface. Nontropical shelf seas could have been mixed by storms, perhaps seasonally. In addition, because the oxygen content of these shelf seas was greater than that of the tropical shelf seas, relatively more oxygen was available for benthic organisms living in marine shelf-sea environments in nontropical latitudes. During the Ordovician, waters analogous to those of the modern oceanic-oxygen minimum zone may have been anoxic. Furthermore, such oxygen-depleted waters probably extended markedly deeper in the Ordovician oceans than they do in modern oceans. Certain Ordovician deep-seafloor sediments, notably red mudstones and cherts found in western Newfoundland, suggest that the deep oceans were at least moderately ventilated. Ordovician life The close of the Cambrian Period was marked by a mass mortality among the trilobites. As they had been the predominant shelf-sea marine invertebrate, the sharp reduction in their numbers opened many marine environments to colonization by other animal forms. Orthid brachiopods radiated (i.e., rapidly dispersed into different environments) noticeably during the Ordovician. Strophomenid and rhynchonelloid brachiopods appeared and moved into many shelf-sea environments. Bryozoans, crinoids, and both tabulates and tetracorals appeared for the first time. Their appearance and initial radiation was in Ordovician tropical shelf-sea environments. Trilobites reradiated, particularly in the siliclastic, nontropical shelf seas. Bivalve mollusks diversified modestly in nearshore siliclastic environments. Nautiloids radiated significantly in the shallow, algae-rich tropical shelf seas early in the Ordovician. During that time, many nautiloids evolved to nektonic modes of life, possibly as a result of the ability to generate gas in their shells. Snails and monoplacophorans lived in the same shallow-shelf, carbonate environments as the nautiloids. Graptolites with planktonic modes of life appeared during the Tremadocian Age. During the remainder of the Ordovician, planktonic graptolites underwent a major transformation, changing from colonies with many branches to those with only a few. In connection with this development, graptolite colonies that formed from two branches appeared. In the early varieties of such graptolite colonies, the two branches were oriented in such a way that the zooids along them faced downward from the sea surface. In the later varieties, the zooids faced toward the sea surface. Zooids in the many-branched colonies faced downward from the sea surface. The geologically oldest remains of nearly complete fish skeletons occur in Early Ordovician strata in Australia. These fish fossils are imprints on sandstones formed in nearshore marine environments. The remains are of ostracoderms, jawless armoured fish. Other, but disarticulated, ostracoderm remains are known from rocks of slightly younger Ordovician age in the western United States. The Ordovician ostracoderms occur in nearshore tropical marine settings. All marine animals suffered mass mortalities during the Late Ordovician. Presumably, these large-scale die-offs, observed among both shelly-fossil organisms and graptolites, were the result of environmental changes associated with the prolonged continental glaciation in the Southern Hemisphere. Mass mortalities at the close of the Cambrian and late in the Ordovician resulted in the unique aspects of the Ordovician fauna. The Late Ordovician mortalities created new opportunities for benthic and planktonic marine organisms. Reradiation during post-Ordovician glaciation led to many new taxathose characteristic of the Silurian. Some form of terrestrial plant life may have developed during the mid-Ordovician. Spores suggestive of land, but not vascular, plants occur in siliclastic Ordovician rocks on those lithospheric plates that were in the tropics at the time. Distribution patterns of Ordovician marine organisms are consistent with plate distributions deduced from studies of remanent magnetism. The Ordovician tropical shelf seas were sites of a warmwater fauna distinct from that found in the nontropical shelf-sea rock suites. Studies of both benthic and planktonic organisms suggest that the several tropical plates were separated from one another. Faunas suggestive of distinct provinces may be found in rock suites on each plate. Faunal provincialism was marked during the Early Ordovician. As the plate bearing what is now the Baltic region and Scandinavia moved into the tropics during the mid-Ordovician, many new taxa were introduced to tropical environments. A marked incursion of such new taxa, seen in both planktonic and benthic faunas, typifies the basal part of the Caradoc Series. The ancestors of some of these taxa are found in the BaltoScanian rock suites of Early Ordovician age. Ordovician faunal distribution patterns appear to reflect plate positions and relative plate motions, documenting the influence of plate tectonics on the distributions of organisms. William B.N. Berry Ordovician rocks Types and distribution Fossiliferous Ordovician strata occur on all present-day continents. In general, these rocks may be divided into shelly-fossil-bearing facies and graptolitic facies, as noted above. The shelly facies may be subdivided into two major rock suites. One of them, the carbonate suite, includes limestones and dolomites formed in supratidal to moderate-depth shelf-sea environments. Such environments and sediment accumulations occur most commonly in the latitudinal range of 3035 S to 3035 N latitude. The carbonate rock suite formed in shelf-sea environments on lithospheric plates that were within the tropics during the Ordovician. The second shelly-fossil-bearing rock suite consists primarily of siliclastic rocks, including various types of sandstones and siltstones and some mud rocks. The siliclastic shelly-fossil rock suite formed in marine shelf environments around Gondwana in nontropical latitudes and in environments where plate motion led to the development of a landmass. Such a landmass was formed along what is today the Appalachian Mountain region in eastern North America and the site of the ancient Caledonian orogenic belt that extended from Ireland and Scotland northeastward through Scandinavia. Thinly laminated, nonbioturbated (sediments undisturbed by organisms), black, graptolite-bearing shales are found rimming many successions of siliclastic rock containing shelly fossils. Similarly, thinly laminated, black, nonbioturbated graptolitic limestones rim shelly-fossil-bearing carbonates. The rocks with graptolites accumulated in marine environments that ranged from the deeper parts of the continental shelf, slope, and rise to parts of the open sea. The source of the graptolite-bearing rocks was primarily the materials accumulating in the shallow shelf and intertidal environments. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Several platesincluding Laurentia, Siberia, Kazakhstania, North China, part of Southeast Asia, Australia, and part of Antarcticathat were aligned in the tropics during the Ordovician (seefigure) are sites of Ordovician carbonate rock suites. Dolomites and evaporites (in the main gypsum and anhydrite) commonly occur in the central parts of these plates. Limestones rim the dolomite and gypsumanhydritesalt sequences. The dolomitegypsumanhydritesalt sequences appear to have formed in the shallowest tropical shelf-sea environments. Since these sequences may be more than 2,000 metres thick and may have accumulated during only a part of the Ordovician, the plates are thought to have been subsiding throughout the period. Whether or not this plate subsidence is related to some form of lateral plate motion has not been documented. The movement of the BaltoScanian plate into the tropics during the Ordovician was accompanied by the accumulation of carbonates in shelf-sea environments on that plate as it moved northward. Siliclastic shelly-fossil rocks occur in western South America, North Africa, the Middle East, southern Europe and the British Isles, and parts of southern China. Glacio-marine sequences are common in the Late Ordovician (Late Caradoc and Ashgill) parts of the rock sequence in these areas. The Late Ordovician segment of the rock succession in central northern Africa contains glacial materials, including moraines and kame terrace deposits. Correlation Correlations among the Ordovician facies and rock suites are difficult, because graptolites rarely occur with shelly fossils and shelly-fossil-bearing rocks are seldom found in rocks with graptolitic facies. Most of the synchroneities that have been established between the facies are achieved by finding rock debris that contains shelly fossils or gravity-flow materials in graptolitic sequences. Because the several carbonate suite-bearing plates appear to have been separated longitudinally, synchroneities among rock suites on each plate have proved to be difficult to achieve. A set of Ordovician divisions is recognized for application in rock sequences on each plate. Correlations among these divisions have not been established definitively, however. Certain correlations among rock suites are shown in the Table.