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Ordovician

The second-oldest period in the Paleozoic Era. The Ordovician is remarkable because not only did one of the most significant Phanerozoic radiations of marine life take place (early Middle Ordovician), but also one of the two or three most severe extinctions of marine life occurred (Late Ordovician). The early Middle Ordovician radiation of life included the initial colonization of land. These first terrestrial organisms were nonvascular plants. Vascular plants appeared in terrestrial settings shortly afterward.  See also: Geologic time scale

The rocks deposited during this time interval (these are termed the Ordovician System) overlie those of the Cambrian and underlie those of the Silurian. The Ordovician Period was about 7 × 107 years in duration, and it lasted from about 5.05 × 108 to about 4.35 × 108 years ago.

The British geologist Charles Lapworth named the Ordovician in 1879, essentially as a resolution to a long-standing argument among British geologists over division of the Lower Paleozoic. Until that time, one school of thought, that of R. I. Murchison and his followers, had maintained that only a Silurian Period encompassed the lower part of the Paleozoic. Adam Sedgwick and his followers advocated that two intervals, the Cambrian and the Silurian, could be recognized in the Lower Paleozoic. By 1879 Lapworth observed that “three distinct faunas” had been recorded from the Lower Paleozoic, and he pointed out that each was as “marked in their characteristic features as any of those typical of the accepted systems of later age.”

To the stratigraphically lowest and oldest of the three, Lapworth suggested in 1879 that the appellation Cambrian be restricted. To the highest and youngest, Lapworth stated that the name Silurian should be applied. To the middle or second of three, Lapworth gave the name Ordovician, taking the name from an ancient tribe renowned for its resistance to Roman domination.

The type area for the Ordovician System is those parts of Wales and England that include rocks bearing the fossils that composed the second of the three major Lower Paleozoic faunas cited by Lapworth. The Ordovician System in Britain was divided into six major units called series, each distinguished by a unique fossil fauna. The time intervals during which each series formed are epochs. From oldest to youngest, the epochs and series of the British Ordovician are Tremadoc, Arenig, Llanvirn, Llandeilo, Caradoc, and Ashgill. Each of them has a type area in Britain where their characteristic faunas may be collected.

Intervals of shorter duration than those of the epoch are recognized as well in Britain. One set of such intervals is based on the evolutionary development of the fossils called graptolites. These intervals are the graptolite zones recognized by Lapworth and his associates Ethel M. R. Wood and Gertrude Elles. Each graptolite zone was about 3 × 106 to 5 × 106 years in duration. The boundary between the Ordovician and superjacent Silurian System has been designated as the base of the Parakidograptus acuminatus graptolite zone by international agreement. The type locality for that boundary is at Dob's Linn, near Moffat, southern Scotland. Black, graptolite-bearing shales are exposed there.

The Ordovician System is recognized in nearly all parts of the world, including the peak of Mount Everest, because the groups of fossils used to characterize the system are so broadly delineated. The British epochs and zones may not be recognized in all areas where Ordovician rocks are found because the fossils used to characterize them are limited to certain geographic areas. Biogeographic provinces limited the distribution of organisms in the past to patterns similar to those of modern biogeographic provinces. Three broadly defined areas of latitude—the tropics, the midlatitudes (approximately 30–60°S), and the Southern Hemisphere high latitudes—constitute the biogeographic regions. Provinces may be distinguished within these three regions based upon organismal associations unique to each province. Epochs and zones are limited to a single region, and consequently each region has a unique set of epochs and zones for the Ordovician. 

Dynamic interrelationships

The Earth's crust is essentially a dynamic system that is ceaselessly in motion. Plate positions and plate motions are linked closely with, and potentially exert a primary driving force that underlies, ocean circulation, ocean-atmosphere interactions, climates and climate change, and expansion and reduction of environments. Life responds to these physical aspects of the Earth's crust.

Several lines of evidence, including remanent magnetism, distributions of reefs and other major accumulations of carbonate rocks, positions of shorelines, and sites of glacial deposits, may be used to deduce many aspects of Ordovician paleogeography and paleogeographic changes. The Ordovician configuration of land and sea was markedly different from today. Much of the Northern Hemisphere above the tropics was ocean. The giant plate Gondwana was the predominant feature of the Southern Hemisphere. Modern Africa and South America were joined and occupied most the Southern Hemisphere high latitudes. The South Pole lay approximately in north-central Africa. A massive lobe of the plate extended northward from eastern Africa into the tropics. The modern Middle East, Turkey, India, Antarctica, and Australia constituted much of that huge lobe. A number of small plates lay on the margins of Gondwana. Certain of them may have been joined to Gonwana, and others were close to it. Some of these high-latitude plates included Perunica (modern Czech Republic); Avalonia (possibly in two parts, an eastern and a western), which included parts of Wales, southern Britain, Newfoundland, and Maritime Canada; and a number of plates that today make up southern Europe, including those found in the Alps, those that constitute the Iberian Peninsula, and those that made up Armorica (much of France). Plates that were within about 30–55°S latitude included Baltica (modern Scandinavia and adjacent eastern Europe east to the Urals), the Argentine Precordillera, South China, Tarim (in Asiatic China), the Exploits, and perhaps similar small plates. Parts of the Andes within modern northern Argentina, Peru, and Bolivia were within this midlatitudinal interval. Plates in tropical latitudes included Laurentia (modern North America, Greenland, Scotland, and some of northern Ireland), North China, Siberia, one or more plates that made up the Kazakh plate, and several plates that were close to or attached to northern or tropical Gondwana (these make up modern southeastern Asia and southeastern China).  See also: Paleogeography; Paleomagnetism; Plate tectonics

During the early part of the Ordovician (Tremadoc-Arenig), prior to a significant number of plate movements, siliclastic materials (sand, silts, muds) spread northward from a Gondwana landmass into river, delta, and nearshore marine environments on the Gondwana plate and on those plates close to it, especially those in high latitudes. Coeval tropical environments were sites of extensive carbonate accumulations. Most midlatitude plates were sites of siliclastic and cool-water carbonate deposition.

Extensive plate motions and major volcanic activity at the margins of many plates characterize the Arenig-Llanvirn boundary interval of the early Middle Ordovician. Many plates on the Gondwanan margins began a northward movement that continued for much of the remainder of the Paleozoic. In addition, Laurentia bulged upward to such an extent that marine environments, which had covered most of the plate early in the Ordovician, were driven to positions on the plate margins. The Avalonian plates joined and moved relatively quickly northward to collide with the eastern side of Laurentia near the end of the Ordovician. Prior to that collision, the Popelogan or Medial New England plate collided with the Laurentian plate at about the position of modern New England. That collision, which occurred about 455 million years ago, classically has been called the Taconic orogeny. Baltica not only moved northward relatively rapidly, but also rotated about 90° during the latter part of the Ordovician. The Argentine Precordillera plate moved southward across a midlatitudinal interval of ocean to collide with what is today the western side of Argentina in the Middle Ordovician. Africa shifted northward during the Ordovician with the result that northern Africa and the regions adjacent to the Middle East shifted into cool-temperate conditions.

Life and environments

As plate motions took place, environments changed significantly, as well as life in them. Both oceanic and terrestrial settings became the sites of significant radiations.

Early Ordovician (Tremadoc-Arenig) environmental conditions in most areas were similar to those of the Late Cambrian. Accordingly, Early Ordovician life was similar to that of the latter part of the Cambrian. Trilobites were the prominent animal in most shelf sea environments. Long straight-shelled nautiloids, certain snails, a few orthoid brachiopods, sponges, small echinoderms, algae, and bacteria flourished in tropical marine environments. Linguloid brachiopods and certain bivalved mollusks inhabited cool-water, nearshore environments.

Middle Ordovician plate motions were acompanied by significant changes in life. On land, nonvascular, mosslike plants appeared in wetland habitats. Vascular plants appeared slightly later in riverine habitats. The first nonvascular plants occurred in the Middle East on Gondwanan shores. The Middle Ordovician radiation of marine invertebrates is one of the most extensive in the record of Phanerozoic marine life. Corals, bryozoans, several types of brachiopods, a number of crinozoan echiniderms, conodonts, bivalved mollusks, new kinds of ostracodes, new types of trilobites, and new kinds of nautiloids suddenly developed in tropical marine environments. As upwelling conditions formed along the plate margins, oxygen minimum zones—habitats preferred by many graptolites—expanded at numerous new sites. Organic walled microfossils (chitinozoans and acritarchs) radiated in mid- to high-latitude environments. Ostracoderms (jawless, armored fish) radiated in tropical marine shallow-shelf environments. These fish were probably bottom detritus feeders.  See also: Paleoecology

Glaciation

When the Avalon plate collided with the Laurentian, a major mountain chain developed in a tropical setting. Vast quantities of siliclastic materials were shed from that land to form what is called the Queenston delta in the present-day Appalachians. As the Queenston delta grew, glaciation commenced at or near the South Pole. Continental glaciation spread from its North African center for a 1–2-million-year interval late in the Ordovician. Glacially derived materials (including many drop-stones) occurred in the Late Ordovician strata in Morocco, Algeria, southern France, Germany, Spain, Portugal, and the Czech Republic. Sea level dropped by at least 70 m at the glacial maximum. As a result, most shallow to modest-depth marine environments were drained. Karsts formed across many carbonates that had accumulated in the shallow marine settings. Upwelling along many platform margins ceased or became quite limited. As a consequence, former extensive oxygen minimum zones were markedly diminished. The loss of wide expanses of shallow marine environments and extensive oxygen minimum zones led to massive extinctions of benthic marine organisms, as well as those graptolites living near oxygen minimum zones. These extinctions took place over a 1–2-million-year interval as environments shifted, diminished, or eventually were lost. Oxygen isotope studies on brachiopod shells suggest that tropical sea surface temperatures dropped by as much as 4°C.  See also: Geomorphology; Paleoceanography

The latest Ordovician stratigraphic record suggests that the ice melted relatively quickly, accompanied by a relatively rapid sea-level rise in many areas. Some organisms—certain conodonts, for example—did not endure significant extinctions until sea levels began to rise and shelf sea environments began to expand.  See also: Stratigraphy

Ocean surface circulation

Surface circulation in Ordovician seas was controlled in the tropics by the several platforms and in the Southern Hemisphere by Gondwanaland. Equatorial surface currents flowed east to west, but they were deflected by the shallow shelf environments. The tropical or warm-water faunal provinces were influenced by these deflections. Homogeneity of the tropical faunas was maintained by the surface water currents. Southern Hemisphere currents were influenced by the relatively long west coast of Gondwanaland and of the Baltoscanian Plate. Upwelling conditions would have been generated along these coasts. Location, size, and relief on Gondwanaland probably led to monsoonal seasonal reversals in surface ocean currents near what is today South China. Absence of lands or shallow shelf seas north of the Northern Hemisphere tropics would permit oceanic surface circulation to be zonal; that is, currents flowed from east to west north of 30° north latitude, and they flowed from west to east between 30 and 60° north latitude.

Economic resources

Ordovician shelf and shelf margin rock sequences in areas where there has been little post-Ordovician volcanic activity or severe deformation have yielded petroleum and natural gas. Quartzites interbedded with carbonates formed in shelf sea environments have been used as a source of silica for glass manufacture. Ordovician carbonates are hosts for lead-zinc-silver ores mined in the western United States, including Missouri and Washington. Significant quantities of gold were recovered from Ordovician graptolite-bearing strata in eastern Australia in the late 1800s. Gold-bearing Ordovician rocks occur in Nevada (western United States) where they are part of one of the most prolific gold-producing areas in