THE PALEOZOIC ERA PERIODS

ORDOVICIAN PERIOD
SILURIAN PERIOD
DEVONIAN PERIOD
CARBONIFEROUS PERIOD
PERMIAN PERIOD

ORDOVICIAN PERIOD

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 level--about 100 m (330 feet) or more--occurred 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).

SILURIAN PERIOD
Silurian Period, interval of geologic time from 438 to 408 million years ago. It is often divided into the Early Silurian Period (438 to 421 million years ago) and the Late Silurian Period (421 to 408 million years ago). As the third period of the Paleozoic Era, the Silurian Period follows the Ordovician Period and precedes the Devonian. The rocks that originated during the period make up the Silurian System. 


The Silurian System was named by Sir Roderick Impey Murchison in 1835 after the Silures, the ancient British tribe that occupied much of central and southeastern Wales, where rocks of the aforementioned age are well developed. Silurian rocks are found all over the world. The Silurian Period marks an important episode in Earth history at the end of early Paleozoic times, with the first appearance of land plants and jawed fish. Important, too, are the graptolites (especially the single-branched monograptids, whose fossils are useful for correlating Silurian rocks in widely separated areas) and many other marine invertebrate groups, such as the trilobites, brachiopods, conodonts, corals, stromatoporoids, crinoids, and mollusks. 

In North America the Silurian System was formerly divided into the Alexandrian below the Niagaran and the Cayugan above. Silurian rocks are extensively exposed in the Appalachians and across the Midwest, especially in the vicinity of the Great Lakes. Large coral-stromatoporoid-algal reef developments in the middle time interval of the system have often formed oil reservoirs. These roughly correspond, on a much grander scale, to the reef developments on the Swedish island of Gotland and on a smaller scale, to the reef developments on Wenlock Edge in Shropshire in the Welsh borderland of England. 

Niagaran limestones are probably best known in the escarpment that forms Niagara Falls, between Lakes Erie and Ontario. These limestones are followed, notably in the Michigan Basin, by tremendous thicknesses of salt deposits, which are evidence of a widespread regression of the seas at the end of Silurian times. This regression also is seen in Europe in the form of transitional beds yielding fossils such as the large arthropods known as eurypterids. These salt deposits grade into continental facies known as Old Red Sandstone, which are of Devonian age and are found across northern Europe from Ireland to Russia, and in more distant places such as eastern Canada and Kashmir. 

During the Silurian Period the continents were distributed in a totally different way than they are today. Such areas as Arctic Canada, Scandinavia, and Australia were probably in the tropics; Japan and the Philippines may have been inside the Arctic Circle; South America and Africa were most likely in the region of the South Pole, with either present-day Brazil or western Africa as the locus of the pole. In any case, the land surface was buried by an ice sheet that may have been about as large as that covering Antarctica today. 

Volcanic activity was comparatively minor during the Silurian Period, but there were important earth movements, notably the Taconian orogeny (mountain-building event) of North America, which marks the boundary between the Ordovician and the Silurian, and the Caledonian orogeny in northwestern Europe, which probably reached its climax toward the end of Silurian time. The latter orogeny, the effects of which are particularly evident in Scandinavia and the northwestern part of the British Isles, probably resulted from the closing of an early Paleozoic precursor of the Atlantic Ocean.

DEVONIAN PERIOD
Devonian period, interval of geologic time from 408 to 360 million years ago. The fourth period of the Paleozoic era, it follows the Silurian and precedes the Carboniferous. The Devonian is often divided into the Early Devonian epoch (408 to 387 million years ago), Middle Devonian epoch (387 to 374 million years ago), and Late Devonian epoch (374 to 360 million years ago). The rocks that originated during the period comprise the Devonian system. 
Rocks of the Devonian system are found in every continent, but they were first recognized as a major group in the counties of Devon and Cornwall in southwestern England. The research of the early geologists Sir Roderick I. Murchison and Adam Sedgwick established the Devonian in 1839. Their work was soon corroborated by that of European and American investigators studying similar strata elsewhere. 

The period was one of significant changes in world paleogeography. The physiognomy of the Earth was substantially different from what it is today; a giant continent was situated in the Southern Hemisphere, and other land masses were located in the equatorial regions. Siberia was separated from Europe by a broad ocean. The continents of North America and Europe had collided during the Early Devonian, resulting in the Caledonian orogeny (mountain-building episode). This event involved much igneous activity, as, for example, the intrusion of huge granitic bodies known as plutons. In the Middle Devonian, the Acadian mountain-building episode took place in northern Appalachia in what is now the eastern United States. There were also major movements of the tectonic plates (those that make up the Earth's crust and upper mantle) in northern and western North America, western South America, East Asia, and eastern Australia. Many parts of the world also experienced intense volcanism and seismic activity.

The equatorial seas separating the continents contained extensive reefs built by corals and other organisms. Large landlocked areas of shallow sea in North America, Central Asia, and Australia became evaporite basins in which great quantities of rock salt, gypsum, and other minerals were precipitated. On the continental platforms themselves and at their margins, vigorous erosion of the uplifted mountainous areas produced great volumes of coarse sediment, which were deposited in the lowlands and seas to become the Old Red Sandstone formations. Desert conditions prevailed in many areas, whereas in others there was abundant rainfall. The Devonian year is thought to have had some 400 days, each consisting of about 21 hours. This would have had a small but not insignificant effect on tides and perhaps rates of coastal erosion, aiding the rapid spread of the seas. 

Evidence indicates that the volcanic activity along the fissures of oceanic ridges was influential in the great flooding of the continents in Devonian times. As the ridges grew with the extrusion of molten rock, the ocean waters rose onto the margins of the continents, covering by Late Devonian times a substantially larger percentage of the Earth than today. 

The Devonian was characterized by varied faunal and floral life forms. Among the marine invertebrates, the bivalve brachiopods and conodonts became highly diversified and abundant. The mollusks made evolutionary strides, and the ammonoids (a group of now-extinct cephalopods) emerged, evolving from their continuing nautiloid ancestors. Corals and similar animals were abundant, and reef habitats for other invertebrates became widespread. By contrast, the graptolites became extinct and the trilobites declined (only the proetaceans survived). 

Many types of primitive fish appeared and proliferated in both marine and freshwater environments during the Devonian. Because of this, the period is commonly referred to as the "Age of Fishes." Jawless, heavily armoured varieties (those of the class Agnatha) developed, as did sharklike forms and lungfish. By the end of the period the rhipidistians, a group of air-breathing, lobe-finned bony fishes, apparently gave rise to four-footed amphibians. 

Vascular plants underwent tremendous diversification during Devonian times. The colonization of ferns and primitive gymnosperms resulted in the formation of the first forests, such as the Gilboa forest in the northeastern United States. These were the forerunners of the great swamp forests of the Carboniferous Period (from 360 to 286 million years ago). 

Toward the end of the Devonian, a number of marine invertebrates suffered extinction. Particularly hard hit were the colonial tabulate corals, stromatoporoids, and various other groups associated with the reef environment. 

CARBONIFEROUS PERIOD
Carboniferous Period, interval of geologic time from 360 to 286 million years ago. It is often divided into the Early Carboniferous Epoch (360 to 320 million years ago) and the Late Carboniferous Epoch (320 to 286 million years ago). The fifth period of the Paleozoic Era, the Carboniferous Period follows the Devonian and precedes the Permian Period. The rocks that originated during Carboniferous time make up the Carboniferous System. 
A brief treatment of the Carboniferous Period follows. For full treatment, see Geochronology: Carboniferous Period . 

The Carboniferous System was identified by William Daniel Conybeare and John Phillips in Great Britain in 1822. Its name refers to the presence of coals in many parts of the succession of strata between the Old Red Sandstone (Devonian) and the New Red Sandstone (Permian) formations in Britain. In North America the Carboniferous is often represented by the Mississippian and Pennsylvanian periods, which are named after the states in which the strata are widely developed and which correspond to the Early Carboniferous and Late Carboniferous time periods. In Europe the Carboniferous System is often subdivided into the Lower Carboniferous and Upper Carboniferous, which correspond to the time units Early and Late. The classification of the Carboniferous rock units and their corresponding time units had become so complex by the end of the first quarter of the 20th century that the International Geological Congress and later the Carboniferous Subcommission of the International Union of Geological Sciences set out to establish criteria by which to define an international time scale for the period. The boundaries of the system and their time equivalents have been generally agreed on, but minor subdivisions are still subject to debate. 

The distribution of the continental landmasses continued to change during the Carboniferous Period. All the landmasses drew closer together as tectonic plate movements gathered them toward the equatorial regions and the southern half of the globe. The enormous continent of Gondwana, made up of what is now Africa, South America, India, the Middle East, Australia, and Antarctica, occupied much of the Southern Hemisphere. Early in the period, Laurussia, consisting principally of present-day North America, Greenland, and northern Europe, advanced toward Gondwana's northern margin. By the end of the Carboniferous Period, most of Laurussia had collided with Gondwana, closing the Tethys seaway between them. The ensuing Appalachian-Hercynian orogeny fused the two continents together. Siberia and China (including Southeast Asia), which were individual continents at this time, remained at high latitudes in the Northern Hemisphere. 

In Gondwana a heavy and prolonged continental glaciation occurred at a south polar centre situated in the vicinity of southern Africa. Later, as the ice diminished, the sea level was restored and the climate ameliorated. Swamps became widespread in both northern and southern continental areas and forest vegetation thrived. The coal deposits of the later Carboniferous times were formed from the debris of these forests, and the widespread and repeated growth of the coal swamp forests was a most characteristic feature of the Carboniferous Period. This episode of coal formation was terminated by a rise in the landmasses and an increasingly arid climate. The end of these environments presaged the great crisis that was to affect much of the living world in the following Permian Period. 

In addition to the great advances made by plants with their complex forest assemblages, the vertebrates were undergoing an evolutionary radiation. Amphibians became widespread and diverse, with some as large as 2 m (6 feet). Reptiles appeared for the first time and rapidly adapted to many habitats. 

PERMIAN PERIOD
Permian Period, interval of geologic time from 286 to 245 million years ago. The Permian is the last of the six periods of the Paleozoic Era; it follows the Carboniferous and precedes the Triassic of the subsequent Mesozoic Era. The Permian is often divided into the Early Permian Epoch (286 to 258 million years ago) and the Late Permian Epoch (258 to 245 million years ago). The rocks that originated during the period make up the Permian System. 
A brief treatment of the Permian Period follows. For full treatment, see Geochronology: Permian Period . 

The Permian was named in 1841 by the early British geologist Sir Roderick I. Murchison for the Perm region of the Russian Urals, whose rocks became a standard reference for the Permian System. The rocks of the system are characterized by an abundance of red land-laid sediments and evaporites (salts formed by a sequence of chemical precipitation in an acid environment). 

Massive movements of crustal plates continued from the preceding Carboniferous Period into the Permian. The northern hemispheric continent of Laurasia (composed essentially of present-day North America, Greenland, Europe, and Asia) became linked to the southern hemispheric landmass of Gondwana (composed of what is now South America, Africa, India, Australia, and Antarctica), forming one single supercontinent by the mid-Permian. This immense assembly of landmasses, known as Pangaea, stretched across all the climatic temperature zones virtually uninterrupted from pole to pole. The extensive glaciation that had begun in the Carboniferous continued into the Permian, mainly in the southern portion of Gondwana that was located around the south paleopole. Hot, dry conditions prevailed almost everywhere else on Pangaea, and, by the Late Permian, deserts had become widespread in several tropical and subtropical areas on the supercontinent. 

Laurasia and Gondwana remained separated at their eastern extent by the Tethys Sea, and the rest of the Earth's surface was covered by a vast ocean known as Panthalassa. Orogenies (mountain-building episodes) associated with the formation of Pangaea by the collision of these continents continued into the Permian, further forming the Appalachian mountain chain of North America and the Hercynian mountain chains of Europe and Asia. 

In eastern Europe the stable Russian Platform in the east flanked a deep-sea trough, the Ural Geosyncline, which was compressed, folded, and uplifted to form the Ural Mountains when the Angaran segment of western Siberia collided with eastern Laurasia. 

In western Europe continental and restricted basin deposits, such as red sandstone, conglomerate, shale, dolomitic limestone, and evaporites, predominated. Permian deposits are widespread in North America, particularly in Texas, New Mexico, Nevada, and Utah, where marine and continental deposits accumulated in great thicknesses. North American Permian deposits are principally sandstones, shales, limestones, and evaporites. Deposits of limestone, sandstone, shale, cherts, and basalt pillow lavas were also formed in the shallow and deepwater environments of the Tethys Sea. The Permian deposits on the landmasses that made up Gondwana exhibit similar characteristics; they contain tillites (consolidated sedimentary rocks formed by the lithification of glacial till) and sediments deposited by glacial meltwater, as well as sandstones, shales, and coal. This rock sequence indicates that during the Permian the world climate became milder through alternating warm and cool stages. Late Permian deposits in Pangaea are characterized by dune sands, evaporites, and red beds (e.g., shale and sandstone of reddish colour), indicating the onset of desert conditions at the close of the period. 

Economically important mineral deposits of Permian age consist of evaporite minerals (such as halite, sylvite, and gypsum), coal, petroleum, and copper ore. Significant evaporite deposits of rock salt and potash minerals are found in the southwestern United States, Russia, Poland, The Netherlands, Denmark, and England. Important Permian coals are found in northern Siberia, China, Korea, India, Australia, and southern Africa. Permian rocks also hold significant oil and natural-gas deposits in the southwestern United States (western Texas, Oklahoma, and New Mexico) and in Russia. 

The fossil record indicates that life evolved from the preceding Carboniferous into the Permian as a continuation of established faunas. The marine invertebrates, mainly brachiopods, ammonoids, bryozoans, bivalves, and foraminiferans, evolved into several lineages during the period. The rapid evolutionary rate of the ammonoids and fusulinid foraminiferans make them extremely useful for geologic dating in the Permian. Marine and freshwater fish lineages and the amphibians thrived in the Permian. Of the vertebrates, the reptiles evolved into three markedly distinct groups: the cotylosaurs, the pelycosaurs, and the therapsids (mammallike reptiles). Land plants evolved from the Carboniferous ferns and seed ferns to the conifers and adapted to drier and well-drained land conditions. An abrupt change occurred in Gondwana with the widespread development of the Glossopteris flora, which were tongue ferns adapted to the colder climates of the Southern Hemisphere. 

Toward the close of the Permian, however, many forms of life suffered mass extinction. In Gondwana the Glossopteris flora disappeared, but the coniferous flora of the Northern Hemisphere survived into the Triassic. Many animal groups did not survive the Permian. Trilobites and fusulinids became extinct, and the brachiopods, ammonoids, and bryozoans were severely reduced. Uncertainty still exists over the events that led to these extinctions at the close of the period.

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