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. No hypothesis explaining this worldwide event has been entirely acceptable. More plausible causes include the deepening of the shallow shelf seas, which led to the destruction of the reefs and related fauna, or the widespread occurrence of anoxic (oxygen-deficient) conditions following the major transgression and regression of oceanic waters at this time. Additional reading For the Devonian rocks, environment, and life-forms, see D.L. Dineley, Aspects of a Stratigraphic System: The Devonian (1984); M.R. House, C.T. Scrutton, and M.G. Bassett (eds.), The Devonian System: A Palaeontological Association International Symposium (1979); W.S. McKerrow and C.R. Scotese (eds.), Palaeozoic Palaeogeography and Biogeography (1990); and N.J. Mcmillan, A.F. Embry, and D.J. Glass (eds.), Devonian of the World: Proceedings of the Second International Symposium on the Devonian System, 3 vol. (1988). Michael R. House Devonian environment Rock types of the Devonian System indicate that most environments of the present day were represented but that they were very differently distributed. During Devonian times the equator is thought to have passed across Laurussia, and so the Old Red Sandstone continent was essentially tropical or subtropical. Paleomagnetic evidence, however, is not clear, and various positions for the equator have been proposed. Furthermore, paleomagnetic evidence can suggest former magnetic latitudes, but it does not indicate longitudinal position. For Gondwana, the evidence suggests a pole probably in the South African area during the Devonian. Paleogeography A description of the physical geography of the Devonian can be attempted using evidence from paleomagnetism, paleoclimate, paleobiogeography, and tectonic reconstruction. Because the paleomagnetic data for the Devonian remains problematic, recent efforts to elucidate paleogeographic position have concentrated on the rock types associated with particular environments and to a lesser extent on faunal distributional data. Such methods use the distribution of evaporites, shelf carbonates, and hermatypic corals, since the present-day aspects of these deposits have specific climatic constraints. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during Early The reconstruction for the later part of the Early Devonian (shown in the figure) reflects one interpretation of continental distribution. There is general agreement that the equator crossed the northern part of the Old Red Sandstone continent during the Devonian but that it migrated southward over the course of the period. This is indicated by the reduction in evaporitic environments in western Canada and the onset of humid and moist conditions in the area of New York. Evidence of nonmarine fish and marine invertebrates provides links across the northern area between Europe, Siberia, and the Canadian Arctic islands. Positioning in relation to Gondwana is more difficult. Some interpretations favour a wide ocean separating Gondwana from Laurussia. This arrangement is thought unlikely because of the remarkable occurrences of similar corals, brachiopods, and ammonoids between eastern North America, Morocco, and Spain. Yet, even if they were close together, precise positioning is a matter for dispute, and using the preceding argument some would have North Africa adjacent to the eastern North American seaboard. There is general agreement that the southern continents of today were united during the Devonian along the lines of their present-day continental slopes. Paleomagnetic evidence is, however, inconsistent on the position of the South Pole; some suggestions favour central South America, while others advance positions in South Africa or sites off the southeast coast. The late Devonian reef developments in Western Australia suggest a near tropical site. During the Devonian, Asia was composed of many separate microplates that are now joined together. Of these, Siberia and Kazakhstania began fusing during the late Devonian and later joined Laurussia, forming the Ural Mountains along the junction. The positions of the other microcontinents are rather uncertain, but many of them were probably either attached or adjacent to the northern margin of Gondwana and migrated north to fuse with growing Asia at several junctures during the later Phanerozoic. Devonian life A highly varied invertebrate fauna derived from that of the Silurian continued in the Devonian, and most ecological niches of shallow and deep marine water were exploited. The remarkable proliferation of primitive fish, which has given the period the name the Age of Fishes, occurred in both fresh and marine waters. Derivation of carnivorous fish from mud-eating forms occurred early in the period, and the tetrapods were derived from the fish near the close of the period. Also remarkable is the rise to dominance of the vascular plants. By the mid-Devonian the first tree forests are known in place, but rich groves must have occurred earlier to provide the widespread plant debris. Invertebrates The Devonian invertebrate faunas are essentially of the type established by the Ordovician. In nearshore sandy and silty environments bivalves, burrowing organisms, brachiopods, and simple corals abounded. In offshore reefs, free from land detritus, biostromes and bioherms flourished, rich in corals, stromatoporoids, crinoids, brachiopods, trilobites, gastropods, and other forms. In deeper waters the cephalopod goniatites, one of the few new groups to appear, were abundant; and there is evidence that the surface levels of these deep waters were occupied by small dacryoconarids of uncertain affinity and by ostracods (arthropods) later in the period. Both Foraminifera and Radiolaria among the Protozoa are well known, and sponges were locally abundant; the famous dictyospongoids of New York are an example. The corals and stromatoporoids among the coelenterate Hydrozoa were extremely important in the reef facies. Elsewhere, only simple corals are frequently found. The limestone-reef and forereef facies and biostromal limestones are known in many areas of the world. The corals include tabulate corals, such as Favosites and Alveolites, but especially rugose corals, which have been used to establish correlations. Amphipora is a common rock-building type in the mid-Devonian of the Northern Hemisphere, and its twiglike form produces a spaghetti or vermicelli rock. Bryozoa were especially common in shallow shelf seas of the period, and rich faunas are known from North America. Both stony (trepostomatous) and netted forms occurred, but the latter, the fenestellids, became important during the period. The brachiopods of the Devonian show great diversity. The spire-bearing spiriferoids were perhaps the most common and have been used for zonation. Two groups of importance emerged during the Devonian: the loop-bearing terebratulids and the spiny, mud-dwelling productids. At the same time, a number of groups became extinct, including various orthids and the pentamerids. Molluscan groups were well represented. The marine clams (Bivalvia) increased greatly during the period, especially in the nearshore environments. The earliest freshwater bivalves appeared in the Late Devonian. The gastropods were well diversified, particularly in calcareous environments, but much less than in later periods. The Scaphopoda first appeared here. A significant Devonian event was the origin of the ammonoids from their continuing nautiloid ancestors. In the chambered shell of the ammonoids, the siphuncle is ventral or outermost in position (except in Late Devonian clymenids), and the septa commence the elaborate folded patterns that culminate in the ammonites of the Mesozoic. From their appearance, probably in the Emsian, the evolution of the goniatites and later ammonites allows a detailed zonal subdivision to be established through the end of the Cretaceous. Devonian goniatites have been found on all continents except Antarctica. Among the Arthropoda the giant Eurypterida are found in the Old Red Sandstone facies. Some were predacious carnivores and probably lived on fish. The first insect, a supposed collembolan, has been recorded from the Devonian of Russia and other areas of the former Soviet Union. Ostracods were locally very abundant; benthic forms occur in shelf-sea deposits and planktonic forms in the Upper Devonian, where their remains form the widespread ostracod-slate facies or cypridinenschiefer. The trilobites were well developed in terms of size (some up to 61 centimetres long), variety, and distribution. Nearly all have clearly established Silurian ancestors. The most common were the phacopids, which exhibit a curious trend toward blindness in the Late Devonian. Almost all the diverse Lower Paleozoic trilobite stocks that entered the period were extinct before the close, and only the proetaceans survived into the Early Carboniferous. Among the Echinodermata, holothureans, asteroids, and ophiuroids are known, but they are rare. Crinoids were abundant, including free-living types with grapnel-shaped anchors. The blastoids diversified considerably, but the cystoids did not survive the period. Conodonts had perhaps their greatest diversification during the Late Devonian and have proved of major importance for correlation. Devonian rocks Occurrence and distribution It generally is believed that Europe and North America were united approximately along the present continental slope margins during the Devonian Period. At the close of the Silurian and continuing in the Early Devonian, considerable igneous activity occurred in the belt including New England, Nova Scotia, Newfoundland, Scotland, Scandinavia, and eastern Greenland. With North America and Europe joined as described, the belt thus indicated formed a mountain tract of active uplift. This is the Caledonian mountain belt that resulted from the Caledonian orogeny. The deposits of the Old Red Sandstones appear to be the detritus produced by the erosion of these mountain areas. The marine Devonian rocks of western Canada and those in a belt from Montana to New York in North America, in Europe from Devon to the Holy Cross Mountains of Poland, on the Russian Platform and Novaya Zemlya, and, again, in the Arctic Islands of Canada appear to provide evidence that marine waters encircled the Old Red Sandstone continent. The accompanying world map shows the distribution of most of the major outcrops of Devonian rocks. In many areas the Devonian rocks have been much disturbed tectonically by subsequent deformation. These fold belts may be distinguished from cratonic areas where sediments remain much as they were when formed. The main fold belts in North America are the Cordillera (western mountain ranges, including the Rocky Mountains) and the Appalachian belts in the east. In contrast, the Devonian of the Midwest and adjoining areas is flat-lying. In South America, the main fold belt is the Andes and sub-Andes, and east of this line the Devonian rocks are little disturbed. In Australia the main fold belt is in the east from Queensland to Tasmania. In Europe the Armorican fold belt stretches eastward from Cornwall and Brittany. To the south of this line from the Pyrenees to Malaysia, Devonian rocks are caught up in the Alpine-Himalayan fold belt. Similarly, the Devonian of the Ural Mountains is disturbed, whereas to the west, on the Russian Platform, and to the east there is less deformation. In all these cases the folding occurred well after the Devonian, but there is evidence that Devonian sedimentation contributed to the oceanic belts that were sites of the mountain building that occurred later. In the regions that have suffered severe deformation, the Devonian sediments are frequently metamorphosed into slates and schists and often lose all the characters by which they may be dated. In areas where little change has taken place, all rock lithologies occur, from those characteristic of continental and desert conditions to the varied lithologies associated with shelf and deep-sea accumulation. Contemporary igneous activity is widespread, both in the form of extrusive lavas, submarine pillow lavas, tuffs, agglomerates, and bentonites and also igneous intrusion. Extrusive activity is found in both continental and marine environments, whereas plutonic intrusions are usually linked with areas of uplift such as the Caledonian and Acadian belts of Europe and eastern North America. For convenience in description this account will commence with a brief review of the European and North African sequences and then pass eastward to Russia, China, and Malaysia. Treatment of the southern continents from New Zealand to South America will follow, and North America will be considered last. Europe A line passing from the Bristol Channel eastward to northern Belgium and Germany roughly demarcates the Devonian marine area south of the Old Red Sandstone continental deposits, which characteristically are red-stained with iron oxide. The continental deposits extend also to Greenland, Spitsbergen, Bear Island, and Norway. The British geologist Robert Jameson coined the term Old Red Sandstone in 1808, mistakenly thinking it to be A.G. Werner's Aelter Rother Sandstein, now known to be of Permian age. The rocks of this wide area have a remarkable affinity in both fauna and rock type and are usually considered to have been united in Devonian times. The relations with the underlying Silurian system are seen in the classic Welsh Borderlands, where the Ludlow Bone Bed was taken as the boundary until international agreement placed it somewhat higher. In Wales, southern Ireland, and the Scottish Lowlands, thicknesses of detritus, chiefly sandstones, accumulated to as much as 6,100 metres in places, and widespread volcanics occur in Scotland. These sediments are rich in fish and plants, as are the eastern Greenland and Norwegian deposits. Devonian rocks in Devon and Cornwall are mostly marine, but there are intercalations of terrestrial deposits from the north. In northern Devon at least 3,660 metres of shales, thin limestones, sandstones, and conglomerates occur, the latter two lithologies typical of the Hangman Grits and Pickwell Down Sandstones, which are the main terrestrial intercalations. However, in southern Devon reef limestones are in the Middle Devonian, and the Upper Devonian locally shows very thin sequences formed on submarine rises and contemporary pillow lavas in basinal areas. In northern Cornwall both the Middle and Upper Devonian are primarily in slate facies. Fossils found in these rocks have permitted detailed correlations with the Belgian and German sequences. Devonian rocks of mixed terrestrial and marine type are known from boreholes under London, and these form a link with the Pas de Calais outcrops and to the classic areas of the Ardennes. There, between the Dinant Basin and Namur Basin to the north is evidence of a northward landmass, as in Devon. Both the Lower and Upper Devonian consist of near-shore and terrigenous sediments that reach thicknesses of 2,740 metres and 460 metres, respectively. The Middle Devonian and lower Upper Devonian (i.e., the Eifelian, Givetian, and Frasnian stages, whose type sections are here) consist mainly of limestones and shales and reach at least 1,500 metres in the south. Reefs are especially well developed in the Frasnian and occur as isolated masses, usually less than about 800 metres in length, separated by shales. Equivalents to the north show red and green silts and shales of marginal continental marine type. Because the Belgian Devonian rocks are well exposed along a northsouth line, their changes in thickness, lithology, and fauna have been well-documented. The Eifel forms a natural eastern extension of the Ardennes, and a somewhat similar succession is known. The Lower Devonian is nonmarine, and the Middle Devonian and Frasnian have a poor reef development, but the calcareous shales and limestones carry a rich and famous fauna. The uppermost Devonian is not preserved. The Rhine valley, along with the Rheinisches Schiefergebirge to the east, has been, since the early days of geology, the subject of extensive study by the numerous German universities that surround it. Again, a northern sediment source generally is indicated, but a borehole near Mnster, well to the north, has encountered Middle and lower Upper Devonian marine limestones. To the south also, approaching the Hunsrck-Taunus mountains, there is evidence of a landmass. Between these areas a rich Devonian sequence is exposed in folded terrain. The maximum thickness is 9,140 metres. The Lower Devonian consists of slates and sandstones. The slate has been much worked to clad houses and castles. A ledge of Emsian sandstone in the Rhine gorge is the setting for the Lorelei legend. Limestones are common in the Givetian and are termed Massenkalk. Middle and Upper Devonian areas of thin sedimentation, as in Devon, are interpreted as deposits on submarine ridges. These are commonly nodular limestones rich in cephalopods that occur between thick shale sequences. Evidence of volcanic activity is common, and this has been invoked to explain the concentrations of sedimentary hematite iron ores in the Givetian and Frasnian. The Harz Mountains show a more calcareous Lower Devonian section. Here copper, lead, and zinc are exploited from lodes in the famous Wissenbach Slate. A calcareous Lower Devonian succession, the Bohemian facies, occurs in the Prague Basin of eastern Europe. A continuous marine succession formed from the Silurian into the Devonian, and the boundary is drawn at the top of the Prdol Formation with the crinoid genus Scyphocrinites. The overlying Lochkovian and Pragian formations include the Koneprusy Limestone with substantial reefs. The Upper Devonian is not preserved. In Moravia complete successions of calcareous and basinal volcanic sediments occur. Devonian rocks of a type analogous to those of southern England and the Ardennes crop out in Brittany. Farther south outcrops occur in Spain and Portugal. The successions of the Pyrenees, Montagne Noire, and Carnic Alps include deepwater limestones; and marine deposits are known in the Balkan Peninsula, including Macedonia, and Romania. The southern Polish outcrops of the Holy Cross Mountains are especially famous and include a lower marine and continental series with a calcareous Middle Devonian and an Upper Devonian of reefs and shales rich in ammonoids and trilobites. In Podolia, along the Dniester (Dnestr) River, are fine marine sections going well up into the Lower Devonian and overlain by the Dniester Series of the Old Red Sandstone type. During the entire Devonian the Ural Mountains formed a depressional trough linked northward to Novaya Zemlya and southward to the CrimeanCaucasian geosyncline that, with the southern European outcrops already mentioned, formed part of the original Tethyan sediments of the AlpineHimalayan fold system of the present day. In European Russia, Old Red Sandstone conditions were general, but marine tongues stretched westward from the Urals to reach Moscow in the Middle Devonian and St. Petersburg in the lower Upper Devonian. A remarkable series of boreholes revealed this in great detail, and there is widespread evidence for salt lakes. Apart from the St. Petersburg outcrop and those along the Don River south of Moscow, the salt lakes are known from subsurface data only. Of economic importance here are the Timan-Pechora oil and gas field and the oil and potash of the Pripet Marshes. The North African areas of Algeria and especially Morocco are noted for their wealth of fossils.