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Devonian

The fourth period of the Paleozoic Era, encompassing an interval of geologic time between 418 and 362 million years before present based on radiometric data. The Devonian System encompasses all sedimentary rocks deposited, and all igneous and metamorphic rocks formed, during the Devonian Period. It is conventional that recognition of Devonian time is determined by the definition of Devonian rocks. See also: Paleozoic

The base of the Devonian System has been fixed, by international agreement, at an actual outcrop of sedimentary rocks at Klonk in the Czech Republic, where it corresponds to the base of the Monograptus uniformis graptolite zone. The top of the Devonian System, corresponding to the base of the Carboniferous System, was similarly fixed at LaSerre in southern France, recognized by the base of the Siphonodella sulcata conodont zone.

In 1839 Adam Sedgwick and R. I. Murchison proposed the Devonian System to encompass the marine sedimentary sequence between the Silurian and Carboniferous in the counties of Cornwall and Devon in southwestern England. In early-studied locales such as Wales and Scotland, only terrestrial Old Red Sandstone lies above the Silurian. When fossil corals found in marine rocks in Devonshire were considered by William Lonsdale to be intermediate in character between those of the better-known Silurian and Carboniferous marine deposits, these Devonshire rocks and the marine fossils they included were determined to be in the stratigraphic position of the Old Red Sandstone. This stage-of-evolution judgment was made in 1837, before Darwin's theory of natural selection had appeared. Because the rocks and faunas of the structurally complex type region were poorly known, Murchison and Sedgwick traveled to Germany in 1839 to see if their new system could be identified in the richly fossiliferous sequences below the Carboniferous in the Rhine Valley. Subsequently the German and Belgian Devonian sequences have served as worldwide standards of comparison.

About the same time, work in America by the paleontologists James Hall, John Clarke, and others was making known the superb physical and faunal development of Devonian rocks in New York, where there are few structural complications. If the Devonian had not been defined until 1842, it might now be called “Erian” after the New York deposits, which were described in much detail in the early reports of the New York State Museum of Natural History.

In modern times, it has become evident that even the Rhenish and New York sequences represent the Devonian System less than adequately. These sequences have been supplemented by the complete Lower Devonian sequence in the Barrandian region of the Czech Republic, and by other Devonian sequences in Iowa, Nevada, Australia, Canada, China, Morocco, and Siberia.

Subdivisions

The Devonian is customarily divided into Lower, Middle, and Upper series and their corresponding epochs. These, in turn, have been divided into stages and their corresponding ages (Fig. 1). The base of the Middle Devonian is at the base of the German Eifelian, or base of the partitus conodont zone. The base of the Upper Devonian is near the base of the Belgian Frasnian, or base of the former lower asymmetricus conodont zone (MN Zone 1), defined by the first occurrence of Ancyrodella rotundiloba. See also: Conodont

Fig. 1 Biostratigraphic zones based on conodonts (left) plotted to show approximate correspondence to the chronostratigraphic stages and series which compose the Devonian System (right) and relative sea-level changes (center) showing major fluctuations during two depositional phases (roman numerals).

Determination of time equivalency (correlation) and age dating in the Devonian are usually accomplished biostratigraphically, by utilizing zones based on the evolutionary successions in individual fossil groups, or on a composite zonal framework based on several fossil groups.

A standard biochronology for the Devonian was first developed by utilizing ammonoid cephalopods, and this was most successful in the Upper Devonian. During the 1960s and 1970s, a standard biochronology was developed for most of the Lower Devonian, based on graptolites. Many local correlations within the Devonian have depended upon the faunal succession of other animal groups (for example, brachiopods, corals, ostracods, trilobites, dacryoconarid tentaculites, fishes) or on plant fossils. A microfossil zonal biochronology has been developed for conodonts to a point where it is superior to any other for the Devonian, in terms of both precision and wide applicability (Fig. 1).

Lithofacies and paleogeography

Because certain sedimentary rock types form under limited ranges of climatic conditions, mapping the nature and distribution of these sedimentary rock types provides a view of ancient climatic distribution. Climate is controlled by intensity of solar insolation and its effect on atmospheric circulation, which is related directly to latitude and direction of Earth's rotation as modified by position of landmasses. The abundance of tropical-climate rocks in present northern temperate continents, combined with lack of these tropical-climate rocks in large portions of southern tropical continents during much of the Paleozoic, has constituted one of the lines of evidence supporting the concept of continental drift. This is particularly obvious in the Devonian, and a recent reconstruction of continental positions has accommodated these climatic inconsistencies in an actualistic view of the Devonian world (Fig. 2), which also explains the heretofore enigmatic faunal distribution patterns. See also: Paleogeography

North America

Devonian rocks across most of the interior of North America, from Hudson Bay to the Ohio Valley across to Nevada and north to the Mackenzie Basin, consist of a predominantly marine carbonate suite, including both limestone and dolomite, commonly with abundant fossils, but containing relatively minor amounts of sandstone and shale. Interbedded with the carbonates in several areas are evaporites ranging from least soluble sulfates, the most common and widespread, through halite, common and widespread in western Canada and present in the Hudson Bay area and Michigan, to the most soluble potash salts, which are rare among evaporites of all ages, but which form a thick local sequence that is mined in the Canadian province of Saskatchewan. This carbonate-evaporite suite indicates a dry tropical climate for central North America in the Devonian. Shale- and sandstone-rich sequences characterize two belts in the North American Devonian. One extends along the Pacific coast from California through the Yukon to the Canadian Arctic islands, and probably represents the equatorial rainfall belt. The other extends along the Appalachians, and probably represents the southern warm temperate rainfall belt. This belt in New York and Pennsylvania includes widespread black shales and the famous Catskill deltaic complex (clastic wedge) that figured strongly in the early history of Devonian investigation in North America.

Europe

During the Devonian, Europe was apparently joined to North America as a result of the Silurian Caledonian orogeny to form a larger continent known variously as Euramerica, Laurussia, or the Old Red Continent. The last name derives from the Old Red Sandstone, among the first Devonian formations described, which is the coarse nonmarine detrital deposit occurring throughout most of Britain, Scandinavia, Spitsbergen, and eastern Greenland and giving evidence of the Caledonian mountains along the west. A dry tropical carbonate-evaporite sequence covers much of Russia, with equatorial bauxite-bearing thick carbonate and detrital sequences along the Ural mountain chain, the site of a seaway during the Devonian. Sequences of detrital rocks and tropical reefy carbonates characterize central Europe, particularly Germany and Belgium, with possibly deeper-water deposits in parts of southern Europe.

Asia

The largest modern continent apparently consisted of as many as 11 microcontinents during the Devonian. The largest of these was Siberia, which has dry tropical carbonate-evaporite sequences and redbeds across much of its area and more humid detrital and reefy carbonates along certain edges. The other Asian fragments also have warm-climate carbonate and detrital sequences, with equatorial bauxites in southern China and Kazakhstan, and evaporites in Iran and other parts of Kazakhstan.

Gondwana

During much of the Paleozoic this giant continent consisted of Africa, South America, Antarctica, Australia, India, and Arabia fitted together, with smaller fragments such as Madagascar, like pieces of a jigsaw puzzle. Over most of Gondwana, including all of South America, sub-Saharan Africa, and Antarctica, Devonian rocks consist entirely of nonred sandstone and shale sequences. These indicate a humid climate, and the absence of any warm-climate rocks such as carbonates, evaporites, or bauxites strongly suggests a cool climate. Thus Gondwana was apparently centered near the South Pole in the Devonian, with most of it in the cold temperate humid belt. Only around the outer fringes in the warm temperate belt do carbonate rocks appear, in northwestern Africa, the northern Indian subcontinent, and most of Australia. Evaporites in parts of Australia indicate that this part of Gondwana extended farthest from the pole, into the dry tropical zone.

Biogeography and ocean currents

Devonian fossils are found to be distributed in three realms. Within each realm there is taxonomic similarity, which indicates that there was reproductive interchange among members of the same phyletic groups, but between each two realms there are various degrees of taxonomic dissimilarity, which indicates that there were various degrees of reproductive isolation among members of the same phylogenetic groups.

The largest realm covered Australia, Asia, Europe, western North America, and the Morocco-India fringe of Gondwana, and is termed the Old World Realm. It was unified by relatively free flowage of the warm equatorial currents and their immediate branches among the continental masses throughout this tropical to subtropical region (Fig. 2).

The Appalachian Realm covered most of eastern North America and the Colombia-Venezuela-Amazon part of northern South American Gondwana, which was adjacent to Appalachian North America during the Devonian. This region was bathed by the temperate southern west-wind current, which crossed a sufficiently broad stretch of ocean so that many Old World larvae could not make the journey, allowing endemic Appalachian forms to develop locally.

The Malvinokaffric Realm covered central Gondwana, including southern South America, southern Africa, and Antarctica. Absence of certain carbonate-secreting groups such as stromatoporoids and green algae, and great reduction of others such as corals suggest that this was a cold-water fauna, which accords with the position of this part of Gondwana suggested by the exclusively detrital rock suite. This region was bathed by a small subpolar ocean current, which was derived from the temperate west-wind current, but in which colder temperatures provided a barrier to Appalachian organisms.

Tectonics

Devonian mountain building was particularly noticeable along the margins of Euramerica. The Acadian orogeny formed mountainous highlands accompanied by a chain of granitic intrusions from Nova Scotia to Pennsylvania during much of Devonian time. These mountains formed a barrier that prevented mixing between organisms of the Appalachian Realm and those of the Old World Realm at the same latitude in central Europe. Erosion from the Acadian mountains produced the thick Catskill deltaic complex of New York and Pennsylvania, which spread its fine-grained sediments far into the interior of eastern North America during later Devonian time. Simultaneously, along the Arctic margin of Canada, the Ellesmerian orogeny was producing folded mountains whose erosional products formed a clastic wedge that was a mirror image of the Catskill deposits.

During latest Devonian time, the Roberts Mountains thrust, of the Antler orogeny in Nevada and Idaho, formed at the top of a subduction zone along which continental crust and overlying sediments were descending beneath oceanic sediments. A Late Devonian orogeny also affected eastern Australia. See also: Orogeny; Plate tectonics

Sea-level changes

Times of active plate movement, such as the Middle and Late Devonian, were times during which the oceanic rise systems formed very large submarine mountain chains, like the Mid-Atlantic Ridge of today. Expansion of these rise systems reduced the volumetric capacity of the ocean basins, so that marine waters rose eustatically and spread as transgressions across low-lying continental platforms, forming broad epeiric seas. This not only affected greatly the physical surface of the Devonian world, but was a major environmental factor in the evolution of plants and animals. During the earliest Devonian, the world's oceans were eustatically low, so that epeiric seas were absent from continental interiors, a continuation from the regression during the Late Silurian. Offshore marine organisms of the earliest Devonian had much in common with their predecessors.

Epeiric seas then expanded sporadically during the remainder of the Early Devonian. This transgressive episode affected Gondwana, in addition to the Euramerican platforms. The general transgressive trend greatly increased near the beginning of the Middle Devonian (transgressive part of transgressive-regressive cycle Ic, Fig. 1), and culminated, after a series of transgressive-regressive cycles, at the end of the Frasnian Age of the Late Devonian. During latest Devonian time, epeiric seas were extensive, but less evenly continuous than before orogenic movements had begun to modify the crustal surface. The interior of Gondwana did not undergo transgression after the early Middle Devonian.

As the Devonian transgressions progressed, many offshore marine animals adapted themselves to the expanding habitats of epeiric seas and then migrated widely as barriers were inundated. This led to loss of isolation, rise of competition, lowering of overall diversity, and loss of the separate realms, with replacement by a cosmopolitan Frasnian marine fauna derived from the Old World Realm. At the end of the Frasnian, still unexplained extinctions of many marine groups further reduced the organic diversity of the Devonian world.

Life

Among the marine invertebrates, trilobites (Arthropoda) were much less abundant than during the Cambrian. The planktic members of the extinct graptolites died out during the Early Devonian, at about the same time as the pelagic ammonoid cephalopods first evolved. The externally two-shelled brachiopods were at their greatest diversity, being represented by more than 900 genera. Lime-secreting corals and stromatoporoids were important and widespread in warm-water environments, and formed reefs during the Middle and Late Devonian. The extinct microfossil group known as conodonts was abundant, widespread, and rapidly evolving during the Devonian, so that conodont fossils are now regarded as the principal tools to be used for international correlation and relative age determination (Fig. 1). See also: Brachiopoda; Conodont; Graptolithina; Micropaleontology; Stromatoporoidea; Trilobita

The great diversification and radiation of fish in the Devonian has led to the term “Age of Fishes” for the period. Placoderm fish, among the most primitive of the jawed vertebrates, were successful predators in Devonian waters, and some grew to lengths up to 8 m (25 ft) just before their extinction at the end of the Devonian. The sharks, with a cartilaginous skeleton but lacking a swimbladder, may have evolved from an early placoderm. See also: Placodermi

Bony fishes or Osteichthyes, a class that includes all modern fish other than sharks and agnathans, were represented in the Devonian by the primitive acanthodians, but more modern groups of bony fishes appeared in the Early Devonian. Lobe-finned bony fishes, or sarcopterygians, include both the lungfish (Dipnoi) and crossopterygians in the Devonian. The oldest known amphibians, including Acanthostega and Ichthyostega, which evolved from the rhipidistian crossopterygians, occur in strata thought to be high Upper Devonian. See also: Dipnoi; Osteichthyes; Sarcopterygii

Many Lower and Middle Devonian fish fossils are now known from rocks deposited in open marine environments, indicating that their habitat was marine as well as estuarine or fresh water. The vertebrates appear to have made the complete transition from ocean to dry land within the Devonian, perhaps as a result of the evolution of land plants during that period.

Land plants began to flourish near the beginning of Devonian time, and were exemplified by the vascular genus Psilophyton of the phylum Psilopsida. The latter gave rise in the Devonian to the Lycopsida (scale trees) and Pteropsida (true ferns). The pteropsids remain important in the world flora of today. See also: Lycophyta; Paleobotany; Psilotophyta; Pteropsida

J. G. Johnson

P. H. Heckel

D. J. Over

Bibliography

M. R. House, C. T. Scrutton, and M. G. Bassett (eds.), The Devonian System, Palaeontol. Ass. Int. Symp. Spec. Pap. 23, 1979
J. G. Johnson, G. Klapper, and C. A. Sandberg, Devonian Eustatic Fluctuations in Euramerica, Geol. Soc. Amer. Bull. 96, 1985
G. R. McGhee, Jr., The Late Devonian Mass Extinction, Columbia Press, 1996
N. J. McMillan, A. F. Embry, and D. J. Glass (eds.), Devonian of the World, Canadian Soc. Petrol. Geol. Mem. 14, 3 vols., 1988
M. A. Murphy, W. B. N. Berry, and C. A. Sandberg (eds.), Western North America: Devonian, Univ. Calif. Riverside Campus Mus. Contrib. 4, 1977
J. B. Roen and R. C. Kepferle (eds.), Petroleum Geology of the Devonian and Mississippian Black Shale of North America, 1994

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