masi eng 47,042 اشتراک گذاری ارسال شده در 13 دی، ۱۳۹۲ Cambrian An interval of time in Earth history (Cambrian Period) and its rock record (Cambrian System). The Cambrian Period spanned about 60 million years and began with the first appearance of marine animals with mineralized (calcium carbonate, calcium phosphate) shells. The Cambrian System includes many different kinds of marine sandstones, shales, limestones, dolomites, and volcanics. Apart from the occurrence of an alkaline playa containing deposits of trona (hydrated basic sodium carbonate) in the Officer Basin of South Australia, there is very little provable record of nonmarine Cambrian environments. The concept that great systems of rocks recorded successive periods of Earth history was developed in England in the early nineteenth century. The Cambrian, which was one of the first systems to be formally named, was proposed by the Reverend Adam Sedgwick in 1835 for a series of sedimentary rocks in Wales that seemed to constitute the oldest sediments in the British Isles. At that time, there was no real idea of the antiquity of Cambrian rocks. They were recognized by distinctive fossils and by their geologic relations to other systems. In the early part of the twentieth century, radiometric techniques for obtaining the ages of igneous and metamorphic rocks evolved. Because of the difficulty of finding rocks that can be dated radiometrically in association with rocks, usually zircon bearing volcanic ashes, that can be dated empirically by fossils, the age in years of most Cambrian deposits is only approximate. The best present estimates suggest that Cambrian time began about 545 million years ago (Ma; earliest date close to the base of the Cambrian is 543 ± 0.2 Ma) and ended at about 485 Ma (latest date in the Upper Cambrian is 491 ± 1 Ma, but this is not terminal Cambrian). It is the longest of the Paleozoic periods and the fourth long est of the Phanerozoic periods. Geography Knowledge of Cambrian geography and of the dynamic aspects of evolution and history in Cambrian time is derived from rocks of this age that have been exposed by present-day erosion or penetrated by borings into the Earth's surface. Despite the antiquity of Cambrian time, a surprisingly good record of marine rocks of Cambrian age has been preserved at many localities throughout the world. Each of the different rock types contains clues about its environment of deposition that have been derived from analogy with modern marine environments. From this information, together with knowledge gained from fossils of about the same age within the Cambrian and information about the present geographic distribution of each Cambrian locality, a general picture of world geography and its changes through Cambrian time is available. Plate tectonics The theory of plate tectonics has provided criteria whereby ancient continental margins can be identified. By using these criteria and the spatial information about marine environments derived from study of the rocks, the Cambrian world can be resolved into at least four major continents that were quite different from those of today (Fig. 1). These were (1) Laurentia, which is essentially North America, minus a narrow belt along the eastern coast from eastern Newfoundland to southern New England that belonged to a separate microcontinent, Avalonia. This microcontinent, which also included present-day England, and another microcontinent now incorporated in South Carolina were originally marginal to Gondwana; (2) Baltica, consisting of present-day northern Europe north of France and west of the Ural Mountains but excluding most of Scotland and northern Ireland, which are fragments of Laurentia; (3) Gondwana, a giant continent whose present-day fragments are Africa, South America, India, Australia, Antarctica, parts of southern Europe, the Middle East, a nd Southeast Asia; and (4) Siberia, including much of the northeastern quarter of Asia. Unfortunately, there is not enough reliable information to accurately locate these continents relative to one another on the Cambrian globe. Current Cambrian reconstructions rely on similarities of fossil faunas and on studies of magnetic polarity reversals in rock sequences through time (magnetostratigraphy). These have mostly been concentrated across the Precambrian-Cambrian boundary in Australia, Morocco, Siberia, and south China, and across the Cambrian-Ordovician boundary in Australia, North America, Kazakhstan, and north China. See also: Continental margin; Continents, evolution of; Plate tectonics Fig. 1 Reconstruction of the Lower Cambrian world. (After W. S. McKerrow, C. R. Scotese, and M. D. Brasier, Early Cambrian continental reconstructions. J. Geol. Soc., 149:599–606, 1992) Time divisions For most practical purposes, rocks of Cambrian age are recognized by their content of distinctive fossils. On the basis of the successive changes in the evolutionary record of Cambrian life that have been worked out during the past century, the Cambrian System has been divided globally into three or four series, each of which has been further divided on each continent into stages, each stage consisting of several zones (Fig. 2). Despite the amount of work already done, precise intercontinental correlation of series and stage boundaries, and of zones, is still difficult, especially in the Early Cambrian due to marked faunal provinciality. Refinement of intercontinental correlation of these ancient rocks is a topic of research. Fig. 2 North American divisions of the Cambrian S ystem. Asterisks denote levels of major trilobite extinctions. Life The record preserved in rocks indicates that essentially all Cambrian plants and animals lived in the sea. The few places where terrestrial sediments have been preserved suggest that the land was barren of major plant life, and there are no known records of Cambrian insects or of terrestrial vertebrate animals of any kind. Plants The plant record consists entirely of algae, preserved either as carbonized impressions in marine black shales or as filamentous or blotchy microstructures within marine buildups of calcium carbonate, called stromatolites, produced by the actions of these organisms. Cambrian algal stromatolites were generally low domal structures, rarely more than a few meters high or wide, which were built up by the trapping or precipitation of calcium carbonate by one or more species of algae. Such structures, often composed of upwardly arched laminae, were common in regions of carbonate sedimentation in the shallow Cambrian seas. See also: Stromatolite Animals The animal record is composed almost entirely of invertebrates that had either calcareous or phosphatic shells (Fig. 3). The fossils of shell-bearing organisms include representatives of several different classes of arthropods, mollusks, echinoderms, brachiopods, and poriferans. Coelenterates, radiolarians, and agglutinated foraminiferans are extremely rare, and bryozoans are unknown from Cambrian rocks. Rare occurrences of impressions or of carbonized remains of a variety of soft-bodied organisms, including worms and a group of soft-bodied trilobites, indicate that the fossil record, particularly of arthropods, is incomplete and biased in favor of shell-bearing organisms. Some widespread fossil groups, such as Archaeocyatha, are known only from Cambrian rocks, and several extinct groups of Paleozoic organisms, such as hyolithids and conodo nts, first appear in Cambrian rocks. Conodonts are thought by some specialists to have affinity with vertebrates, but others prefer to relate them to cephalochordates. Dermal plates recovered from the Late Cambrian of North America and Australia are considered to represent the earliest fish remains. See also: Arthropoda; Conodont; Porifera Fig. 3 Representative Cambrian fossils: (a–c) trilobites; (d–f) brachiopods; (g) hyolithid; (h–i) mollusks; (j–l) echinoderms; and (m, n) archaeocyathids. Diversity Although the record of marine life in the Cambrian seems rich, one of the dramatic differences between Cambrian marine rocks and those of younger periods is the low phyletic diversity of most fossiliferous localities. The most diverse faunas of Cambrian age have been found along the ocean-facing margins of the shallow seas that covered large areas of the Cambrian continents. Because these margins were often involved in later geologic upheavals, their rich record of Cambrian life has been largely destroyed. Only a few localities in the world remain to provide a more accurate picture of the diversity of organisms living in Cambrian time. These are known as Konservat Lagerstätten—conservation deposits containing occurrences of extraordinary preservation, particularly of soft body parts. Globally, they are known from more than 35 localities to date if the “Orsten”-type preservation in the Swedish Alum Shale and elsewhere are considered as Lagerstätten. Orsten is an organic-rich, anthraconitic, concretionary limestone in which phosphatized cuticle-bearing organisms are exquisitely preserved in three dimensions. In Laurentia, the richest localities are in the Kinzers Formati on of southeastern Pennsylvania, the Spence Shale of northern Utah, the Wheeler Shale and Marjum Formation of western Utah, the Buen Formation of northern Greenland, and the Burgess Shale of British Columbia. The last is the largest such deposit, containing about 152 mostly monospecific genera of Middle Cambrian age. Equally spectacular is the Chengjiang fauna found at Maotianshan in Yunnan, southwest China, which contains in excess of 70 arthropod-dominated species of Early Cambrian age. However, for extremely fine morphological detail, Orsten-type preservation in the Lower Cambrian of England, the Middle Cambrian of Russia and Australia, and the Upper Cambrian of Poland and Sweden is unsurpassable. Trilobites The most abundant remains of organisms in Cambrian rocks are of trilobites (Fig. 3a–c). They are present in almost every fossiliferous Cambrian deposit and are the principal tools used to describe divisions of Cambrian time and to correlate Cambrian rocks. These marine arthropods ranged from a few millimeters to 20 in. (50 cm) in length, but most were less than 4 in. (10 cm) long. Although some groups of trilobites such as the Agnostida (Fig. 3a) were predominantly pelagic in habitat, most trilobites seem to have been benthic or nektobenthic and show a reasonably close correlation with bottom environments. For this reason, there are distinct regional differences in the Cambrian trilobite faunas of the shallow seas of different parts of the Cambrian world. See also: Trilobita Brachiopods The next most abundant Cambrian fossils are brachiopods (Fig. 3d–f). These bivalved animals were often gregarious and lived on the sediment surface or on the surfaces of other organisms. Brachiopods with phosphatic shells, referred to the Acrotretida (Fig. 3f), are particularly abundant in many limestones and can be recovered in nearly perfect condition by dissolving these limestones in acetic or formic acids. Upper Cambrian limestones from Texas, Oklahoma, and the Rocky Mountains yield excellent silicified shells of formerly calcareous brachiopods when they are dissolved in dilute hydrochloric acid. See also: Brachiopoda Archaeocyathids Limestones of Early Cambrian age may contain large reeflike structures formed by an association of algae and an extinct phylum of invertebrates called Archaeocyatha (Fig. 3m and n). Typical archeocyathids grew conical or cylindrical shells with two walls separated by elaborate radial partitions. The walls often have characteristic patterns of perforations. See also: Archaeocyatha Mollusks and echinoderms The Cambrian record of mollusks and echinoderms is characterized by many strange-looking forms (Fig. 3g–l). Some lived for only short periods of time and left no clear descendants. Representatives of these phyla, such as cephalopods, clams, and true crinoids, which are abundant in younger rocks, are rare in Cambrian rocks; but rostroconch mollusks are known from the Early, Middle, and Late Cambrian at various times in Laurentia, Australia, Siberia, north China, and Korea. Snails, however, are found throughout the Cambrian. Discoveries of primitive clams have been made in Early Cambrian beds, but they are apparently absent from the later record of life for tens of millions of years until post-Cambrian time. See also: Echinodermata; Mollusca Corals Except for rare jellyfish impressions, the Coelenterata were thought to be unrepresented in Cambrian rocks. Corals have now been discovered in early Middle Cambrian rocks in Austral ia. However, like clams, they are not seen again as fossils until Middle Ordovician time, many tens of millions of years later. See also: Cnidaria Extinction The stratigraphic record of Cambrian life in Laurentia (North America) shows perhaps five major extinctions of most of the organisms living in the shallow seas. These extinction events form the boundaries of evolutionary units called biomeres (Fig. 2). Their cause, and their presence in the Cambrian records of other continents, is under investigation. At least one of these extinction events, that at the Marjuman-Steptoean boundary, coincides with a large positive carbon isotope anomaly in Laurentia, Australia, south China, and Kazakhstan. However, perhaps it was these periodic disasters that prevented clear continuity in the evolutionary records of many groups and which led, particularly, to the discontinuous records of the echinoderms, corals, and mollusks. See also: Animal evolution; Extinction (biology) Faunal origin One major unsolved problem is the origin of the entire Cambrian fauna. Animal life was already quite diverse before Cambrian time. The earliest Cambrian beds contain representatives of more than 20 distinctly different invertebrate groups. All of these have calcified shells, but none of the Precambrian organisms have any evidence of shells. There is still no clear evidence to determine whether shells evolved in response to predation or to environmental stress, or as the result of some change in oceanic or atmospheric chemistry. See also: Precambrian History At the beginning of Cambrian time, the continents were largely exposed, much as they are now. Following some still-unexplained event, the seas were suddenly populated by a rich fauna of shell-bearing invertebrates after 3 billion years of supporting only simple plants and perhaps 100 million years with shell-less invertebrates. See also: Precambrian Belts of volcanic islands comparable to those of the western Pacific Ocean today fringed eastern Laurentia, the Australian and western Antarctic margins of Gondwana, and southern Siberia. These belts suggest that crustal plates analogous to those of the present day were in motion at that time. Thick evaporites in Siberia and the Middle Eastern and Indian parts of Gondwana suggest regions of warm temperature and high evaporation rate. Absence of significant development of limestones around Baltica suggest that it was a cool region, probably at high latitudes. Near the continental margins of eastern and western Laurentia, on and around Siberia, and on the western Antarctic, eastern Australian, northwestern African, and southern European margins of Gondwana, archaeocyathid bioherms developed and flourished. By the end of Early Cambrian time, archaeocyathids had become extinct, and shell-bearing organisms capable of building bioherms did not reappear until Middle Ordovician time, at least 45 million years later. Volcanism and evaporitic conditions continued into the Middle Cambrian in Siberia and parts of Gondwana, and evaporites of this age are also known from northern Canada. However, a dramatic change took place in the southern European and northwestern African parts of Gondwana. Carbonate sedimentation virtually ceased throughout that region as those parts of Gondwana reached areas of cooler water and probably higher latitudes. Sea level was rising over much of the world throughout Middle Cambrian time, flooding the interiors of most continents. In the Late Cambrian, parts of western Baltica and eastern Laurentia began to show signs of crustal deformation suggesting that Iapetus, the ocean between Laurentia, Gondwana, and Baltica, was beginning to close. Crustal deformation was also taking place in southern Siberia, eastern Australia, and western Antarctica. In the broad, shallow seas over all of the contin ents except Baltica and the southern European and northwestern African parts of Gondwana, extensive areas of carbonate sediments developed. At least five times in the shallow seas covering Laurentia, large parts of the animal populations became extinct and had to be replenished from the oceanic regions. The last of these extinction events marks the end of Cambrian time in Laurentia. Throughout Cambrian time, terrestrial landscapes were stark and barren. Life in the sea was primitive and struggling for existence. Only in post-Cambrian time did the shallow marine environment stabilize and marine life really flourish. Only then did vertebrates evolve and plants and animals invade the land. C. H. Holland (ed.), Cambrian of the British Isles, Norden and Spitzbergen, 1974 C. H. Holland (ed.), Cambrian of the New World, 1971 C. H. Holland (ed.), Lower Paleozoic of the Middle East, Eastern and Southern Africa, and Antarctica, 1981 W. S. McKerrow, C. R. Scotese, and M. D. Brasier, Early continental reconstructions, J. Geol. Soc., 149:559–606, 1992 M. A. McMenamin and D. L. McMenamin, The Emergence of Animals: The Cambrian Breakthrough, 1990 A. R. Palmer, A proposed nomenclature for stages and series for the Cambrian of Laurentia, Can. J. Earth Sci., 35(4):323–328, 1998 A. R. Palmer, Search for the Cambrian world, Amer. Sci., 62:216–224, 1974 R. A. Robison and C. Tiechert (eds.), Treatise on Invertebrate Paleontology, pt. A: Biogeography, 1979 J. A. Secord, Controversy in Victorian Geology: The Cambrian-Silurian Dispute, 1986 H. B. Whittington, The Burgess Shale, 1985 Additional Readings M. D. Brasier, The basal Cambrian transition and Cambrian bio-events (from terminal Proterozoic extinctions to Cambrian biomeres), in O. H. Walliser (ed.), Global Events and Event Stratigraphy in the Phanerozoic, Springer, Berlin, 1995 M. D. Brasier, Towards a carbon isotope stratigraphy of the Cambrian System: Potential of the Great Basin succession, in E. A. Hailwood and R. B. Kidd (eds.), High Resolution Stratigraphy, Geol. Soc. Spec. Publ., no. 70, 1992 S. Conway Morris and H. B. Whittington, The animals of the Burgess Shale, Sci. Amer., 241(1):122–133, 1979 P. J. Cook and J. H. Shergold (eds.), Phosphate Deposits of the World, 1. Proterozoic and Cambrian Phosphorites, Cambridge University Press, Melbourne, 1986 J. W. Cowie and M. D. Brasier (eds.), The Precambrian-Cambrian Boundary, Oxford Monogr. Geol. Geophy., no. 12, Clarendon Press, Oxford, 1989 S. J. Gould, Wonderful Life, W. W. Norton, New York, 1989 Hou Xianguang, L. Ramsköld, and J. Bergström, Composition and preservation of the Chengjiang fauna: A Lower Cambrian soft-bodied biota, Zoologica Scripta, 20(4):395–411, 1991 C. E. Isachsen et al., New constraint on the division of Cambrian time, Geology, 22:496–498, 1994 P. Janvier, Vertebrate origins: Conodonts join the club, Nature, 374:761–762, 1995 E. Landing et al., Duration of the Early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana, Can. J. Earth Sci., 35(4):329–338, 1998 D. Walossek and K. J. Müller, Cambrian “Orsten”-type arthropods and the phylogeny of Crustacea, in R. A. Fortey and R. H. Thomas (eds.), Arthropod Relationships, Systematics Ass. Spec. Vol. Ser., no. 55, 1997 H. B. Whittington, Trilobites, Boydell Press, Woodbridge, 1992 G. C. Young and J. R. Laurie (eds.), An Australian Phanerozoic Time-scale, Oxford University Press, Melbourne, 1996 International Union of Geological Sciences, Commission on Stratigraphy, Subcommission on Cambrian Stratigraphy نقل قول لینک به دیدگاه
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