Dictionary:
Cam·bri·an (kăm'brē-ən, kām'-) 
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[From Medieval Latin Cambria, Wales, alteration of Cumbria, from Welsh Cymry. Adj., sense 2, and n., sense 2, after CAMBRIA.]
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Cambrian |
Sci-Tech Encyclopedia:
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 best present estimates suggest that Cambrian time began about 545 million years ago (Ma) and ended at about 485 Ma. It is the longest of the Paleozoic periods and the fourth longest of the Phanerozoic periods.
The Cambrian world can be resolved into at least four major continents that were quite different from those of today. 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, and Southeast Asia; and (4) Siberia, including much of the northeastern quarter of Asia. See also Continental margin; Continents, evolution of; Plate tectonics.
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. 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.
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.
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.
The most abundant remains of organisms in Cambrian rocks are of trilobites. 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. The next most abundant Cambrian fossils are brachiopods. These bivalved animals were often gregarious and lived on the sediment surface or on the surfaces of other organisms. Limestones of Early Cambrian age may contain large reeflike structures formed by an association of algae and an extinct phylum of invertebrates called Archaeocyatha. The Cambrian record of mollusks and echinoderms is characterized by many strange-looking forms. Some lived for only short periods of time and left no clear descendants. 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 Australia. However, like clams, they are not seen again as fossils until Middle Ordovician time, many tens of millions of years later. See also Archaeocyatha;
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.
Geography Dictionary:
Cambrian |
The oldest period of Palaeozoic time stretching approximately from 570 to 500 million years bp.
WordNet:
Cambrian |
The noun has 2 meanings:
Meaning #1:
from 500 million to about 544 million years ago; marine invertebrates
Synonym: Cambrian period
Meaning #2:
a native or resident of Wales
Synonyms: Welshman, Welsh
The adjective Cambrian has one meaning:
Meaning #1:
of or relating to or characteristic of Wales or its people or their language
Synonym: Welsh
Pertains to noun: Cambria (meaning #1)
Wikipedia:
Cambrian |
| Cambrian period 542 - 488.3 million years ago |
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| Mean atmospheric O2 content over period duration | ca. 12.5 Vol %[1] (63 % of modern level) |
| Mean atmospheric CO2 content over period duration | ca. 4500 ppm[2] (16 times pre-industrial level) |
| Mean surface temperature over period duration | ca. 21 °C [3] (7 °C above modern level) |
| Sea level (above present day) | Rising steadily from 30m to 90m[4] |
The Cambrian is the first geological period of the Paleozoic era, lasting from million years ago to million years ago(ICS, 2004)[5]; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for Wales, where Britain's Cambrian rocks are best exposed.[6]
The Cambrian is unique in its unusually high proportion of lagerstätten. These are sites of exceptional preservation, where 'soft' parts of organisms are preserved as well as their more resistant shells. This means that, paradoxically, our understanding of the Cambrian biota surpasses that of later periods.[7]
The Cambrian period marked a profound change in life on Earth. Before the Cambrian, life was on the whole small and simple. Complex organisms became gradually more common in the millions of years immediately preceding the Cambrian, but it wasn't until this period that mineralised - hence readily fossilised - organisms became common.[8] This diversification of lifeforms was relatively rapid, and is termed the Cambrian explosion. This explosion produced the first representatives of most modern phyla, but on the whole, most Cambrian animals look alien to today's eyes, falling in the evolutionary stems of modern groups. While life prospered in the oceans, the land was barren - with nothing more than a microbial 'crud' gracing the soils. Apart from tentative evidence suggesting that some animals floundered around on land, most of the continents resembled deserts spanning from horizon to horizon. Shallow seas flanked the margins of several continents, which had resulted from the relatively recent breakup of the preceding supercontinent Pannotia. The seas were relatively warm, and polar ice was absent.
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Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the Trichophycus pedum assemblage.[9] Nevertheless, the Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky because of occurrence of very similar trace fossils belonging to the Treptichnids group well below the T. pedum in Namibia, Spain and Newfoundland, and possibly, in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.[10][11]
The Cambrian period follows the Ediacaran and is followed by the Ordovician period. The Cambrian is divided into four epochs or series and ten ages or stages. Currently only two series and four stages are named and have a GSSP.
Since the international stratigraphic subdivision is not yet complete, many local subdivisions are still widely used. In some of these subdivisions the Cambrian is divided into three epochs with locally differing names — the Early Cambrian (Caerfai or Waucoban, million years ago to million years ago), Middle Cambrian (St Davids or Albertian, million years ago to million years ago) and Furongian ( million years ago to million years ago ; also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.
Each of the local epochs are divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:
| Chinese | North American | Russian-Kazakhian | Australian | Regional | ||
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| C A M B R I A N |
Furongian | Ibexian (part) | Ayusokkanian | Datsonian | Dolgellian (Trempealeauan, Fengshanian) | |
| Payntonian | ||||||
| Sunwaptan | Sakian | Iverian | Festiniogian (Franconian, Changshanian) | |||
| Steptoan | Aksayan | Idamean | Maentwrogian | |||
| Marjuman | Batyrbayan | Mindyallan | ||||
| Middle Cambrian |
Maozhangian | Mayan | Boomerangian | |||
| Zuzhuangian | Delamaran | Amgan | Undillian | |||
| Zhungxian | Florian | |||||
| Templetonian | ||||||
| Dyeran | Ordian | |||||
| Early Cambrian |
Longwangmioan | Toyonian | Lenian | |||
| Changlangpuan | Montezuman | Botomian | ||||
| Qungzusian | Atdabanian | |||||
| Meishuchuan | Tommotian | |||||
| PRECAMBRIAN | Nemakit-Daldynian* | |||||
*In Russian tradition the lower boundary of the Cambrian is suggested to be defined at the base of the Tommotian Stage which is characterized by diversification and global distribution of organisms with mineral skeletons and appearing of first Archaeocyath bioherms.[12][13][14]
The time range for the Cambrian has classically been thought to have been from about 500 Ma to about 570 Ma. The lower boundary of the Cambrian was traditionally set at the earliest appearance of trilobites and also unusual forms known as archeocyathids (literally 'ancient cup') that are thought to be the earliest sponges and also the first non-microbial reef builders.
The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radiometric dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Ma are common between authors. Framing dates of ca. 545 to 490 Ma were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.
A radiometric date from New Brunswick puts the end of the Lower Cambrian around 511 Ma. This leaves 21 Ma for the other two series/epochs of the Cambrian.
A more precise date of 542 ± 0.3 Ma for the extinction event at the beginning of the Cambrian has recently been submitted.[15] The rationale for this precise dating is interesting in itself as an example of paleological deductive reasoning. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13, a "reverse spike" that paleontologists call an excursion. It is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that this well-established carbon-13 excursion occurs is in Oman. Amthor (2003) describes evidence from Oman that indicates the carbon-isotope excursion relates to a mass extinction: the disappearance of distinctive fossils from the Precambrian coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a volcanic ash horizon from which zircons provide a very precise age of 542 ± 0.3 Ma (calculated on the decay rate of uranium to lead). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in Siberia and Namibia. It is presented here as likely to become accepted as the definitive age for the start of the Phanerozoic eon, and thus the start of the Paleozoic era and the Cambrian period.
Reconstructions of Cambrian geography contain relatively large sources of error. They suggest that a global supercontinent, Pannotia, was in the process of breaking up,[16][17] with Laurentia (North America) and Siberia having separated from the main mass of the Gondwana supercontinent to form isolated landmasses.[18] Most continental land mass was clustered in the southern hemisphere.[18]
With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted[19] – the sea level was high, which led to large areas of the continents being flooded in warm, shallow seas ideal for thriving life. The sea levels fluctuated somewhat, suggesting that there were 'ice ages', associated with pulses of expansion and contraction of a south polar ice cap.[20]
Whilst the Cambrian period was, on the whole, rather warm, it was not entirely without glaciation.[21]
The Cambrian marked a steep change in the diversity and composition of Earth's biosphere. The incumbent Ediacaran biota suffered a mass extinction at the base of the period, which corresponds to an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate, and occurred around the same time as the Cambrian explosion saw the seemingly rapid appearance of representatives of all but one of the modern phyla. There are even suggestions that some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites.
Many modes of preservation are unique to the Cambrian period, resulting in an unusually high proportion of lagerstatte; see the list at the end of the article.
Generally it is accepted that there were no land plants at this time, although it is likely that a microbial "scum" comprising fungi, algae, and possibly lichens covered the land.[22]
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| Preceded by Proterozoic eon | 542 Ma - Phanerozoic eon - Present | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 542 Ma - Paleozoic era - 251 Ma | 251 Ma - Mesozoic era - 65 Ma | 65 Ma - Cenozoic era - Present | ||||||||||
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian | Triassic | Jurassic | Cretaceous | Paleogene | Neogene | Quaternary | |
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved. Read more |
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