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The Cambrian is a major division of the geologic timescale that begins about 542 ± 1.0 Mya (unit) (million years ago) at the end of the Proterozoic eon (geology) and ended about 488.3 ± 1.7 Ma with the beginning of the Ordovician Geologic period (International Commission on Stratigraphy, 2004). It is the first period of the Paleozoic Era (geology) of the Phanerozoic eon. The Cambrian is named for Cambria, the classical name for Wales, the area where rocks from this time period were first studied.

The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable Metazoa organisms that are more complex than sea sponges or Medusa (biology). This sudden appearance of hard body fossils is referred to as the Cambrian explosion.

Cambrian subdivisions The Cambrian period follows after the Neoproterozoic and is followed by the Ordovician period. The Cambrian is divided into three geologic time scale — the Early Cambrian (Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (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 epochs are divided into two faunal stages. Only one, the Paibian, has been recognized by the International Commission on Stratigraphy, and others are still unnamed. However, the Cambrian is divided into several regional faunal stages:

{]|| Ibexian (part)]| Idamean|-|| [Sunwaptan| [Mindyallan|-|| [Steptoan| [Payntonian|-|| rowspan="2" | [Marjuman|||-! rowspan="5" | [Middle Cambrian| [Mayan stage| Boomerangian| [Delamaran| [Undillian||| [Florian stage||-|||| Templetonian|| rowspan="2" | [Ordian| [Longwangmioan| [Lenian| [Montezuman|||-| [Qungzusian|||-| [Meishuchuan|||-||| [Nemakit-Daldynian|||}

Cambrian dating The time range for the Cambrian has classically been thought to have been from about 1 E16 s to about 1 E16 s. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early arthropods known as trilobites and of primitive reef-forming animals known as Archeocyatha. The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radioactive dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Mega-annum are common between authors. Framing dates of ca. () 545 to 490 mya were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.

A radiometric date from New Brunswick puts the end of the first stage of the Cambrian around 1 E16 s. This leaves 21 Ma for the other two stages of the Cambrian.

A more precise date of 1 E16 s for the extinction event at the beginning of the Cambrian has recently been submitted.{{cite book| author = Gradstein, F.M. | coauthors = Ogg, J.G., Smith, A.G., others | year = 2004 | title = A Geologic Time Scale 2004 | publisher = Cambridge University Press | isbn = --> The rationale for this precise dating is interesting in itself as an example of [Paleontology [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.

Cambrian paleogeography Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Gondwana remained the largest supercontinent after the breakup of Pannotia. It is thought that Cambrian climates were significantly warmer than those of preceding times that experienced extensive ice ages discussed as the Cryogenian. Also there was no glaciation at the poles. Continental drift rates in the Cambrian may have been anomalously high. Laurentia, Baltica and Siberia (continent) remained independent continents since the break-up of the supercontinent of Pannotia. Gondwana started to drift towards the South Pole. Panthalassa covered most of the southern hemisphere, and minor oceans included the Proto-Tethys Ocean, Iapetus Ocean, and Khanty Ocean, all of which expanded by this time.

Cambrian fauna Redlichia chinensis from the Cambrian of ChinaAside from a few enigmatic forms that may or may not represent animals, all modern animal phyla with any fossil record to speak of (except bryozoans) appear to have representatives in the Cambrian, and of these most except Porifera seem to have originated just after or just before the start of the period. However, several modern phyla, primarily those with small and/or soft bodies, have no fossil record, in the Cambrian or otherwise. Many Extinction phyla and odd animals that have unclear relationships to other animals also appear in the Cambrian. The apparent "sudden" appearance of very diverse faunas over a period of no more than a few tens of millions of years is referred to as the "Cambrian Explosion". Also, the first possible tracks on land, such as Protichnites and Climactichnites, dating to about 530 mya and found in Ontario, Canada, and northern United States, appeared at this time. The conodonts, small predatory primitive chordates known from their fossilised teeth, also appeared during the Furongian epoch of the Cambrian period. The conodonts thrived throughout the Paleozoic and the early Mesozoic until they completely disappeared during the Late Triassic period when the first mammals were evolving.

The best studied sites where the soft parts of organisms have fossilized are in the Burgess shale of British Columbia. They represent strata from the Middle Cambrian and provide us with a wealth of information on early animal diversity. Similar faunas have subsequently been found in a number of other places — most importantly in very early Cambrian shales in the People's Republic of China's Yunnan Province (see Maotianshan shales). Fairly extensive Precambrian Ediacaran biota faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.

Cambrian flora Generally it is accepted that there were no land plants at this time although molecular dating suggests that simple land plants appeared earlier, in the Precambrian about 700 Mega-annum and fungi about 1 billion years ago also in the Precambrian. The land at this time was barren, mostly desert and badlands. Marine green algae probably appeared at this time, and they eventually evolved into land plants, in the Late Ordovician.

See also

References
 

Cambrian



 
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