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{{Geological period
|image=EarlyCambrianGlobal.jpg
|o2=12.5
|co2=4500
|temp=21
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The '''Cambrian''' is a [[geologic period|geologic period and system]] that began about {{period start|Cambrian}}{{period start error|cambrian}} [[Mya (unit)|Ma]] (million years ago) at the end of the [[Proterozoic]] [[eon (geology)|eon]] and ended about {{period end|cambrian}}{{period start error|ordovician}} Ma with the beginning of the [[Ordovician]] period {{ICS 2004}}. It was the first period of the [[Paleozoic]] [[Era (geology)|era]] of the [[Phanerozoic]] eon. The Cambrian takes its name from [[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|multicellular]] organisms.  This sudden appearance of hard body fossils is referred to as the [[Cambrian explosion]]. 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. This assemblage is distinct from anything in the Precambrian as it has ecologically tiered vertical burrows which are absent from the Precambrian. 

== Cambrian subdivisions ==
The Cambrian period follows the [[Ediacaran]] and is followed by the [[Ordovician]] period. The Cambrian is divided into three [[geologic time scale|epochs]] — 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 several 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]] of which the Russian-Kazakhian system is most used in international parlance: 

{| class="wikitable" |
!
! Chinese
! North American
! Russian-Kazakhian
! Australian
! Regional
|-
! rowspan="4" | [[Furongian]]
|
| [[Ibexian]] (part)
| [[Ayusokkanian]]
| [[Idamean]]
| [[Dolgellian]]
|-
|
| [[Sunwaptan]]
| [[Sakian]]
| [[Mindyallan]]
| [[Festiniogian]]
|-
|
| [[Steptoan]]
| [[Aksayan]]
| [[Payntonian]]
| [[Maentwrogian]]
|-
|
| rowspan="2" | [[Marjuman]]
| [[Batyrbayan]]
|
|
|-
! rowspan="5" | [[Middle Cambrian]]
| [[Maozhangian]]
| [[Mayan stage|Mayan]]
| [[Boomerangian]]
|
|-
| [[Zuzhuangian]]
| [[Delamaran]]
| [[Amgan]]
| [[Undillian]]
|
|-
| [[Zhungxian]]
|
|
| [[Florian stage|Florian]]
|
|-
|
|
|
| [[Templetonian]]
|
|-
| &nbsp;
| rowspan="2" | [[Dyeran]]
|
| rowspan="2" | [[Ordian]]
|
|-
! rowspan="5" | [[Early Cambrian]]
| [[Longwangmioan]]
| [[Toyonian]]
| [[Lenian]]
|-
| [[Changlangpuan]]
| [[Montezuman]]
| [[Botomian]]
|
|
|-
| [[Qungzusian]]
|
| [[Atdabanian]]
|
|
|-
| [[Meishuchuan]]
|
| [[Tommotian]]
|
|
|-
|
|
| [[Nemakit-Daldynian]]
|
|
|}

===Cambrian dating===
The time range for the Cambrian has classically been thought to have been from about 500 mya to about 570 mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early [[arthropod]]s known as [[trilobite]]s and also unusual forms known as [[Archeocyatha|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 [[Mega-annum|Ma]] 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 511 mya. This leaves 21 Ma for the other two stages of the Cambrian.

A more precise date of 542 ± 0.3 mya for the extinction event at the beginning of the Cambrian has recently been submitted.<ref name=Gradstein2004>{{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 = 
 }}</ref> The rationale for this precise dating is interesting in itself as an example of [[Paleontology|paleological]] [[deductive reasoning]]. Exactly at the Cambrian boundary there is a marked fall in the abundance of [[carbon-13]], a "reverse spike" that [[paleontologist]]s 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 [[zircon]]s 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==
[[Image:Cambrianmap.svg|thumb|325px|The continental distribution in the early Cambrian epoch, 514 million years ago.]]
Cambrian [[continent]]s 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 [[climate]]s were significantly warmer than those of preceding times that experienced extensive [[ice age]]s discussed as the [[Cryogenian|Varanger glaciation]]. Also there was no glaciation at the poles. [[Continental drift]] rates in the Cambrian may have been anomalously high. [[Laurentia]], [[Baltica]] and [[Siberia (continent)|Siberia]] 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==
[[Image:Trilobite Redlichia.jpg|thumb|Fossil [[trilobite]] ''Redlichia chinensis'' from the Cambrian of China]]
Of those modern [[animal]] phyla that fossilize easily, all save the [[bryozoan]]s have claimed representatives in the Cambrian. Many [[Extinction|extinct]] 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 &mdash; most importantly in very early Cambrian [[shale]]s in the [[People's Republic of China]]'s [[Yunnan]] Province (see [[Maotianshan shales]]). Fairly extensive Precambrian [[Ediacaran biota|Ediacaran]] 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 land plant ancestors diverged from the algae much earlier, in the [[Neoproterozoic]] about 700 [[Mega-annum|ma]]{{Fact|date=December 2007}}, and that [[fungi]] diverged from the animals about 1 billion years ago{{Fact|date=December 2007}}. The land at this time was barren, mostly [[desert]] and [[badlands]].

==See also==
* [[List of fossil sites]] ''(with link directory)''
* [[Cambrian-Ordovician extinction events]]

==References==
{{reflist|2}}
* [[Stephen Jay Gould|Gould]], Stephen Jay; Wonderful Life: the [[Burgess Shale]] and the Nature of Life (New York: Norton, 1989)
* {{cite journal
 | last = Amthor | first =  J. E.
 | coauthors = and others
 | year = 2003
 | title= Extinction of ''[[Cloudina]]'' and ''[[Namacalathus]]'' at the Precambrian-Cambrian boundary in Oman
 | journal = Geology
 | volume = 31 | pages = pp 431–434
 | id = {{doi|<nowiki>10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2 </nowiki>}}
 | doi= 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2
 | doilabel= 10.1130/0091-7613(2003)031&#60;0431:EOCANA&#62;2.0.CO;2
 }}
* Ogg, Jim; June, 2004, ''Overview of Global Boundary Stratotype Sections and Points (GSSP's)'' http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

== External links ==
{{Commonscat|Cambrian}}
* [http://www.astrobio.net/news/article251.html Weird Life on the Mats]
* [http://www.trilobite.info  Dr. Sam Gon's trilobite pages] (contains numerous Cambrian trilobites)
** [http://www.trilobites.info/biostratigraphy.htm Biostratigraphy] - includes information on Cambrian trilobite [[biostratigraphy]]
* [http://www.earth-pages.com/archive/geobiology.asp Report on the web on Amthor and others from ''Geology'' vol. 31]
* [http://www.scotese.com/ Paleomap Project]
* [http://www.geo-lieven.com/erdzeitalter/kambrium/kambrium.htm Examples of Cambrian Fossils]

{{Cambrian Footer}}
{{Paleozoic Footer}}

[[Category:Cambrian| ]]

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[[he:קמבריון]]
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[[vi:Kỷ Cambri]]
[[tr:Kambriyen dönem]]
[[uk:Кембрійський період]]
[[vo:Kambrium]]
[[zh:寒武纪]]