Cambrian

For other uses, see Cambrian (disambiguation).

Cambrian Period 542–488.3 million years ago PreЄ Є O S D C P T J K Pg N
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 542 ± 0.3 to 488.3 ± 1.7 Mya (ICS, 2004,^[5] chart); 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 Latin name for Wales, where Britain's Cambrian rocks are best exposed.^[6] It should not be confused with the Cambrian supereon, the current supereon of the planet.

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 our understanding of the Cambrian biology surpasses that of some later periods.^[7]

The Cambrian Period marked a profound change in life on Earth; prior to the Cambrian era, life was on the whole was small and simple. Complex organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not 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 vast increase in diverse forms of life produced the first representatives of many modern phyla, representing the evolutionary stems of modern groups of species. While life prospered in the oceans, the land was barren – with nothing more than a microbial 'crud' known as soil crust covering the land. Apart from some tentative evidence suggesting that a few animals foundered around on land, most of the continents resembled deserts spanning from horizon to horizon. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period.

The United States Federal Geographic Data Committee uses a "barred capital C" character similar to the capital letter Ukrainian Ye ‹Є› to represent the Cambrian Period.^[9] The proper^[10] glyph is a proposed addition to the Unicode standard at code point A792.^[11]

Stratigraphy

Further information: Stratigraphy of the Cambrian

Key events in the Cambrian

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Ediacaran

C
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Ordovician

Drumian (de)

Guzhangian (de) (Dresbachian)

Paibian

Jiangshanian (de)

Stage 10

Nemakit-
Daldynian

Tommotian

Atdabanian

Botomian

Toyonian

Toyonian

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Orsten Fauna

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Burgess Shale

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Kaili biota

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Archaeocyatha extinction

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Emu Bay Shale

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Chengjiang and Sirius Passet biotas

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First Trilobites

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SSF diversification, first brachiopods & archaeocyatha

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First halkieriids, mollusсs, hyoliths SSF

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Treptichnus pedum trace

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First Treptichnus sp. trace

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First Cloudina & Namacalathus mineral skeletal fossils

Stratigraphic scale of the ICS with Russian Lower Cambrian subdivision and Precambrian/Cambrian boundary.

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 Treptichnus pedum assemblage.^[12] Nevertheless, the usage of Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, 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.^[13][14]

Subdivisions

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.

Because 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, 542 ± 0.3 to 513 ± 1.7), Middle Cambrian (St Davids or Albertian, 513 ± 0.3 to 499 ± 1.7) and Furongian (499 ± 0.3 to 488.3 ± 1.7 ; also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.

Trilobite zones allow biostratigraphic correlation in the Cambrian.

Each of the local epochs is 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
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 the appearance of the first Archaeocyath bioherms.^[15][16][17]

Cambrian dating

Archeocyathids from the Poleta formation in the Death Valley area

The time range for the Cambrian has classically been thought to have been from about 570 Mya to about 500 Mya. 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 Mya 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 Lower Cambrian around 511 Mya. This leaves 21 Mya for the other two series/epochs 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.^[18] 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 Mya (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.

Geography

Laurentia

Siberia

Baltica

Gondwana

Continental distribution in 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,^[19][20] with Laurentia (North America) and Siberia having separated from the main mass of the Gondwana supercontinent to form isolated landmasses.^[21] Most continental land mass was clustered in the southern hemisphere.^[21] Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.^[22]

With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted^[23] – 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.^[24]

Climate

While the Cambrian period was, on the whole, rather warm, it was not entirely without glaciation.^[25]

Fauna

Main article: Cambrian explosion

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 which transformed the seabed ecosystems. Before Cambrian, the sea floor was covered by microbial mats. By the end of the period, burrowing animals had destroyed the mats through bioturbation, and gradually turned the seabeds into what they are today. As a consequence, many of those organisms who were dependent on the mats went extinct, while the other species adapted to the changed environment who now offered new ecological niches.^[26] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla.^[27] However, many of these phyla were represented only by stem-group forms; and since mineralized phyla generally have a benthic origin, they may not be a good proxy for (more abundant) non-mineralized phyla.^[28]

There are also suggestions that some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites.

In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;^[29] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry.^[29] (Most Cambrian carbonates were formed by microbial or non-biological processes.)^[29]

Many modes of preservation are unique to the Cambrian period, resulting in an abundance of lagerstätte; see the list at the end of the article.

Flora

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.^[30]

See also

• End Botomian extinction event—circa 517 m.y.a.
• Dresbachian extinction event—circa 502 m.y.a.
• Cambro-Ordovician extinction event – circa 488 m.y.a.
• List of fossil sites (with link directory)
• Type locality (geology), the locality where a particular rock type, stratigraphic unit, fossil or mineral species is first identified

References

1. ^ Image:Sauerstoffgehalt-1000mj.svg
2. ^ Image:Phanerozoic Carbon Dioxide.png
3. ^ Image:All palaeotemps.png
4. ^ Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes". Science 322 (5898): 64–8. Bibcode 2008Sci...322...64H. doi:10.1126/science.1161648. PMID 18832639. 
5. ^ Gradstein, Felix M.; Ogg, J. G.; Smith, A. G. (2004). A Geologic Time Scale 2004. Cambridge: Cambridge University Press. ISBN 0521786738. 
6. ^ Sedgwick, A. (1852). "On the classification and nomenclature of the Lower Paleozoic rocks of England and Wales". Q. J. Geol. Soc. Land. 8: 136–138. doi:10.1144/GSL.JGS.1852.008.01-02.20. 
7. ^ Orr, P. J.; Benton, M. J.; Briggs, D. E. G. (2003). "Post-Cambrian closure of the deep-water slope-basin taphonomic window". Geology 31 (9): 769–772. Bibcode 2003Geo....31..769O. doi:10.1130/G19193.1. http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FG19193.1. Retrieved 2008-06-28.  edit
8. ^ Butterfield, N. J. (2007). "MACROEVOLUTION AND MACROECOLOGY THROUGH DEEP TIME". Palaeontology 50 (1): 41–55. doi:10.1111/j.1475-4983.2006.00613.x. } edit
9. ^ Federal Geographic Data Committee, ed (August 2006) (PDF). FGDC Digital Cartographic Standard for Geologic Map Symbolization FGDC-STD-013-2006. U.S. Geological Survey for the Federal Geographic Data Committee. p. A–32–1. http://ngmdb.usgs.gov/fgdc_gds/geolsymstd/fgdc-geolsym-all.pdf. Retrieved August 23, 2010. 
10. ^ Priest, Lorna A.; Iancu, Laurentiu; Everson, Michael (October 2010). "Proposal to Encode C WITH Bar" (PDF). http://std.dkuug.dk/jtc1/sc2/wg2/docs/n3896.pdf. Retrieved 6 April 2011. 
11. ^ "Proposed New Characters: Pipeline Table". Unicode Consortium Web Site. Unicode, Inc.. http://www.unicode.org/alloc/Pipeline.html. Retrieved 6 April 2011. 
12. ^ A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy.
13. ^ M.A. Fedonkin, B.S. Sokolov, M.A. Semikhatov, N.M.Chumakov (2007). "Vendian versus Ediacaran: priorities, contents, prospectives." In: edited by M. A. Semikhatov "The Rise and Fall of the Vendian (Ediacaran) Biota. Origin of the Modern Biosphere. Transactions of the International Conference on the IGCP Project 493, August 20-31, 2007, Moscow." Moscow: GEOS.
14. ^ A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "Treptichnus pedum and the Vendian-Cambrian boundary". 33 Intern. Geol. Congr. August 6–14, 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.
15. ^ A.Yu. Rozanov, V.V. Khomentovsky, Yu.Ya. Shabanov, G.A. Karlova, A.I. Varlamov, V.A. Luchinina, T.V. Pegel’, Yu.E. Demidenko, P.Yu. Parkhaev, I.V. Korovnikov, N.A. Skorlotova (2008). "To the problem of stage subdivision of the Lower Cambrian". Stratigraphy and Geological Correlation 16 (1): 1–19. Bibcode 2008SGC....16....1R. doi:10.1007/s11506-008-1001-3. http://www.springerlink.com/content/v6785v3x25263l85/. 
16. ^ B. S. Sokolov, M. A. Fedonkin (1984). "The Vendian as the Terminal System of the Precambrian". Episodes 7 (1): 12–19. http://www.episodes.org/backissues/71/ARTICLES--12.pdf. 
17. ^ V. V. Khomentovskii and G. A. Karlova (2005). "The Tommotian Stage Base as the Cambrian Lower Boundary in Siberia". Stratigraphy and Geological Correlation 13 (1): 21–34. http://www.maikonline.com/maik/showArticle.do?auid=VAE43XYML4. 
18. ^ Gradstein, F.M.; Ogg, J.G., Smith, A.G., others (2004). A Geologic Time Scale 2004. Cambridge University Press. 
19. ^ Powell, C.M.; Dalziel, I.W.D.; Li, Z.X.; McElhinny, M.W. (1995). "Did Pannotia, the latest Neoproterozoic southern supercontinent, really exist". EOS (Transactions, American Geophysical Union) 76: 46–72. 
20. ^ Scotese, C.R. (1998). "... supercontinents: The assembly of Rodinia, its break-up, and the formation of Pannotia during the Pan...". Journal of African Earth Sciences 27 (1): 171. 
21. ^ ^{a b} Mckerrow, W. S.; Scotese, C. R.; Brasier, M. D. (1992). "Early Cambrian continental reconstructions". Journal of the Geological Society 149 (4): 599–593. doi:10.1144/gsjgs.149.4.0599.  edit
22. ^ Mitchell, R. N.; Evans, D. A. D.; Kilian, T. M. (2010). "Rapid Early Cambrian rotation of Gondwana". Geology 38 (8): 755. doi:10.1130/G30910.1.  edit
23. ^ Smith, A.G. (in press (2008)). "Neoproterozoic time scales and stratigraphy". Geol. Soc. (Special publication). 
24. ^ Brett, C. E.; Allison, P. A.; Desantis, M. K.; Liddell, W. D.; Kramer, A. (2009). "Sequence stratigraphy, cyclic facies, and lagerstätten in the Middle Cambrian Wheeler and Marjum Formations, Great Basin, Utah". Palaeogeography Palaeoclimatology Palaeoecology 277: 9–33. doi:10.1016/j.palaeo.2009.02.010.  edit
25. ^ Landing, E.; MacGabhann, B. �N. A. (2009). "First evidence for Cambrian glaciation provided by sections in Avalonian New Brunswick and Ireland: Additional data for Avalon–Gondwana separation by the earliest Palaeozoic". Palaeogeography, Palaeoclimatology, Palaeoecology 285 (3-4): 174. doi:10.1016/j.palaeo.2009.11.009.  edit
26. ^ As the worms churn
27. ^ Landing, E.; English, A.; Keppie, J. D. (2010). "Cambrian origin of all skeletalized metazoan phyla--Discovery of Earth's oldest bryozoans (Upper Cambrian, southern Mexico)". Geology 38 (6): 547. doi:10.1130/G30870.1.  edit
28. ^ Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society 75 (2): 253–95. doi:10.1017/S000632310000548X. PMID 10881389. 
29. ^ ^{a b c} Munnecke, A.; Calner, M.; Harper, D. A. T.; Servais, T. (2010). "Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001.  edit
30. ^ Gray, J.; Chaloner, W. G.; Westoll, T. S. (1985). "The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970–1984 [and Discussion]". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (1934–1990) 309 (1138): 167–195. Bibcode 1985RSPTB.309..167G. doi:10.1098/rstb.1985.0077. JSTOR 2396358. 

Further reading

• Amthor, J. E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31 (5): 431–434. Bibcode 2003Geo....31..431A. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. 
• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSPs) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
• Gould, Stephen Jay; Wonderful Life: the Burgess Shale and the Nature of Life (New York: Norton, 1989)

External links

• Cambrian period on In Our Time at the BBC. (listen now)
• Biostratigraphy – includes information on Cambrian trilobite biostratigraphy
• Dr. Sam Gon's trilobite pages (contains numerous Cambrian trilobites)
• Examples of Cambrian Fossils
• Paleomap Project
• Report on the web on Amthor and others from Geology vol. 31
• Weird Life on the Mats

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