Cambrian explosion
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2008-07-16T13:30:00Z
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/* Notes */
{{underconstruction}}
The '''[[Cambrian]] explosion''' or '''Cambrian radiation''' was the seemingly rapid appearance of most major groups of complex [[animals]] around {{Ma|530}}, as evidenced by the [[fossil record]].<ref name="BerkeleyCambrian">[http://www.ucmp.berkeley.edu/cambrian/camb.html The Cambrian Period]</ref><ref name="BristolUCEtiming">[http://palaeo.gly.bris.ac.uk/Palaeofiles/Cambrian/timing/timing.html The Cambrian Explosion – Timing]</ref> This was accompanied by a major diversification of other organisms.<ref group=note>This included at least the animals, phytoplankton and calcimicrobes.<ref name=Butterfield2001/></ref><ref name=Butterfield2001>{{cite journal
| author = Butterfield, N.J.
| year = 2001
| title = Ecology and evolution of Cambrian plankton
| journal = The Ecology of the Cambrian Radiation. Columbia University Press, New York
| pages = 200–216
| url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf+
| accessdate = 2007-08-19
}}</ref> Before about {{Ma|580}}<!--At 610, Aspidella disks appeared, but it is not clear that these represented complex life forms.-->, most organisms were simple, composed of individual cells occasionally organised into [[colony (biology)|colonies]]. In the following 70 million to 80 million years, the rate of [[evolution]] accelerated by an order of magnitude,<ref group=note>As defined in terms of the extinction and origination rate of species.<ref name=Butterfield2007/></ref> and the diversity of life began to resemble today’s.<ref name="Bambach2007">{{cite journal
| author = Bambach, R.K.
| coauthors = Bush, A.M., Erwin, D.H.
| year = 2007
| title = Autecology and the filling of Ecospace: Key metazoan radiations
| journal = Palæontology
| volume = 50
| issue = 1
| pages = 1–22
| doi = 10.1111/j.1475-4983.2006.00611.x
}}</ref>
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century,<ref name=Buckland1841>{{cite book
| author = Buckland, W.
| year = 1841
| title = Geology and Mineralogy Considered with Reference to Natural Theology
| publisher = Lea & Blanchard
}}</ref> and [[Charles Darwin]] saw it as one of the main objections that could be made against his theory of evolution by [[natural selection]].<ref name=OriginOfSpecies>{{cite book
| title=On the Origin of Species by Natural Selection
| author=Darwin, C
| authorlink=Charles Darwin
| year=1859
| pages=315–316
| publisher=Murray, London, United Kingdom
}}</ref>
The long-running puzzlement about the appearance of the Cambrian [[fauna]], seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid evolution; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks.
{{-}}
{{Cambrian explosion graphical timeline}}
==History and significance==
{{main|Evolutionary history of life | Ediacara biota}}
Geologists as long ago as [[William Buckland|Buckland]] (1784–1856) realised that a dramatic step change in the fossil record occurred around the base of what we now call the Cambrian.<ref name=Buckland1841/> Charles Darwin considered this sudden appearance of many animal groups with few or no antecedents to be the greatest single objection to his theory of evolution: indeed, he devoted a substantial chapter of ''[[The Origin of Species]]'' to this problem.<ref name=OriginOfSpecies>{{cite book
| title=On the Origin of Species by Natural Selection
| author=Darwin, C
| authorlink=Charles Darwin
| year=1859
| pages=315–316
| publisher=Murray, London, United Kingdom
}}</ref>
American palæontologist [[Charles Doolittle Walcott|Charles Walcott]] proposed that an interval of time, the “Lipalian”, was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time.<ref>{{cite journal|authorlink=Charles Walcott| title=Cambrian Geology and Paleontology|journal=Smithsonian Miscellaneous Collections|volume=57|pages=14|author=Walcott, C.D.|year=1914}}</ref>
More recently it was discovered that the history of life on earth goes back at least {{Ma|3550|million years}}: rocks of that age at [[Warrawoona]] in [[Australia]] contain fossils of [[stromatolite]]s, stubby pillars that are formed by colonies of [[micro-organism]]s. Fossils (''[[Grypania]]'') of more complex [[eukaryotic]] cells, from which all animals, plants and fungi are built, have been found in rocks from {{Ma|1400}}, in [[China]] and [[Montana]]. Rocks dating from {{Ma|565|543}} contain fossils of the [[Ediacara biota]], organisms so large that they must have been multi-celled, but very unlike any modern organism. But as recently as the 1970s there was no sign of how the ''relatively'' modern-looking organisms of the Middle and Late [[Cambrian]] arose.<ref name="CowenHistLife" />
[[Image:Opabinia BW.jpg| thumb | right | 250px | ''[[Opabinia]]'' made the largest single contribution to modern interest in the Cambrian explosion. ]]
The intense modern interest in this "Cambrian explosion" was sparked by the work of [[Harry B. Whittington]] and colleagues, who in the 1970s re-analysed many fossils from the [[Burgess Shale]] (see below) and concluded that several were complex as but different from any living animals.<ref name="Whittington1985BurgessShale">{{cite book | title=The Burgess Shale | author=Whittington, H.B. | authorlink=Harry Whittington | coauthors = Geological Survey of Canada | year=1985 | publisher=Yale University Press }}</ref> The most common organism, ''[[Marrella]]'', was clearly an [[arthropod]], but not a member of any known arthropod [[Class (biology)| class]]. Organisms such as the five-eyed ''[[Opabinia]]'' and spiny slug-like ''[[Wiwaxia]]'' were so different from anything else known that Whittington's team assumed they must represent different [[phylum| phyla]], only distantly related to anything known today. [[Stephen Jay Gould]]’s popular 1989 account of this work, ''[[Wonderful Life (book)|Wonderful Life]]'',<ref name="WonderfulLife">{{cite book
| title=Wonderful Life: The Burgess Shale and the Nature of History
| author=Gould, S.J.
| authorlink=Steven Jay Gould
| year=1989
| publisher=W.W. Norton & Company
}}</ref> brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal [[phylum| phyla]] had appeared rather suddenly.
But other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.<ref name="McNamara1996DatingOriginAnimals">{{cite journal
| author = McNamara, K.J.
| title = Dating the Origin of Animals
| journal = Science
| volume = 274
| number= 5295
| pages = 1993–1997
| date = [[20 December]] [[1996]]
| doi = 10.1126/science.274.5295.1993f
}}</ref><ref name="AwramikStromatoliteDiversityMetazoanAppearance">
{{cite journal
| author = Awramik, S.M.
| title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
| journal = Science
| volume = 174
| number=4011
| pages = 825–827
| date = [[19 November]] [[1971]]
| doi=10.1126/science.174.4011.825
| year = 1971
| pmid = 17759393
}}
</ref><ref name="FedonkinWaggoner1997KimberellaMollusc" /> There has also been intense debate whether there was a genuine "explosion" of modern from in the Cambrian and, to the extent that there was, how it happened and why it happened then.<ref name="Marshall2006Explaining" />
==Types of evidence==
Deducing the events of half a billion years ago is tricky, and evidence comes from biological and chemical signatures in rocks.
===Dating the Cambrian===
Accurate absolute [[radiometric dating|radiometric]] dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available, and for only a few regions.<ref>e.g. {{cite journal
| title=Recent radiometric dating of some Cambrian rocks in southern Australia: relevance to the Cambrian time scale
| author=Jago, J.B.
| coauthors = Haines, P.W.
| journal=Revista Española de Paleontología
| pages=115–22
| year=1998
}}</ref>
Relative dating (''A'' was before ''B'') is often sufficient for studying processes of evolution, but this too has been difficult, because of the problems involved in matching up rocks of the same age across different [[continent]]s.<ref name=DatingProblems>e.g. {{Cite journal
| last = Gehling
| first = James
| last2 = Jensen
| first2 = Sören
| last3 = Droser
| first3 = Mary
| last4 = Myrow
| first4 = Paul
| last5 = Narbonne
| first5 = Guy
| title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland
| journal = Geological Magazine
| volume = 138
| issue = 2
| pages = 213–218
| date = March 2001
| year = 2001
| url = http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669
| doi = 10.1017/S001675680100509X
}}</ref>
Therefore dates or descriptions of sequences of events should be regarded with some caution until better data become available.
===Body fossils===
Fossils of organisms' bodies are usually the most informative type of evidence. Fossilisation is a rare event, and most fossils are either destroyed by [[erosion]] or [[metamorphosis]] before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, they are often adequate to illustrate the broader patterns of life's history.<ref name="BentonQualityFossilRecord">{{cite journal |author=Benton MJ, Wills MA, Hitchin R |title=Quality of the fossil record through time |journal=Nature |volume=403 |issue=6769 |pages=534–7 |year=2000 |pmid=10676959 |doi=10.1038/35000558}}
: Non-technical [http://palaeo.gly.bris.ac.uk/cladestrat/news.html summary]
</ref> There are also biases in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms.<ref name="Butterfield2003ExceptionalFossilPreservation">{{cite journal | author = Butterfield , N.J. | year =2003 | title = Exceptional Fossil Preservation and the Cambrian Explosion | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 166–177 | doi = 10.1093/icb/43.1.166 | url=http://icb.oxfordjournals.org/cgi/content/abstract/43/1/166 | accessdate=2008-06-28 }}</ref> Further, only the parts of organisms that were already [[Mineralization| mineralised]] are usually preserved, such as the shells of molluscs. Since animal species are soft-bodied, they decay before they can become fossilised. Indeed, of the 30-plus [[phylum|phyla]] of living animals, two-thirds have never been found as fossils.<ref name="CowenHistLife">{{ cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science }}</ref>
[[Image:Marella200x155.png|thumb| This ''[[Marrella]]'' specimen illustrates how clear and detailed the fossils from the [[Burgess Shale]] [[lagerstätte]] are.]]
The Cambrian fossil record includes an unusually high number of [[lagerstätte]]n, which preserve soft tissues. These allow [[Paleontology|paleontologists]] to examine the internal anatomy of animals which in other sediments are only represented by shells, spines, claws, etc – if they are preserved at all. The most significant Cambrian lagerstätten are the early Cambrian [[Maotianshan shale]] beds of Chengjiang ([[Yunnan]], [[China]]) and [[Sirius Passet]] ([[Greenland]]);<ref name=ConwayMorris1979>
{{cite journal
| author = Morris, S.C. | year = 1979 | title = The Burgess Shale (Middle Cambrian) Fauna | journal = Annual Review of Ecology and Systematics
| volume = 10 | issue = 1 | pages = 327–349
| doi = 10.1146/annurev.es.10.110179.001551
}}</ref> the middle Cambrian [[Burgess Shale]] ([[British Columbia]], [[Canada]])<ref name=Yochelson1996>{{cite journal
| author = Yochelson, E.L.
| year = 1996
| title = Discovery, Collection, and Description of the Middle Cambrian Burgess Shale Biota by Charles Doolittle Walcott
| journal = Proceedings of the American Philosophical Society
| volume = 140 | issue = 4 | pages = 469–545
| url = http://links.jstor.org/sici?sici=0003–049X(199612)140%3A4%3C469%3ADCADOT%3E2.0.CO%3B2–8
| accessdate = 2007-04-24
}}</ref>; and the late Cambrian [[Orsten]] ([[Sweden]]) fossil beds.
While lagerstätten preserve far more than the conventional fossil record, they are far from complete. Because lagerstätten are restricted to a narrow range of environments (where soft-bodied organisms can be preserved very quickly, e.g. by mudslides), most animals are probably not represented; further, the exceptional conditions that create lagerstätten probably do not represent normal living conditions.<ref name=Butterfield2001ECR>{{citation
| author = Butterfield, N.J.
| year = 2001
| chapter = Ecology and evolution of Cambrian plankton
| title = The Ecology of the Cambrian Radiation
| publisher = Columbia University Press, New York
| pages = 200–216
| url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf
| accessdate = 2007-08-19
}}</ref> In addition, the known Cambrian lagerstätten are rare and difficult to date; Precambrian lagerstätten have yet to be studied in detail.
The sparseness of the fossil record means that organisms usually exist long before they are found in the fossil record.<ref name=Signor1982>{{cite journal
| author = Signor, P.W. |year= 1982
| title = Sampling bias, gradual extinction patterns and catastrophes in the fossil record
| journal = Geological implications of impacts of large asteroids and comets on the earth
| id = A 84–25651 10–42
| location= Boulder, CO |publisher = Geological Society of America
| pages = 291–296 | url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8425675AH | accessdate = 2008-01-07}}</ref>
===Trace fossils===
[[Image:Cruziana2.jpg|thumb|[[Trace fossil]] of the type called ''Cruziana'', possibly made by a [[trilobite]]. ]][[Trace fossils]] consist mainly of tracks and burrows on and under what was then the seabed.
Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.<ref name=Seilacher1994>e.g. {{cite journal
| author = Seilacher, A.
| year = 1994
| title = How valid is Cruziana Stratigraphy?
| journal = International Journal of Earth Sciences
| volume = 83
| issue = 4
| pages = 752–758
| url = http://www.springerlink.com/index/WP279834395100KH.pdf
| accessdate = 2007-09-09
}}</ref> Whilst exact assignment of trace fossils to their makers is generally impossible, traces may provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).
===Geochemical observations===
{{main|Early Cambrian geochemical fluctuations}}
Several [[Geochemistry| chemical markers]] indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction,<ref name="Knoll1999" /><ref name=Amthor2003>{{cite journal
| author = Amthor, J.E.
| coauthors = Grotzinger, J.P., Schroder, S., Bowring, S.A., Ramezani, J., Martin, M.W., Matter, A.
| year = 2003
| title = Extinction of'' Cloudina ''and ''Namacalathus'' at the Precambrian-Cambrian boundary in Oman
| journal = Geology
| volume = 31
| issue = 5
| pages = 431–434
| doi = 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2
}}</ref> or with a massive warming resulting from the release of [[clathrate|methane ice]].
<ref name=Marshall2006Explaining/>
Such changes may reflect a cause of the Cambrian explosion, although they may also have resulted from an increased level of biological activity – a possible result of the explosion.<ref name=Marshall2006Explaining/> Despite these uncertainties, the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of the likely environmental changes.
===Phylogenetic techniques===
[[Cladistics]] is a technique for working out the “family tree” of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely retated to each other than either is to A. Characters which are compared may be [[Anatomy| anatomical]] (such as the presence of a [[notochord]]) or [[Molecular phylogeny| molecular]], by comparing sequences of [[DNA]] or [[protein]]. The cladistic technique is sometimes fallible, as some features (e.g. wings or [[Evolution of the eye| camera eyes]]) evolved more than once, [[convergent evolution|convergent]]ly – this must be taken into account in analyses.
From the relationships, it may be possible to constrain the date that lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.
It is also possible to ascertain an estimate of the divergence time between two clades – i.e. an approximation of the origination of a lineage – by assuming that DNA [[mutation]]s accumulate at a constant rate. These "[[molecular clock]]s", however, are fallible, and provide only a very approximate timing: they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,<ref>L.A. Hug and A.J.Roger, The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses. Molecular Biology and Evolution 2007 24(8):1889–1897, 2007</ref> and estimates produced by different techniques vary by a factor of two.<ref name=PetersonEtAl2005>{{cite journal
| doi = 10.1073/pnas.0503660102
| pmid = 15983372
| author = Peterson, Kevin J.; Butterfield, Nicholas J.
| journal = Proceedings of the National Academy of Sciences
| volume = 102
| issue = 27
| pages = 9547
| year = 2005
| title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record
}}</ref>
==Explanation of a few scientific terms==
To avoid becoming even longer this article uses some scientific terms, and this is a good place for some simple explanations.<ref name="Marshall2006Explaining">{{cite journal
| author=Marshall, C.R.
| title=Explaining the Cambrian “Explosion” of Animals
| journal=Annu. Rev. Earth Planet. Sci.
| date=2006
| volume=34
| pages=355–384
| doi=10.1146/annurev.earth.33.031504.103001
| url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.33.031504.103001?journalCode=earth
| year=2006
| format=abstract
}}</ref>
A '''[[phylum]]''' is the highest level in the [[Linnaeus| Linnean]] system for classifying animals. Phyla can be thought of as groupings of animals based on general body plan.<ref>{{cite book | last = Valentine
| first = James W. | year = 2004 | title = On the Origin of Phyla | publisher = University Of Chicago Press | location = Chicago | id = 0226845486 | pages = 7
}}"<cite>Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to <strong>their</strong> similarities, and so on, to form a hierarchy.</cite>"</ref> Despite the seemingly different ''external'' appearances of organisms, they are classified into phyla based on their ''internal'' and developmental organizations.<ref>{{cite book
| last = Parker
| first = Andrew
| year = 2003
| title = In the blink of an eye: How vision kick-started the big bang of evolution
| publisher = Free Press
| location = Sydney
| id = 0743257332
| pages = 1–4
}}"<cite>The job of an evolutionary biologist is to make sense of the conflicting diversity of form – there is not always a relationship between internal and external parts. Early in the history of the subject, it became obvious that internal organisations were generally more important to the higher classification of animals than are external shapes. The internal organisation puts '''general restrictions''' on how an animal can exchange gases, obtain nutrients and reproduce.</cite>"</ref> For example, despite their obvious differences, [[spiders]] and [[barnacles]] both belong to the phylum [[Arthropoda]]; but [[earthworms]] and [[tapeworms]], although similar in shape, belong to different phyla.
A phylum is not a fundamental division of nature, such as the difference between [[electron]]s and [[proton]]s. It is simply a very high-level grouping in a [[Linnaean taxonomy| classification system]] created to describe all currently living organisms. This system is imperfect, even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm-like species. As it is based on living organisms, it accommodates extinct organisms poorly, if at all.<ref name="CowenHistLife" /><ref name="Jefferies1979OriginOfChordates">{{ citation | author=Jefferies, R.P.S. | date=1979 | title=The origin of chordates — a methodological essay | editor=House, M.R., | collection=The origin of major invertebrate groups | pages=443–477 | publisher=Academic Press | location=London }} summarised in {{ cite journal | author=Budd, G.E. | title=The Cambrian Fossil Record and the Origin of the Phyla | journal=Integrative and Comparative Biology | date=2003 | volume=43 | issue=1 | pages=157–165 | doi=10.1093/icb/43.1.157 | url=http://icb.oxfordjournals.org/cgi/content/full/43/1/157 | year=2003 }}</ref>
Groups which cannot easily be placed in an existing phylum are considered to be '''[[stem group]]s'''. A stem group annelid, for instance, is any group which was closely related to, but not within, the crown group "annelida". The stem group split off from the lineage that would lead to annelids, and eventually became extinct.
{{coelomate basic}}
'''[[Triploblastic]]''' means consisting of 3 layers, which are formed
in the [[embryo]] (quite early in the animal's development from a single-celled egg to a larva or juvenile form). The innermost layer forms the [[digestive tract]] (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic – the best-known exceptions are [[Porifera]] (sponges) and [[Cnidaria]] (jellyfish, sea anemones, etc.).
The '''[[bilateria]]ns''' are animals which are approximately symmetrical (by reflection) at some point in their life history. This implies that they have top and bottom surfaces and, importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian. Living [[Echinoderm]]s ([[starfish]], [[sea urchin]]s, [[sea cucumber]]s, etc.) ''look'' radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical.<ref>{{ cite journal | authors=Dominguez, P., Jacobson, A.G., Jefferies, R.P.S. | title=Paired gill slits in a fossil with a calcite skeleton | journal=Nature | issue=417 | pages=841-844 | date=June 2002 | doi=10.1038/nature00805 | url=http://www.nature.com/nature/journal/v417/n6891/abs/nature00805.html }}</ref> [[Porifera]] and [[Cnidaria]] are radially symmetrical, non-bilaterian and non-triploblastic..
'''[[Coelomate]]''' means having a body cavity (coelom) which contains the internal organs. Most of the phyla featured in the debate about the Cambrian explosion are coelomates: [[arthropods]], [[annelid]] worms, [[molluscs]], [[echinoderms]] and [[chordate]]s – the non-coelomate [[priapulid]]s are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have a coelom – for example [[flatworms]], whose organs are surrounded by [[parenchyma| unspecialized tissues]]).
==Precambrian life==
Our understanding of the Cambrian explosion relies upon knowing what was there beforehand – did the event herald the sudden appearance of a wide range of animals and behaviours, or did such things exist beforehand?
===Evidence of animals around {{Ma|1000}}===
:''For further information, see [[Acritarch]] and [[Stromatolite]]
[[Image:Stromatolites in Sharkbay.jpg|right|thumb|200px|Modern [[stromatolites]] in [[Shark Bay]], [[Western Australia]].]]
Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. [[Stromatolite]]s, stubby pillars built by colonies of [[microorganism]]s, are a major constituent of the fossil record from about {{Ma|2700}}, but their abundance and diversity declined steeply after about {{Ma|1250}}. This decline has been attributed to disruption by grazing and burrowing animals.<ref name="McNamara1996DatingOriginAnimals">
{{cite journal
| author = McNamara, K.J.
| title = Dating the Origin of Animals
| journal = Science
| volume = 274
| number= 5295
| pages = 1993–1997
| date = [[20 December]] [[1996]]
| url=http://www.sciencemag.org/cgi/content/full/274/5295/1993f
| accessdate=2008-06-28
| doi = 10.1126/science.274.5295.1993f
| year = 1996
}}</ref><ref name="AwramikStromatoliteDiversityMetazoanAppearance">
{{cite journal
| author = Awramik, S.M.
| title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
| journal = Science
| volume = 174
| number=4011
| pages = 825–827
| date = [[19 November]] [[1971]]
| url = http://www.sciencemag.org/cgi/content/abstract/174/4011/825
| accessdate=2007-12-01
| doi=10.1126/science.174.4011.825
| year = 1971
| format = abstract
| pmid = 17759393
}}
</ref><ref name="Bengtson2002OriginsOfPredation">{{ Citation
| author=Bengtson, S.
| date=2002
| contribution=Origins and early evolution of predation
| title=The fossil record of predation. The Paleontological Society Papers 8
| editors=Kowalewski, M., and Kelley, P.H.
| pages=289– 317
| publisher=The Paleontological Society
| url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
| accessdate=2007-12-01
| format = Free full text
}}</ref>
Precambrian marine diversity was dominated by small fossils known as [[acritarch]]s. This term describes almost any small organic walled fossil – from the egg cases of small [[metazoans]] to resting [[cysts]] of many different kinds of [[chlorophyta|green algae]]. After appearing around {{Ma|2000}}, acritarch underwent a boom around {{Ma|1000}}, increasing in abundance, diversity, size, complexity of shape and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from the [[Neoproterozoic]] era also show signs of anti-predator defenses.<ref name="Bengtson2002OriginsOfPredation">{{ Citation
| author=Bengtson, S.
| date=2002
| contribution=Origins and early evolution of predation
| title=The fossil record of predation. The Paleontological Society Papers 8
| editors=Kowalewski, M., and Kelley, P.H.
| pages=289– 317
| publisher=The Paleontological Society
| url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
| format = Free full text
}}</ref> A consideration of taxon longevity appears to support an increase in predation pressure around this time,<ref name=Stanley2008>{{Cite journal
| title = Predation defeats competition on the seafloor
| author = Stanley
| year = 2008
| journal = Paleobiology
| volume = 34
| pages = 1
| url = http://paleobiol.geoscienceworld.org/cgi/content/extract/34/1/1
| format = extract
| doi = 10.1666/07026.1
}}</ref>
However, in general, the rate of evolution in the Precambrian was very slow, with many cyanobacterial species persisting unchanged for billions of years.<ref name=Butterfield2007/>
If these predatory organisms really were metazoans, this means that Cambrian animals didn't appear "from no-where" at the base of the Cambrian - predecessors had existed for hundreds of millions of years.
===Fossils of the Doushantuo formation===
{{main|Doushantuo formation}}
The {{Ma|580|million year old}}<ref name="CondonZhuBowring2005UPbAgesDoushantuo">{{ cite journal
| title=U-Pb Ages from the Neoproterozoic Doushantuo Formation, China
| journal=Science
| date=[[1 April]] [[2005]]
| issue=5718
| pages=95–98
| doi=10.1126/science.1107765
| url=http://www.sciencemag.org/cgi/content/abstract/308/5718/95
| author=Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y.
| year=2005
| volume=308
| format=abstract
| pmid=15731406
}}</ref> [[Doushantuo formation]] harbours microscopic fossils which may represent early bilaterians. Some have been described as animal embryos and eggs, although some of these may represent the remains of giant bacteria.<ref name="DoushantuoFossils">
:{{citation
| author = Xiao, S., Zhang, Y. & Knoll, A. H.
| title = Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite
| journal = Nature
| volume = 391
| pages = 553–558
| doi = 10.1038/35318
| year = 1998
}}
:{{cite journal
| title=Cellular and Subcellular Structure of Neoproterozoic Animal Embryos
| journal = Science
| volume =314
| pages = 291–294
| doi=10.1126/science.1133129
| year=2006
| author=Hagadorn, J. W.
}}
:{{cite journal
| title = Evidence of giant sulphur bacteria in Neoproterozoic phosphorites
| journal = Nature
| volume = 445
| pages = 198–201
| doi = 10.1038/nature05457
| year = 2007
| author = Bailey, Jake V.
}}</ref>
Another fossil, ''[[Vernanimalcula]]'', has been interpreted as a coelomate bilaterian,<ref name="Chen2004">
{{cite journal
| author = Chen, J.Y.
| coauthors = Bottjer, D.J.; Oliveri, P.; Dornbos, S.Q.; Gao, F.; Ruffins, S.; Chi, H.; Li, C.W.; Davidson, E.H.
| date = [[2004-07-09]]
| title = Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian
| journal = Science
| volume = 305
| issue = 5681
| pages = 218–222
| doi = 10.1126/science.1099213
| year = 2004
| pmid = 15178752
}}</ref>
but may simply be an infilled bubble.<ref name="Bengtson2004">
{{cite journal
| author = Bengtson, S.
| coauthors = Budd, G.
| year = 2004
| title = Comment on ‘‘Small bilaterian fossils from 40 to 55 million years before the Cambrian’’
| journal = Science
| volume = 306
| pages = 1291a
| doi = 10.1126/science.1101338
| pmid = 15550644
}}</ref>
These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators.<ref name="DoushantuoFossils"/><ref name=Philip2006>{{cite journal
| author = Philip, C. J.; Neil, J.; John, A.; ,
| year = 2006
| title = Synchrotron X-ray tomographic microscopy of fossil embryos
| journal = Nature
| volume = 442
| issue = 7103
| pages = 680
| doi = 10.1038/nature04890
}}</ref>
===Burrows===
{{main|Cambrian substrate revolution}}
[[Image:Ediacaran trace fossil.jpg|thumb|An Ediacaran trace fossil, made when an organism burrowed below a microbial mat.]]
The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about {{Ma|565}}.<ref group=note>Older marks found in billion-year old rocks<ref name="Seilacher1998"/> have since been shown not to be biogenic.</ref><ref name="Seilacher1998">{{cite journal| title=Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India| journal=Science| volume=282| number=5386| pages=80–83| year=1998| doi = 10.1126/science.282.5386.80 | author=Seilacher, A., Bose, P.K. and Pflüger, F.| format=abstract| pmid=9756480
}}</ref><ref name="Erwin1999OriginOfBodyplans" /> They were probably made by organisms resembling earthworms in shape, size, and how they moved. The burrow-makers have never been found preserved, but because they would need a head and a tail, the burrowers probably had bilateral symmetry – which would in all probability make them bilaterian animals.<ref>{{citation
| author = Fedonkin, M.A.
| year = 1992
| chapter = Vendian faunas and the early evolution of Metazoa
| title = Origin and early evolution of the Metazoa
| location = New York
| publisher = Springer
| editors = Lipps, J., and Signor, P. W.
| pages = 87–129
| url = http://books.google.co.uk/books?id=gUQMKiJOj64C&pg=PP1&ots=BkpdtmDml1&sig=ap0OD3JXuSkTVhJTSqQbT5uC2P8
| isbn = 0306440679
| accessdate = 2007-03-08
}}</ref> They fed above the sediment surface, but were forced to burrow to avoid predators.<ref name=Dzik2007>{{The Rise and Fall of the Ediacaran Biota|Dzik, J|The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|405|414|30}}</ref>
Around the start of the Cambrian (about {{Ma|542}}) many new types of traces first appear, including well-known vertical burrows such as ''Diplocraterion'' and ''Skolithos'', and traces normally attributed to [[arthropod]]s, such as ''Cruziana'' and ''Rusophycus''. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard (although not necessarily mineralised) [[exoskeletons]].<ref name=Jensen2003>{{cite journal| title=The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives| author=Jensen, S.| journal=Integrative and Comparative Biology| doi = 10.1093/icb/43.1.219| year=2003| volume=43| pages=219}}</ref>
Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile bottom-dwelling organisms.{{Fact|try Budd|date=June 2008}} They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artefact.<ref name=Seilacher2005/>
Indeed, as burrowing became established, it allowed an explosion of its own,<!--this is carefully worded so as not to imply that was neither a part of the Cambrian explosion, or necessarily different from it - please take this into account when re-wording!--> for as burrowers disturbed the sea floor, they aerated it, mixing oxygen into the toxic muds. This made the bottom sediments more hospitable, and allowed a wider range of organisms to inhabit them - creating new niches and the scope for higher diversity.<ref name=Seilacher2005>{{cite journal | author = Seilacher, Adolf | coauthors = Luis A. Buatoisb, M. Gabriela Mángano | date = [[2005-10-07]] | title = Trace fossils in the Ediacaran–Cambrian transition: Behavioral diversification, ecological turnover and environmental shift | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 227 | issue = 4 | pages = 323–356 | doi = 10.1016/j.palaeo.2005.06.003 | year = 2005 }}</ref>
===Ediacaran organisms===
[[Image:DickinsoniaCostata.jpg|200px|thumb|right|''[[Dickinsonia|Dickinsonia costata]]'', an Ediacaran organism of unknown affinity, with a quilted appearance.]]
{{main|Ediacara biota|Cloudinid|Kimberella|Spriggina}}
At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species which would prove far more ephemeral.<ref name=Butterfield2007/> This radiation, the first in the fossil record,<ref name=Butterfield2007/> is followed soon after by an array of unfamiliar, large, fossils dubbed the [[Ediacara biota]],<ref>
{{ cite journal | title=The Avalon Explosion: Evolution of Ediacara Morphospace
| date=January 2008 | volume=319| issue=5859 |pages=81–84
| doi=10.1126/science.1150279
| url=http://www.sciencemag.org/cgi/content/abstract/319/5859/81
| author=Shen, B., Dong, L., Xiao, S. and Kowalewski, M. | year=2008
| journal=Science | format=abstract | pmid=18174439
}}</ref> which flourished for 40 million years until the start of the Cambrian.<ref name=Grazhdankin2004>{{cite journal | author = Grazhdankin | year = 2004 | doi = 10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 | issn = 0094–8373 | volume = 30 | pages = 203 | title = Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution | journal = Paleobiology }}</ref> Most of this "[[Ediacara biota]]" were at least a few centimeters long, significantly larger than any earlier fossils. The organisms form three distinct assemblages, increasing in size and complexity as time progresses.<ref name="Erwin1999OriginOfBodyplans">
{{ cite journal | author=Erwin, D.H. | title=The origin of bodyplans
| journal=American Zoologist | date=June 1999 | volume=39 | issue=3
| pages=617–629 | doi=10.1093/icb/39.3.617
| url=http://findarticles.com/p/articles/mi_qa3746/is_199906/ai_n8843690/pg_1
| year=1999 | format=free full text }}</ref>
Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses – one palæontologist proposed that the strangest organisms should be classified as a separate [[Kingdom (biology)| kingdom]], Vendozoa.<ref name="Seilacher1992">{{cite journal
| author = Seilacher, A.
| year = 1992
| title = Vendobionta and Psammocorallia: lost constructions of Precambrian evolution
| journal = Journal of the Geological Society, London
| volume = 149
| issue = 4
| pages = 607–613
| url = http://jgs.lyellcollection.org/cgi/content/abstract/149/4/607
| doi = 10.1144/gsjgs.149.4.0607
| issn = 0016–7649
| accessdate = 2007-06-21
| format = abstract
}} </ref>
[[Image:Kimberella blue.jpg| thumb | right | 200px | Fossil of ''[[Kimberella]]'', a triploblastic bilaterian , and possibly a mollusc. ]]
At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs (''[[Kimberella]]''),<ref name=Martin2000>{{cite journal
| author = Martin, M.W.
| coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
| year = 2000
| date = [[2000-05-05]]
| title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
| journal = Science
| volume = 288
| issue = 5467
| pages = 841
| doi = 10.1126/science.288.5467.841
| url = http://www.scienceonline.org/cgi/content/abstract/288/5467/841
| accessdate = 2007-05-10
| format = abstract
| pmid = 10797002
}}</ref><ref name="FedonkinWaggoner1997KimberellaMollusc">{{cite journal
| author =Fedonkin, M. A.
| coauthors =Waggoner, B.
| title =The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
| journal =Nature
| volume =388
| pages =868–871
| date =1997
| url =http://www.nature.com/nature/journal/v388/n6645/abs/388868a0.html
| doi = 10.1038/42242
| format =abstract
}}</ref> echinoderms (''[[Arkarua]]'');<ref>{{cite journal | author=Mooi, R. and Bruno, D. | title=Evolution within a bizarre phylum: Homologies of the first echinoderms | journal=American Zoologist | volume=38 | pages=965–974 | date=1999 | url=http://icb.oxfordjournals.org/cgi/reprint/38/6/965.pdf
}}</ref> and arthropods (''[[Spriggina]]'',<ref>{{cite journal | author =McMenamin, M.A.S | title =''Spriggina'' is a trilobitoid ecdysozoan | journal =Abstracts with Programs | publisher=Geological Society of America | volume =35 | issue=6 | pages =105 | date =2003 | url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm | format =abstract
}}</ref> ''[[Parvancorina]]''<ref>{{ cite journal | title= A ''Parvancorina''-like arthropod from the Cambrian of South China | journal=Historical Biology: A Journal of Paleobiology | volume=18 | issue=1 | date=March 2006 | pages=33–45 | author=Lin, J-P., Gon, S.M., Gehling, J.G., Babcock, L.E., Zhao, Y-L., Zhang, X-L,, Hu, S-X., Yuan, J-L., Yu, M-Y., and Peng, J. | doi= 10.1080/08912960500508689 | year= 2006 }}</ref>). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans.<ref name=Butterfield2006>{{cite journal
| author = Butterfield, N.J.
| year = 2006
| title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
| journal = Bioessays
| volume = 28
| issue = 12
| pages = 1161–6
| doi = 10.1002/bies.20507
| accessdate =
}}</ref> However there seems little doubt that ''Kimberella'' was at least a triploblastic bilaterian animal.<ref name=Butterfield2006/> These organisms are central to the debate about how abrupt the Cambrian explosion was. If some were early members of the animal phyla seen to-day, the "explosion" looks a lot less sudden than if these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon (40M years is "soon" by evolutionary and geological standards).
Fossils of ''[[Cloudina]]'' and ''[[Sinotubulites]]'' have been found in sediments formed near the end of the Ediacaran period. Although they are as hard to classify as most other Ediacaran organisms, they are important in two other ways. First, they are the earliest known calcifying organisms (organisms that built shells out of [[calcium carbonate]]).<ref name=Miller2004>{{citation
| author = Miller, A.J.
| year = 2004
| title = A Revised Morphology of Cloudina with Ecological and Phylogenetic Implications
| url = http://ajm.pioneeringprojects.org/files/CloudinaPaper_Final.pdf
| accessdate = 2007-04-24
}}</ref><ref name=ConwayMorris1990>{{cite journal
| author = Conway Morris, S.
| coauthors = Mattes, B.W., Chen, M.
| year = 1990
| title = The early skeletal organism'' Cloudina'': new occurrences from Oman and possibly China
| journal = American Journal of Science
| volume = 290
| pages = 245–260
| accessdate =
}}</ref><ref name=Grant1990>{{cite journal
| author = Grant, SW
| year = 1990
| title = Shell structure and distribution of'' Cloudina'', a potential index fossil for the terminal Proterozoic
| journal = American Journal of Science
| issue = 290-A
| pages = 261–294
| accessdate =
}}</ref> Even more striking is the fact that ''[[Cloudina]]'' specimens show evidence of borings by predators, while ''[[Sinotubulites]]'' fossils found in the same locations do not. This suggests that: there were predators that were sufficiently advanced to penetrate shells; these predators found ''Cloudina'' a more inviting target than ''Sinotubulites''.<ref name="BengtsonZhao1992PredatorialBorings">{{cite journal
| date = [[17 July]] [[1992]]
| title = Predatorial Borings in Late Precambrian Mineralized Exoskeletons
| journal = Science
| volume = 257
| issue = 5068
| pages = 367
| doi = 10.1126/science.257.5068.367
| accessdate =
| author = Bengtson, S. and Zhao, Y.
| pmid = 17832833
}}</ref> A possible "arms race" between predators and prey is one of the most promising components of theories that attempt to explain the Cambrian explosion.<ref name="Bengtson2002OriginsOfPredation" />
==Cambrian life==
===Small shelly fauna===
{{Main|Small shelly fauna}}
Fossils known as “[[small shelly fauna]]” have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the [[Nemakit-Daldynian]] and [[Tommotian]] ages; see [[#Timeline|timeline]]). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, [[archeocyathid]]s (sponge-like animals) and small shells very like those of [[brachiopod]]s and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.<ref name=Matthews1975>{{cite journal
| author = Matthews, S.C.
| coauthors = Missarzhevsky, V.V.
| date = [[1975-06-01]]
| title = Small shelly fossils of late Precambrian and early Cambrian age: a review of recent work
| journal = Journal of Geological Society
| volume = 131
| issue = 3
| pages = 289
| doi = 10.1144/gsjgs.131.3.0289
| year = 1975
}}</ref>
While small, these fossils are far more common than complete fossils of the organisms that produced them; crucially, they cover the window from the start of the Cambrian to the first lagerstatten: a period of time that is otherwise lacking in fossils. Hence they supplement the conventional fossil record, and allow the fossil ranges of many groups to be extended.
===Early Cambrian trilobites and echinoderms===
[[Image:LeggedTrilobite2.jpg|thumb|A fossilized [[trilobite]], an ancient type of [[arthropod]]. This specimen, from the Burgess shale, preserves "soft parts" – the antennae and legs. ]]
The earliest Cambrian [[trilobite]] fossils are about 530 million years old, but the class was already quite diverse and world-wide, suggesting that they had been around for quite some time.<ref name="Lieberman1999Trilobites">{{cite journal
| author =Lieberman, BS
| title =Testing the Darwinian Legacy of the Cambrian Radiation Using Trilobite Phylogeny and Biogeography
| journal =Journal of Paleontology
| volume =73
| issue =2
| date =1999
| url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176
| format =abstract
}}</ref>
The earliest generally-accepted [[echinoderm]]s appeared at about the same time; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.<ref name="DornbosBottjer2000Helicoplacoids">{{cite journal
| author =Dornbos, S.Q. and Bottjer, D.J.
| title =Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution
| journal =Geology
| volume =28
| issue =9
| pages =839–842
| date =2000
| url =http://geology.geoscienceworld.org/cgi/content/full/28/9/839
| doi =10.1130/0091-7613(2000)28<839:EPOTEE>2.0.CO;2
| year =2000
}}</ref>
These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.
===Burgess shale type faunas===
{{main|Burgess shale type preservation}}
The Burgess shale and similar lagerstatten preserve the soft parts of organisms, which provides a wealth of data to aid in the classification of enigmatic fossils. It often preserved complete specimens of organisms only otherwise known from dispersed parts, such as loose scales or isolated mouthparts. Further, the majority of organisms and taxa in these horizons are entirely soft bodied - hence absent from the rest of the fossil record.<ref name=Butterfield2003>{{cite journal
| title = Exceptional Fossil Preservation and the Cambrian Explosion
| year = 2003
| journal = Integrative and Comparative Biology
| volume = 43
| issue = 1
| pages = 166–177
| doi = 10.1093/icb/43.1.166
| author = Butterfield, Nicholas J.
}}</ref> Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed.{{Verify source|Caron 2006 needs incorporating - see talk page|date=July 2008}}
However, the assemblages may represent a "museum": a deep water ecosystem that is evolutionarily "behind" the rapidly diversifying faunas of shallower waters.<ref name=ConwayMorris2008>{{cite journal
| author = Conway Morris, Simon
| year = 2008
| title = A Redescription of a Rare Chordate, Metaspriggina Walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada
| journal = Journal of Paleontology
| volume = 82
| pages = 424
| doi = 10.1666/06-130.1
}}</ref>
Because the lagerstatten provide a mode and quality of preservation that's virtually absent outside of the Cambrian, lots of organisms appear completely different to anything known from the conventional fossil record. This led early workers in the field to attempt to shoehorn the organisms into extant phyla; the shortcomings of this approach led them to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged from [[stem group| lineages]] before they established the phyla we know today{{unclear}} – slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did.
The preservational mode is rare in the preceding Ediacaran period, but those assemblages known show no trace of animal life - perhaps implying a genuine absence of macroscopic metazoans.<ref name=Xiao2002>{{cite journal
| title = A Reassessment of the Neoproterozoic Miaohe Carbonaceous Biota in South China
| author = Xiao, Shuhai; M. Steiner; A. H. Knoll.
| year = 2002
| journal = Journal of Paleontology
| volume = 76
| pages = 345–374
| doi = 10.1666/0022-3360(2002)076<0347:MCCIAT>2.0.CO;2
}}</ref>
===Early Cambrian crustaceans===
{{More|Orsten}}
Crustaceans, one of the three great modern groups of [[arthropods]], are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans".<ref name=Harvey2008>{{cite journal
| url = http://www.nature.com/nature/journal/vaop/ncurrent/full/nature06724.html
| doi = 10.1038/nature06724
| author = Harvey, T.H.; Butterfield, N.J.
| journal = Nature
| year = 2008
| title = Sophisticated particle-feeding in a large Early Cambrian crustacean
| volume = 452
| pages = 868
| issue = 7189
| accessdate = 2008-06-11
}}</ref> The Cambrian record of crown group crustaceans comes from microfossils. The Swedish [[Orsten]] horizons contain later Cambrian crustacea, but only organisms smaller than 2 mm are preserved. This restricts the data set to juveniles and miniaturised adults.
A more informative data source is the organic microfossils of the Mount Cap formation, Canada. This late Early Cambrian assemblage ({{Ma|510|515}}) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with [[HF acid|a strong acid]]. The diversity of this assemblage is similar to that of modern crustacean faunas. Most interestingly, analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to a large (~30 cm)<ref>{{PalAss2007|author=Harvey, T.P.H.}}</ref> organism, and would have provided great potential for diversification: specialised feeding apparatus allows a number of different approaches to feeding to develop, and creates a number of different approaches to avoiding being eaten!<ref name=Harvey2008/>
===Early Ordovician radiation===
{{Main|Ordivician radiation}}
After a [[Cambrian-Ordovician extinction events| mass extinction]] at the Cambrian-Ordovician boundary, another radiation occurred, which established the taxa which would dominate the Palaeozoic.<ref name=Droser2003/>.
A new phylum, the [[Bryozoa]], is first observed after this Ordovician radiation; the total number of orders doubled, and families tripled,<ref name=Droser2003>{{cite journal
| author = Droser, Mary L.; Finnegan, Seth
| year = 2003
| title = The Ordovician Radiation: A Follow-up to the Cambrian Explosion?
| journal = Integrative and Comparative Biology
| volume = 43
| issue = 1
| pages = 178
| doi = 10.1093/icb/43.1.178
}}</ref> increasing marine diversity to levels typical of the Palaeozoic,<ref name="Marshall2006Explaining" /> and disparity to levels approximately equivalent to today's.<ref name=Bambach2007/>
==How real was the explosion?==
The fossil record as Darwin knew it seemed to suggest that the major [[metazoan]] groups appeared in a few million years of the early to mid-Cambrian, and even in the 1980s this still appeared to be the case.<ref name="Whittington1985BurgessShale" /><ref name="WonderfulLife" />
However, evidence of Precambrian metazoa is gradually accumulating. If the Ediacaran ''Kimberella'' was a mollusc-like [[protostome]] (one of the two main groups of [[coelomate]]s),<ref name=Martin2000 /><ref name="FedonkinWaggoner1997KimberellaMollusc" /> the protostome and [[deuterostome]] lineages must have split significantly before {{Ma|550}} (deuterostomes are the other main group of coelomates).<ref name="ErwinDavidson2002LastCommonBilaterianAncestor" /> Even if it is not a protostome, it is widely accepted as a [[bilaterian]].<ref name="ErwinDavidson2002LastCommonBilaterianAncestor" /><ref name=Butterfield2006 /> Since fossils of rather modern-looking [[Cnidaria]]ns ([[jellyfish]]-like organisms) have been found in the [[Doushantuo formation| Doushantuo]] [[lagerstätte]], the Cnidarian and bilaterian lineages must have diverged well over {{Ma|580}}.<ref name="ErwinDavidson2002LastCommonBilaterianAncestor">{{ cite journal | title=The last common bilaterian ancestor | author=Erwin, D.H., and Davidson, E.H. | journal=Development |volume=129 | pages=3021-3032 | date=2002 | url=http://dev.biologists.org/cgi/content/full/129/13/3021 | accessdate=2008-07-10 }}</ref>
Trace fossils<ref name="Erwin1999OriginOfBodyplans" /> and predatory borings in ''[[Cloudina]]'' shells provide further evidence of Ediacaran animals.<ref name="BengtsonZhao1992PredatorialBorings" /> Some fossils from the Doushantuo formation have been interpeted as embryos and one (''[[Vernanimalcula]]'') as a bilaterian coelomate, although these interpretations are not universally accepted.<ref name="Chen2004" /><ref name="Bengtson2004" /><ref name="ChenDefendVernanimacula">{{ cite journal
| date=2004
| title=Response to Comment on “Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian”
| doi = 10.1126/science.1102328
| accessdate=
| author=Chen, J.Y., Oliveri, P., Davidson, E. and Bottjer, D.J.
| journal=Science
| volume=306
| pages=1291b
}}</ref> Earlier still, predatory pressure has acted on [[stromatolite]]s and [[acritarch]]s since around {{ma|1250}}.<ref name="Bengtson2002OriginsOfPredation" />
The presence of Precambrian animals somewhat dampens the "bang" of the explosion: not only was the appearance of animals gradual, but their [[evolutionary radiation]] ("diversification") may also not have been as rapid as once thought. Indeed, statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals' history.{{ref|pulse|4}}
There is little doubt that disparity – that is, the range of different organism "designs" or "ways of life" – rose sharply in the early Cambrian.<ref name=Bambach2007>{{cite journal | author = Bambach | year = 2007 | doi = 10.1111/j.1475-4983.2006.00611.x | title = Autecology And The Filling Of Ecospace: Key Metazoan Radiations | journal = Palaeontology | volume = 50 | pages = 1 }}</ref> However recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing.<ref>{{cite journal | author = Erwin, D.H.| year = 2007| doi = 10.1111/j.1475-4983.2006.00614.x| title = Disparity: Morphological Pattern And Developmental Context| journal = Palaeontology| volume = 50| pages = 57 }}</ref> In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation.<ref name=Bambach2007/>
The diversity of many Cambrian assemblages is similar to today's.<ref>{{citation|doi=10.1666/0094-8373(2003)029<0349:MDOCAA>2.0.CO;2|issn=0094-8373|year=2003|volume=29|pages=349|title=Morphological diversity of Carboniferous arthropods and insights on disparity patterns through the Phanerozoic|author=Lofgren, Andrea Stockmeyer|journal=Paleobiology}}</ref><ref name=Harvey2008/>
==Possible causes of the “explosion”==
Despite the evidence that moderately complex animals ([[triploblastic]] [[bilaterians]]) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental, and ecological changes. Any explanation must explain the timing and magnitude of the explosion. It is also possible that the "explosion" requires no special explanation.
===Changes in the environment===
====Increase in oxygen levels====
[[Earth's atmosphere| Earth’s earliest atmosphere]] contained no free [[oxygen]]; the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of [[photosynthesis]]. As a general trend, the [[concentration]] of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.<ref name ="CowenHistLife"/>
Shortage of oxygen might well have prevented the rise of large, complex animals. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones); but the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal’s size increases equally in all directions. An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen. However, members of the [[Ediacara biota]] reached metres in length; clearly oxygen did not limit ''their'' growth.<ref name=Knoll1999>e.g. {{cite journal
| author = Knoll, A.H.
| coauthors = Carroll, S.B.
| date = [[1999-06-25]]
| title = Early Animal Evolution: Emerging Views from Comparative Biology and Geology
| journal = Science
| volume = 284
| issue = 5423
| pages = 2129
| doi = 10.1126/science.284.5423.2129
| year = 1999
| pmid = 10381872
}}</ref> Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as [[collagen]], required for the construction of complex structures,<ref name=Towe1970>{{cite journal
| author = Towe, K.M.
| date = [[1970-04-01]]
| title = Oxygen-Collagen Priority and the Early Metazoan Fossil Record
| journal = Proceedings of the National Academy of Sciences
| volume = 65
| issue = 4
| pages = 781–788
| doi = 10.1073/pnas.65.4.781
| url=http://www.pnas.org/cgi/content/abstract/65/4/781
| year = 1970
| format = abstract
| pmid = 5266150
}}</ref> or to form molecules for the construction of a hard exoskeleton.<ref name=CatlingEtAl2005>{{cite journal
| doi = 10.1089/ast.2005.5.415
| author = Catling, D.C.; Glein, C.R.; Zahnle, K.J.; McKay, C.P.
| journal = Astrobiology
| volume = 5
| issue = 3
| pages = 415–438
| year = 2005
| title = Why {{O2}} Is Required by Complex Life on Habitable Planets and the Concept of Planetary "Oxygenation Time"
| accessdate =
}}</ref>
====Snowball Earths====
{{main|Snowball Earth}}
In the late [[Neoproterozoic]] (extending into the early [[Ediacaran]] period), the Earth suffered [[Snowball Earth|massive glaciations]] in which most of its surface was covered by ice. This may have caused a mass extinction, creating a genetic bottleneck; the resulting diversification may have given rise to the Ediacara biota, which appears soon after the last "Snowball Earth" episode.<ref name="HoffmanKaufman1998NeoproterozoicSnowball">{{ cite journal
| title=A Neoproterozoic Snowball Earth
| journal=Science
| date=[[28 August]] [[1998]]
| volume=281
| issue=5381
| pages=1342–1346
| doi=10.1126/science.281.5381.1342
| url=http://www.sciencemag.org/cgi/content/abstract/281/5381/1342
| author=Hoffman, P.F., Kaufman, A.J., Halverson, G.P., and Schrag, D.P.
| year=1998
| format=abstract
| pmid=9721097
}}</ref>
However, the snowball episodes occurred a long time before the start of the Cambrian, and it is hard to see how so much diversity could have been caused by even a series of bottlenecks;<ref name="Marshall2006Explaining" /> the cold periods may even have ''delayed'' the evolution of large size.<ref name="Bengtson2002OriginsOfPredation" />
===Developmental Explanations===
A range of theories are based on the concept that minor modifications to animals' development as they grow from [[embryo]] to adult may have been able to cause very large changes in the final adult form.
The [[hox]] genes, for example, control which organs individual regions of an embryo will develop into. For instance, if a certain hox gene is expressed, a region will develop into a limb; if a different hox gene is expressed in that region (a minor change), it could develop into an eye instead (a phenotypically major change).
Such a system allows a large range of disparity to appear from a limited set of genes, but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity. Evidence of Precambrian metazoans<ref name="Marshall2006Explaining" /> combines with molecular data<ref name=deRosa1999>{{cite journal | author = Renaud de Rosa, Jennifer K. Grenier5,2, Tatiana Andreeva3, Charles E. Cook, André Adoutte1, Michael Akam4, Sean B. Carroll2 and Guillaume Balavoine | year = 1999 | doi = 10.1038/21631 | journal = Nature | volume = 399 | pages = 772|title=Hox genes in brachiopods and priapulids and protostome evolution}}</ref> to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian.
===Ecological Explanations===
These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the [[food chain]]; some suggest [[Evolutionary arms race| arms races]] between predators and prey, and others focus on the more general mechanisms of [[coevolution]]. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they must explain why the "explosion" happened when it did.<ref name="Marshall2006Explaining" />
====End-Ediacaran mass extinction====
{{main|End-Ediacaran extinction}}
Evidence for such an extinction includes the disappearance from the fossil record of the [[Ediacara biota]] and shelly fossils such as ''[[Cloudina]]'', and the accompanying perturbation in the {{d13c|link}} record.
Mass extinctions are often followed by adaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction. However, once the dust had settled, overall disparity and diversity returned to the pre-extinction level in each of the Phanerozoic extinctions.<ref name="Marshall2006Explaining" />
====Evolution of eyes====
{{main|Evolution of the eye}}
Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time hunting and evading were both close-range affairs – smell, vibration, and touch were the only senses used. When predators could see their prey froma distance, new defensive strategies were needed. Armor, spines, and similar defenses may also have evolved in response to vision.<ref>{{ cite book | author=Parker, Andrew | title=In The Blink Of An Eye |publisher= Perseus Books |location=Cambridge, Massachusetts, USA | date=2003 }}</ref> Nevertheless many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian.<ref name=McCall2006>{{cite journal | author = McCall | year = 2006 | doi = 10.1016/j.earscirev.2005.08.004 | title = The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion | journal = Earth-Science Reviews | volume = 77 | pages = 1 }}</ref> It is also difficult to understand why the evolution of eyesight would have caused an explosion, since other senses such as smell and pressure detection can detect things further away than they can be seen under the sea, but the appearance of these other senses apparently did not cause an evolutionary explosion.<ref name=Marshall2006Explaining/>
====Arms races between predators and prey====
The ability to avoid or recover from [[predation]] often makes the difference between life and death, and is therefore one of the strongest components of [[natural selection]]. The pressure to adapt is stronger on the prey than on the predator: if the predator fails to win a contest, it loses its lunch; if the prey is the loser, it loses its life.<ref name="DawkinsKrebs1979ArmsRaces">{{ cite journal
| title=Arms races between and within species
| volume=205
| issue=1161
| pages=489–511
| date=[[September 21]] [[1979]]
| url=http://links.jstor.org/sici?sici=0080–4649(19790921)205%3A1161%3C489%3AARBAWS%3E2.0.CO%3B2–1
| author=Dawkins, R. and Krebs, R.J.
| day=21
| doi=10.1098/rspb.1979.0081
| journal=Proceedings of the Royal Society: Biological Sciences Series B
}}</ref>
But there is evidence that predation was rife long before the start of the Cambrian, for example in the increasingly spiny forms of [[acritarchs]] and the holes drilled in ''[[Cloudina]]'' shells. Hence it is unlikely that the ''appearance'' of predation was the trigger for the Cambrian "explosion", although it may well have exhibited a strong influence on the body forms that the "explosion" produced.<ref name="Bengtson2002OriginsOfPredation" /> Alternatively a more subtle aspect, such as the evolution of a new ''style'' of predation, may have played a role.
====Increase in size and diversity of planktonic animals====
[[Geochemical]] evidence strongly indicates that the total mass of [[plankton]] has been similar to modern levels since early in the Proterozoic. Before the start of the Cambrian, their corpses and droppings were too small to fall quickly towards the sea-bed, since their [[Drag (physics)| drag]] was about the same as their weight. This meant they were destroyed by [[scavengers]] or by chemical processes before they reached the sea floor.<ref name=Butterfield2001/>
Mesozooplankton are plankton of a larger size, and early Cambrian specimens [[Filter feeder| filtered]] microscopic plankton from the seawater. These larger organisms would have produced droppings and corpses that were large enough to fall fairly quickly. This provided a new supply of energy and nutrients to the mid-levels and bottoms of the seas, which opened up a huge range of new possible ways of life. If any of these remains sunk uneaten to the sea floor they could be buried; this would have taken some [[carbon]] out of [[Carbon cycle| circulation]], resulting in an increase in the [[concentration]] of breathable oxygen in the seas.<ref name=Butterfield2001/> (carbon readily [[Chemical compound| combines]] with oxygen)
The initial herbivorous mesozooplankton were probably larvae of benthic (sea-floor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the sea-floor during the [[Ediacaran]] period.<ref name=Butterfield2001/><ref name=Peterson2005>{{ cite journal
| title=Tempo and mode of early animal evolution: inferences from rocks, Hox, and molecular clocks
| journal=Paleobiology
| date=June 2005
| volume=31
| issue=2 (Supplement)
| pages=36–55
| doi=10.1666/0094-8373(2005)031[0036:TAMOEA]2.0.CO;2
| publisher=Paleontological Society
| url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/31/2_Suppl/36
| author=Peterson, K.J., McPeek, M.A., and Evans, D.A.D.
| year=2005
| format=abstract
}}</ref>
Metazoans have an amazing ability to increase diversity through [[co-evolution]].<ref name=Butterfield2007/> This means that a trait of one organism can cause another to evolve in response; a number of responses are possible, and a different species can potentially emerge for each. As a simple example, the evolution of predation may have caused one organism to develop defence while another developed motion to flee. This would cause the predator lineage to split into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual co-evolution is somewhat more subtle, but in this fashion, great diversity can arise: three quarters of living species are animals, and most of the rest have formed by co-evolution with animals.<ref name=Butterfield2007/>
===Discredited hypotheses===
{{main|Discredited hypotheses for the Cambrian explosion}}
As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some hypotheses look improbable. Causes that have been proposed but are now discounted include the evolution of herbivory, vast changes in the speed of [[tectonic plate]] movement or of the [[precession| cyclic changes]] in the Earth's orbital motion, or the operation of different evolutionary mechanisms from those that are seen in the rest of the [[Phanerozoic]] eon.
===No explanation required===
The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed; for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed.{{Verify source|date=June 2008}}<ref name="SoléFernándezKauffman2003AdaptiveWalks">{{ cite journal
| title=Adaptive walks in a gene network model of morphogenesis: insights into the Cambrian explosion
| date=2003
| journal=Int. J. Dev. Biol.
| volume=47
| issue=7
| pages=685–693
| pmid=14756344
| author=Solé, R.V., Fernández, P., and Kauffman, S.A.}}</ref>
==Uniqueness of the explosion==
The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a [[co-evolution]]ary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in the water column.<ref name=Butterfield2007>{{cite journal
| doi = 10.1111/j.1475-4983.2006.00613.x
| author = Butterfield, N.J.
| journal = Palaeontology
| volume = 50
| issue = 1
| pages = 41–55
| year = 2007
| title = Macroevolution And Macroecology Through Deep Time
}}</ref> The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all [[metazoan]] [[clade]]s of which Cambrian fossils have been found. Later [[Evolutionary radiation| radiation]]s, such as those of [[fish]] in the [[Silurian]] and [[Devonian]] periods, involved fewer [[taxon| taxa]], mainly with very similar body plans.<ref name="CowenHistLife" />
Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously-unavailable [[ecological niche]]s. When these were all occupied, there was little room for such wide-ranging diversifications to occur again, because there was strong competition in all niches and {{wict|incumbent}}s usually had the advantage. If there had continued to be a wide range of empty niches, clades would be able to continue diversifying and become disparate enough for us to recognise them as different [[phylum| phyla]]; when niches are filled, lineages will continue to resemble one another long after they diverge, as there is limited opportunity for them to change their life-styles and forms.<ref name="Valentine1995WhyNoNewPhyla">{{ cite journal | title=Why No New Phyla after the Cambrian? Genome and Ecospace Hypotheses Revisited | author=Valentine, J.W. | journal=Palaios | voluime=10 | issue=2 | date=April 1995 | pages=190–194 | doi=10.2307/3515182 | url=http://links.jstor.org/sici?sici=0883–1351(199504)10%3A2%3C190%3AWNNPAT%3E2.0.CO%3B2-H#abstract | year=1995 | volume=10 | format=abstract
}}</ref>
There is a similar one-time explosion in the [[Evolutionary history of plants#Colonisation of land|evolution of land plants]]: after a cryptic history beginning about {{Ma|450}}, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about {{Ma|400}}.<ref name="CowenHistLife" />
==Further reading==
* {{cite journal |year=2000|title=A critical reappraisal of the fossil record of the bilaterian phyla |journal=Biological Reviews|volume=75|pages=253–295|author=Budd, G. E. & Jensen, J.|doi=10.1017/S000632310000548X}}
* Collins, Allen G. [http://www.ucmp.berkeley.edu/phyla/metazoafr.html “Metazoa: Fossil record”]. Retrieved Dec. 14, 2005.
* Conway Morris, S. (1997). ''The Crucible of Creation: the Burgess Shale and the rise of animals''. Oxford University Press. ISBN 0–19–286202–2
* {{cite journal
| author = Conway Morris, S.
| year = 2006
| title = Darwin’s dilemma: the realities of the Cambrian ‘explosion’
| journal = Philosophical Transactions of the Royal Society B: Biological Sciences
| volume = 361
| issue = 1470
| pages = 1069–1083
| issn =
| doi =10.1098/rstb.2006.1846
| url = http://www.journals.royalsoc.ac.uk/index/QU151T4722902768.pdf
}} An enjoyable account.
* {{cite journal|author=Kennedy, M., M. Droser, L. Mayer., D. Pevear, and D. Mrofka|year=2006|doi=10.1126/science.311.5766.1341c|journal=Science|volume=311|issue=5766|pages=1341|title=Clay and Atmospheric Oxygen}}
* Knoll,A. H. and Carroll, S. B. (1999). Early Animal Evolution: Emerging Views from Comparative Biology and Geology. ''Science'' '''284''' (5423): 2129 – 2137.
*[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B83WC-4N0HJMK-2&_user=1300184&_coverDate=12%2F31%2F2007&_rdoc=6&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%2333783%232007%23999839995%23671853%23FLA%23display%23Volume)&_cdi=33783&_sort=d&_docanchor=&_ct=9&_acct=C000052237&_version=1&_urlVersion=0&_userid=1300184&md5=d9c2663e7fbd6a77385d61334953d75d Alexander V. Markov, and Andrey V. Korotayev (2007) “Phanerozoic marine biodiversity follows a hyperbolic trend” ''Palaeoworld'' 16(4): pp. 311–318].
* [[Andrew Parker (zoologist)|Parker, A.]] (2004). ''In the Blink of an Eye'', Free Press, ISBN 0–7432–5733–2.
* {{cite journal|author=Wang, D. Y.-C., S. Kumar and S. B. Hedges|year=1999|title=Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi|journal=Proceedings of the Royal Society of London, Series B, Biological Sciences|volume=266|pages=163–71|doi=10.1098/rspb.1999.0617|issue=1415}}
* {{cite journal|author=Xiao, S., Y. Zhang, and A. Knoll|year=1998|title=Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite|journal=Nature|volume=391|pages=553–58|doi=10.1038/35318}}
Timeline References:
* Gradstein and Ogg, “A Phanerozoic time scale”, v.19, no.1&2., 1996.
* {{cite journal | author=Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; [[Mikhail A. Fedonkin|Fedonkin]], M.A.; Kirschvink, J.L. | title=Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution | journal=Science | year=2000 | volume=288 | pages=841–845 | doi=10.1126/science.288.5467.841 | pmid=10797002}}
==External links==
* [http://www.ijdb.ehu.es/web/contents.php?vol=47&issue=7–8&doi=14756326 The Cambrian “explosion” of metazoans and molecular biology: would Darwin be satisfied?]
* [http://genome6.cu-genome.org/andrey/GouldComment.pdf On embryos and ancestors] by [[Stephen Jay Gould]]
* {{citation|url=http://www.pnas.org/cgi/content/full/97/9/4420 |title=The Cambrian “explosion”: Slow-fuse or megatonnage?}}
* [http://www.bbc.co.uk/radio4/history/inourtime/ram/inourtime_20050217.ram The Cambrian Explosion] – ''In Our Time'', [[BBC Radio 4]] broadcast, 17 February 2005
* [http://www.kumip.ku.edu/cambrianlife/ Utah's Cambrian life] – new (2008) website with good images of a range of Burgess-shale-type and other Cambrian fossils.
==Notes==
<references group=note/>
{{note|trace_fossils|3}}Older marks found in billion-year old rocks<ref name="Seilacher1998">{{cite journal
| title=Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India
| journal=Science
| volume=282
| number=5386
| pages=80–83
| year=1998
| url=http://www.sciencemag.org/cgi/content/abstract/282/5386/80
| doi = 10.1126/science.282.5386.80
| accessdate=2007-08-20
| author=Seilacher, A., Bose, P.K. and Pflüger, F.
| format=abstract
| pmid=9756480
}}
</ref> have since been recognised as non-biogenic.<ref name=Budd2000>{{cite journal
| author = Budd, G.E.
| coauthors = Jensen, S.
| year = 2000
| title = A critical reappraisal of the fossil record of the bilaterian phyla
| journal = Biological Reviews
| volume = 75
| issue = 02
| pages = 253–295
| doi = 10.1017/S000632310000548X
| url = http://www.journals.cambridge.org/abstract_S000632310000548X
| accessdate = 2007-06-27
| format = abstract
}}</ref><ref name="Jensen2003">{{cite journal
| author =Jensen, S.
| title =The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives
| journal =Integrative and Comparative Biology
| volume =43
| issue =1
| pages =219–228
| date =2003
| url=http://icb.oxfordjournals.org/cgi/content/abstract/43/1/219
| doi=10.1093/icb/43.1.219
| year =2003
| format=abstract
}}</ref>
{{note|pulse|4}} The analysis considered the bioprovinciality of trilobite lineages, as well as their evolutionary rate.<ref name=Lieberman2003>{{cite journal
| author = Lieberman, B.
| year = 2003
| title = Taking the Pulse of the Cambrian Radiation
| journal = Integrative and Comparative Biology
| volume = 43
| issue = 1
| pages = 229–237
| doi = 10.1093/icb/43.1.229
| url = http://icb.oxfordjournals.org/cgi/content/abstract/43/1/229
}}</ref>
==References==
{{reflist|2}}
[[Category:Paleontology]]
[[Category:Cambrian]]
[[Category:Fossils]]
[[ar:انفجار كامبري]]
[[ca:Explosió cambriana]]
[[da:Kambriske Eksplosion]]
[[de:Kambrische Explosion]]
[[es:Explosión cámbrica]]
[[fr:Explosion cambrienne]]
[[gl:Explosión cámbrica]]
[[ko:캄브리아기의 대폭발]]
[[lt:Kambro sprogimas]]
[[hu:Kambriumi robbanás]]
[[nl:Cambrische explosie]]
[[ja:カンブリア爆発]]
[[no:Den kambriske eksplosjon]]
[[pl:Eksplozja kambryjska]]
[[pt:Explosão Cambriana]]
[[ru:Кембрийский взрыв]]
[[sr:Камбријумска експлозија]]
[[sh:Kambrijska eksplozija]]
[[fi:Kambrikauden räjähdys]]
[[sv:Kambriska explosionen]]
[[tr:Kambrien patlaması]]
[[zh:寒武纪大爆发]]