View source for Earth:Cambrian explosion

{{short description|Period of major evolutionary diversification of animal life}}
The '''Cambrian explosion''',  '''Cambrian radiation,'''<ref>{{cite book |last1=Zhuravlev |first1=Andrey |last2=Riding |first2=Robert |title=The Ecology of the Cambrian Radiation |date=2000 |publisher=Columbia University Press |isbn=978-0-231-10613-9 |url=https://cup.columbia.edu/book/the-ecology-of-the-cambrian-radiation/9780231106139 |quote=The Cambrian radiation was the explosive evolution of marine life that started 550,000,000 years ago. It ranks as one of the most important episodes in Earth history. This key event in the [[Biology:History of life|history of life]] on our planet changed the marine biosphere and its sedimentary environment forever, requiring a complex interplay of wide-ranging biologic and nonbiologic processes.}}</ref> '''Cambrian diversification''', or the '''Biological Big Bang'''<ref>{{Cite journal|last=Koonin|first=Eugene V|date=2007-08-20|title=The Biological Big Bang model for the major transitions in evolution|journal=Biology Direct|volume=2|pages=21|doi=10.1186/1745-6150-2-21|issn=1745-6150|pmc=1973067|pmid=17708768 |doi-access=free }}</ref> refers to an interval of time approximately {{Ma|538.8}} in the Cambrian Period of early [[Earth:Paleozoic|Paleozoic]] when there was a sudden radiation of complex life and practically all major [[Biology:Animal|animal]] [[Biology:Phylum|phyla]] started appearing in the fossil record.<ref name="StratChart 2022">{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |publisher=International Stratigraphic Commission |date=February 2022 |access-date=22 April 2022}}</ref><ref>{{cite journal |last1=Maloof |first1=A. C. |last2=Porter |first2=S. M. |last3=Moore |first3=J. L. |last4=Dudas |first4=F. O. |last5=Bowring |first5=S. A. |last6=Higgins |first6=J. A. |last7=Fike |first7=D. A. |last8=Eddy |first8=M. P. |title=The earliest Cambrian record of animals and ocean geochemical change |journal=Geological Society of America Bulletin |year=2010 |volume=122 |issue=11–12 |pages=1731–1774 |doi=10.1130/B30346.1 |bibcode=2010GSAB..122.1731M}}</ref><ref>{{cite web |title=New Timeline for Appearances of Skeletal Animals in Fossil Record Developed by UCSB Researchers |url=http://www.ia.ucsb.edu/pa/display.aspx?pkey=2364 |publisher=The Regents of the University of California |access-date=1 September 2014 |date=10 November 2010}}</ref> It lasted for about 13<ref>[https://www.science.org/doi/abs/10.1126/science.11539488 Calibrating rates of early Cambrian evolution], Science 1993, 261(5126), s. 1293–1298. SA Bowring, JP Grotzinger, CE Isachsen, AH Knoll, SM Pelechaty, P Kolosov</ref><ref name="dev.biologists.org">{{cite journal|last1=Valentine |first1=JW |last2=Jablonski|first2=D |last3=Erwin |first3=DH|title=Fossils, molecules and embryos: new perspectives on the Cambrian explosion |journal=Development|year=1999 |volume=126 |issue=5 |pages=851–9 |doi=10.1242/dev.126.5.851 |pmid=9927587 |url=http://dev.biologists.org/content/126/5/851.long}}</ref><ref name=Budd>{{cite journal |last1=Budd |first1=Graham|title=At the origin of animals: the revolutionary Cambrian fossil record |journal=Current Genomics |year=2013 |volume=14 |issue=6 |pages=344–354 |doi=10.2174/13892029113149990011|pmid=24396267|pmc=3861885}}</ref>&nbsp;&ndash;&nbsp;25<ref name="The Cambrian conundrum: early diver">{{cite journal | last1=Erwin | first1=D. H. | last2=Laflamme | first2=M. | last3=Tweedt | first3=S. M. | last4=Sperling | first4=E. A. | last5=Pisani | first5=D. | last6=Peterson | first6=K. J. | year=2011 | title=The Cambrian conundrum: early divergence and later ecological success in the early history of animals | journal=Science | volume=334 | issue=6059| pages=1091–1097 | doi=10.1126/science.1206375 | pmid=22116879|bibcode=2011Sci...334.1091E | s2cid=7737847 }}</ref><ref>{{cite journal | last1=Kouchinsky | first1=A. | last2=Bengtson | first2=S. | last3=Runnegar | first3=B. N. | last4=Skovsted | first4=C. B. | last5=Steiner | first5=M. | last6=Vendrasco | first6=M. J. | year=2012 | title=Chronology of early Cambrian biomineralization | journal=Geological Magazine | volume=149 | issue=2| pages=221–251 | doi=10.1017/s0016756811000720| bibcode=2012GeoM..149..221K | doi-access=free }}</ref> million years and resulted in the [[Biology:Divergent evolution|divergence]] of most modern metazoan phyla.<ref>{{cite journal |last1=Conway Morris |first1=S. |title=The Cambrian "explosion" of metazoans and molecular biology: would Darwin be satisfied? |journal=The International Journal of Developmental Biology |year=2003 |volume=47 |issue=7–8 |pages=505–15 |pmid=14756326 |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756326}}</ref> The event was accompanied by major diversification in other groups of organisms as well.{{efn|This included at least animals, [[Earth:Phytoplankton|phytoplankton]] and [[Biology:Calcimicrobe|calcimicrobe]]s.<ref name="Butterfield2001ECR"/>}}

Before early Cambrian diversification,{{efn|At 610 million years ago, ''[[Biology:Aspidella|Aspidella]]'' disks appeared, but it is not clear that these represented complex life forms.}} most organisms were relatively simple, composed of individual cells, or small multicellular organisms, occasionally organized into [[Biology:Colony|colonies]]. As the rate of diversification subsequently accelerated, the variety of life became much more complex, and began to resemble that of today.<ref name="Bambach2007">{{cite journal | author=Bambach, R.K. |author2=Bush, A.M. |author3=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|bibcode=2007Palgy..50....1B | doi-access=free }}</ref> Almost all present-day animal phyla appeared during this period,<ref name=Budd2000/><ref name=Budd2003>{{cite journal | author=Budd, G.E. | title=The Cambrian Fossil Record and the Origin of the Phyla | journal=Integrative and Comparative Biology | volume=43 | issue=1 | pages=157–165 | doi=10.1093/icb/43.1.157 | year=2003 | pmid=21680420| doi-access=free}}</ref> including the [[Biology:Cambrian chordates|earliest chordates]].<ref>{{Cite journal |last=McMenamin |first=Mark A. S. |date=2019 |title=Cambrian Chordates and Vetulicolians |journal=Geosciences |language=en |volume=9 |issue=8 |pages=354 |doi=10.3390/geosciences9080354 |bibcode=2019Geosc...9..354M |issn=2076-3263|doi-access=free }}</ref>

A 2019 paper suggests that the timing should be expanded back to include the late [[Earth:Ediacaran|Ediacaran]], where another diverse soft-bodied biota existed and possibly persisted into the Cambrian, rather than just the narrower timeframe of the "Cambrian Explosion" event visible in the fossil record, based on analysis of chemicals that would have laid the building blocks for a progression of transitional radiations starting with the Ediacaran period and continuing at a similar rate into the Cambrian.<ref>{{cite journal | author1 = Wood, R. |author2 = Liu, A.G. |author3=Bowyer, F. |author4=Wilby, P.R. |author5 = Dunn, F.S. |author6= Kenchington, C.G. |author7 = Cuthill, J.F.H. |author8 = Mitchell, E.G. |author9 = Penny, A. | title=Integrated records of environmental change and evolution challenge the Cambrian Explosion | journal=Nature Ecology & Evolution | volume=3 | issue=4 | pages=528–538 | doi=10.1038/s41559-019-0821-6 | year=2019 |pmid = 30858589 |bibcode = 2019NatEE...3..528W |doi-access=free}}</ref>


==History and significance==
{{Life timeline}}
{{main|Biology:History of life}}
The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s,<ref name="Buckland1841">{{cite book| author=Buckland, W. | year=1841| title=Geology and Mineralogy Considered with Reference to Natural Theology| publisher=Lea & Blanchard| isbn=978-1-147-86894-4 }}</ref> and in his 1859 book ''On the Origin of Species'', [[Biography:Charles Darwin|Charles Darwin]] discussed the then-inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through [[Biology:Natural selection|natural selection]].<ref name="OriginOfSpecies">{{cite book| title=On the Origin of Species by Natural Selection| author=Darwin, C| author-link=Charles Darwin| year=1859| pages=[http://darwin-online.org.uk/content/frameset?pageseq=324&itemID=F373&viewtype=text 302, 306–308]| publisher=Murray| location=London| isbn=978-1-60206-144-6| oclc=176630493}}</ref> The long-running puzzlement about the seemingly-sudden appearance of the Cambrian [[Earth:Fauna|fauna]] without evident precursor(s) centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period during the early Cambrian, what might have caused such rapid change, and what it would imply about the origin of animal life. Interpretation is difficult, owing to a limited supply of evidence based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.

The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698.<ref>{{cite book|last1=Liñán |first1=E.|last2=Gonzalo |first2=R|editor1-last=Rábano |editor1-first=I.|editor2-last=Gozalo|editor2-first=R.|editor3-last=García-Bellido |editor3-first=D.|title=Advances in Trilobite Research|year=2008|publisher=Instituto Geológico y Minero de España |location=Madrid |isbn=978-84-7840-759-0 |page=240 |chapter=Cryptopalaeontology: Magical descriptions of trilobites about two thousand years before scientific references}}</ref> Although their evolutionary importance was not known, on the basis of their old age, William Buckland (1784–1856) realized that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian.<ref name="Buckland1841"/> Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the [[Earth:Cambrian|Cambrian]] and [[Earth:Silurian|Silurian]] periods.<ref>{{cite journal |last1=Bell |first1=Mark |title=Fossil Focus: Trilobites |journal=Palaeontology Online |year=2013 |volume=3 |issue=5 |pages=1–9 |url=http://www.palaeontologyonline.com/articles/2013/fossil-focus-trilobites/}}</ref> By 1859, leading geologists including Roderick Murchison were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In ''On the Origin of Species'', Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, and absent other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection. He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found because of the imperfections of the fossil record.<ref name="OriginOfSpecies"/> In the sixth edition of his book, he stressed his problem further as:<ref>{{cite book |last1=Darwin |first1=Charles R. |title=The origin of Species by Means of Natural Selection |date=1876 |page=286 |edition=6}}</ref>

{{blockquote|To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer.}}

American paleontologist Charles Walcott, who studied the Burgess Shale fauna, 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 | title=Cambrian Geology and Paleontology |journal=Smithsonian Miscellaneous Collections |volume=57 |page=14 |author=Walcott, C.D. |year=1914}}</ref>

Earlier fossil evidence has since been found. The earliest claim is that the history of life on Earth goes back {{Ma|3850|million years}}:<ref>{{cite journal |title=Evidence for life on earth more than 3850 million years ago |journal=Science |author=Holland, Heinrich D |date= January 3, 1997 |doi=10.1126/science.275.5296.38 |volume=275 |pmid=11536783 |issue=5296 |pages=38–9|s2cid=22731126 }}</ref> Rocks of that age at Warrawoona, Australia, were claimed to contain fossil [[Earth:Stromatolite|stromatolite]]s, stubby pillars formed by colonies of [[Biology:Microorganism|microorganism]]s. Fossils (''[[Biology:Grypania|Grypania]]'') of more complex [[Biology:Eukaryote|eukaryotic]] cells, from which all animals, plants, and fungi are built, have been found in rocks from {{Ma|1400}}, in {{wipe|China}} and Montana. Rocks dating from {{Ma|580|543}} contain fossils of the Ediacara biota, organisms so large that they are likely multicelled, but very unlike any modern organism.<ref name="CowenHistLife" /> In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian,<ref>{{cite journal | author=Cloud, P.E. | year=1948 |title=Some problems and patterns of evolution exemplified by fossil invertebrates | journal=Evolution | volume=2 | issue=4 | pages=322–350 | doi=10.2307/2405523 | pmid=18122310 | jstor=2405523 }}</ref> but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late [[Earth:Cambrian|Cambrian]] arose.<ref name="CowenHistLife" />

[[File:20191108 Opabinia regalis.png|thumb|right|''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, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals.<ref name="Whittington1979Lawn">Whittington, H. B. (1979). Early arthropods, their appendages and relationships. In M. R. House (Ed.), The origin of major invertebrate groups (pp. 253–268). The Systematics Association Special Volume, 12. London: Academic Press.</ref><ref name="Whittington1985BurgessShale">{{cite book | title=The Burgess Shale | author=Whittington, H.B. | author2=Geological Survey of Canada | year=1985 | publisher=Yale University Press | isbn=978-0-660-11901-4 | oclc=15630217 }}</ref> The most common organism, ''[[Biology:Marrella|Marrella]]'', was clearly an [[Biology:Arthropod|arthropod]], but not a member of any known arthropod [[Biology:Class|class]]. Organisms such as the five-eyed ''[[Biology:Opabinia|Opabinia]]'' and spiny slug-like ''[[Biology:Wiwaxia|Wiwaxia]]'' were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today. [[Biography:Stephen Jay Gould|Stephen Jay Gould]]'s popular 1989 account of this work, ''Wonderful Life'',<ref name="WonderfulLife">{{cite book| title=Wonderful Life: The Burgess Shale and the Nature of History| author=Gould, S.J.| author-link=Stephen Jay Gould| year=1989| publisher=W. W. Norton & Company| isbn=978-0-393-02705-1| oclc=185746546| bibcode=1989wlbs.book.....G| url=https://archive.org/details/wonderfullifebur00goul}}</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 phyla had appeared almost simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of [[Biology:Punctuated equilibrium|punctuated equilibrium]], which [[Biography:Niles Eldredge|Eldredge]] and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.<ref name="Bengtson2004EarlySkeletalFossils">{{Cite encyclopedia | chapter=Early skeletal fossils
| author = Bengtson, S. |editor1=Lipps, J.H. |editor2=Waggoner, B.M. | title=Neoproterozoic-Cambrian Biological Revolutions
| year = 2004
| encyclopedia = The Paleontological Society Papers
| volume = 10
| pages = 67–78
| chapter-url = http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf}}</ref>

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" /><ref name="AwramikStromatoliteDiversityMetazoanAppearance" /><ref name="FedonkinWaggoner1997KimberellaMollusc"/>

===Dating the Cambrian===
[[Physics:Radiometric dating|Radiometric]] dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only a few regions.

Relative dating (''A'' was before ''B'') is often assumed 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 [[Earth:Continent|continent]]s.<ref name="DatingProblems">e.g. {{Cite journal| last1 = Gehling| first1 = 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| doi = 10.1017/S001675680100509X| bibcode = 2001GeoM..138..213G| s2cid = 131211543}}</ref>

Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available. In 2004, the start of the Cambrian was dated to 542 [[Year#SI prefix multipliers|Ma]].<ref name="USGS_2007">{{cite web | url=https://pubs.usgs.gov/fs/2007/3015/fs2007-3015.pdf | title=Divisions of Geologic Time— Major Chronostratigraphic and Geochronologic Units | publisher=USGS | work=USGS Fact Sheet 2007–3015 | date=March 2007 | access-date=22 April 2022 | author=U.S. Geological Survey Geologic Names Committee}}</ref> In 2012, it was revised to 541 Ma<ref name="Gradstein_et-al_2012">{{cite book | title=The Geologic Timescale 2012  | publisher=Elsevier |editor=Gradstein, F.M. |editor2=Ogg, J.G. |editor3=Schmitz, M.D. |editor4=Ogg, G.M. | year=2012 | isbn=978-0-44-459390-0}}</ref> then in 2022 it was changed again to 538.8 Ma.<ref name="StratChart 2022" />

Some theory suggest Cambrian explosion occurred during the last stages of [[Earth:Gondwana|Gondwana]]n assembly, which is formed following [[Earth:Rodinia|Rodinia]] splitting, overlapped with the opening of the [[Earth:Iapetus Ocean|Iapetus Ocean]] between [[Earth:Laurentia|Laurentia]] and western Gondwana.<ref>{{Cite journal |last1=Miashita |first1=Y. |last2=Yamamoto |first2=T. |date=1996-08-01 |title=Gondwanaland: Its formation, evolution and dispersion |url=https://www.sciencedirect.com/science/article/pii/S0899536297868820 |journal=Journal of African Earth Sciences |language=en |volume=23 |issue=2 |pages=XIX |doi=10.1016/S0899-5362(97)86882-0 |bibcode=1996JAfES..23D..19M |issn=1464-343X}}</ref><ref>{{Cite journal |last1=Meert |first1=Joseph G. |last2=Van Der Voo |first2=Rob |date=1997-05-01 |title=The assembly of Gondwana 800-550 Ma |url=https://www.sciencedirect.com/science/article/pii/S0264370796000464 |journal=Journal of Geodynamics |language=en |volume=23 |issue=3 |pages=223–235 |doi=10.1016/S0264-3707(96)00046-4 |bibcode=1997JGeo...23..223M |issn=0264-3707}}</ref> The largest Cambrian faunal province is located around Gondwana, which extended from the low northern latitudes to the high southern latitudes, just short of the South Pole. By the middle and later parts of the Cambrian, continued rifting had sent the paleocontinents of Laurentia, [[Earth:Baltica|Baltica]] and Siberia on their separate ways.<ref>{{Cite web |title=Cambrian Period - The Cambrian environment {{!}} Britannica |url=https://www.britannica.com/science/Cambrian-Period/The-Cambrian-environment |access-date=2023-03-12 |website=www.britannica.com |language=en}}</ref>

===Body fossils===
Fossils of organisms' bodies are usually the most informative type of evidence. Fossilization is a rare event, and most fossils are destroyed by [[Earth:Erosion|erosion]] or [[Earth:Metamorphism|metamorphism]] before they can be observed. Hence, the fossil record is very incomplete, increasingly so as earlier times are considered. Despite this, they are often adequate to illustrate the broader patterns of life's history.<ref name="BentonQualityFossilRecord">{{cite journal |author=Benton MJ |author2=Wills MA |author3=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|bibcode = 2000Natur.403..534B |s2cid=4407172 |url=http://doc.rero.ch/record/13615/files/PAL_E635.pdf }}

: Non-technical [http://palaeo.gly.bris.ac.uk/cladestrat/news.html summary] {{Webarchive|url=https://web.archive.org/web/20070809045303/http://palaeo.gly.bris.ac.uk/cladestrat/news.html |date=2007-08-09 }}</ref> Also, biases exist 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 | pmid=21680421| doi-access = free }}</ref> Further, only the parts of organisms that were already [[Biology:Mineralization|mineralised]] are usually preserved, such as the shells of [[Biology:Mollusca|molluscs]]. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although 30-plus phyla of living animals are known, two-thirds have never been found as fossils.<ref name="CowenHistLife">{{cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science | isbn=978-1-4051-1756-2| year=2002 }}</ref>

[[File:Marrella (fossil).png|thumb| This ''Marrella'' specimen illustrates how clear and detailed the fossils from the Burgess Shale Lagerstätte actually are as well as the oldest evidence for liquid blood in an animal.]]
The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow [[Biology:Paleontology|paleontologists]] to examine the internal anatomy of animals, which in other sediments are only represented by shells, spines, claws, etc.&nbsp;– if they are preserved at all. The most significant Cambrian lagerstätten are the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, {{wipe|China}}) and Sirius Passet ([[Place:Greenland|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 [[Earth:Burgess Shale|Burgess Shale]] (British Columbia, {{wipe|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
| jstor = 987289
|trans-title=2017-01-11
}}</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="Butterfield2001"/> In addition, the known Cambrian lagerstätten are rare and difficult to date, while 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&nbsp;– this is known as the Signor–Lipps effect.<ref name="Signor1982">{{cite book |last1=Signor |first1=P.W. |last2=Lipps|first2=J.H. |chapter=Sampling bias, gradual extinction patterns and catastrophes in the fossil record |editor-last=Silver|editor-first=L.T.|editor2-last=Schulz|editor2-first=P.H. |title=Geological implications of impacts of large asteroids and comets on the earth |id=A 84–25651 10–42|pages=291–296 |publisher=Geological Society of America |year=1982 |chapter-url=https://books.google.com/books?id=efz87W88HawC&pg=291 |isbn=978-0-8137-2190-3}}</ref>

In 2019, a "stunning" find of lagerstätten, known as the [[Earth:Qingjiang biota|Qingjiang biota]], was reported from the Danshui river in Hubei province, {{wipe|China}}. More than 20,000 fossil specimens were collected, including many soft bodied animals such as jellyfish, sea anemones and worms, as well as sponges, arthropods and algae. In some specimens the internal body structures were sufficiently preserved that soft tissues, including muscles, gills, mouths, guts and eyes, can be seen. The remains were dated to around 518 Mya and around half of the species identified at the time of reporting were previously unknown.<ref>{{cite news |title=Huge fossil discovery made in China's Hubei province |url=https://www.bbc.co.uk/news/world-asia-china-47667880 |work=BBC News |access-date=24 March 2019}}</ref><ref>{{cite news |title='Mindblowing' haul of fossils over 500m years old unearthed in China  |url=https://www.theguardian.com/science/2019/mar/21/mindblowing-haul-of-fossils-over-500m-years-old-unearthed-in-china |work=The Guardian |access-date=24 March 2019}}</ref><ref>{{cite journal | last1=Fu | first1=Dongjing | last2=Tong | first2=Guanghui | last3=Dai| first3=Tao M. | last4=Liu | first4=Wei | last5=Yang | first5=Yuning | last6=Zhang | first6=Yuan | last7=Cui | first7=Linhao | last8=Li | first8=Lyoyang | last9=Yun | first9=Hao | last10=Wu | first10=Yu | last11=Sun | first11=Ao | last12=Liu | first12=Cong | last13=Pei | first13=Wenrui | last14=Gaines | first14=Robert R. | last15=Zhang | first15=Xingliang | year=2019 | title= The Qingjiang biota—A Burgess Shale–type fossil Lagerstätte from the early Cambrian of South China | journal=Science | volume=363 | issue=6433| pages=1338–1342 | pmid=30898931 |doi=10.1126/science.aau8800| bibcode=2019Sci...363.1338F | s2cid=85448914 | doi-access=free }}</ref>

===Trace fossils===
[[File:CambrianRusophycus.jpg|thumb|''Rusophycus'' and other trace fossils from the [[Earth:Gog Group|Gog Group]], Middle Cambrian, Lake Louise, Alberta, Canada]]
[[Biology:Trace fossil|Trace fossil]]s consist mainly of tracks and burrows, but also include coprolites (fossil [[Biology:Feces|feces]]) and marks left by feeding.<ref name="UCMPWhatIsPaleo">{{cite web | url=http://www.ucmp.berkeley.edu/faq.php#paleo | access-date=2008-09-18 | title=What is paleontology? | publisher=University of California Museum of Paleontology | archive-url=https://web.archive.org/web/20080916013642/http://www.ucmp.berkeley.edu/faq.php#paleo | archive-date=2008-09-16 }}</ref><ref name="FedonkinGehlingEtAl2007RiseOfAnimals" /> Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts, and 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
| doi=10.1007/BF00251073 | bibcode=1994GeoRu..83..752S| s2cid = 129504434 }}</ref> While exact assignment of trace fossils to their makers is generally impossible, traces may, for example, provide the earliest physical evidence of the appearance of moderately complex animals (comparable to [[Biology:Earthworm|earthworm]]s).<ref name="FedonkinGehlingEtAl2007RiseOfAnimals">{{cite book
| author1=Fedonkin, M.A. |author2=Gehling, J.G. |author3=Grey, K. |author4=Narbonne, G.M. |author5=Vickers-Rich, P. | title=The Rise of Animals: Evolution and Diversification of the Kingdom Animalia
| publisher=JHU Press | year=2007 | isbn=978-0-8018-8679-9 | pages=213–216
| url=https://books.google.com/books?id=OFKG6SmPNuUC&q=trace+fossil+complex+animal&pg=PA213
| access-date=2008-11-14
}}</ref>

===Geochemical observations===
{{main|Chemistry:Early Cambrian geochemical fluctuations}}
Several [[Chemistry: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.
| author2=Grotzinger, J.P.|author3=Schroder, S.|author4=Bowring, S.A.|author5=Ramezani, J.|author6=Martin, M.W.|author7=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
| issn = 0091-7613
| bibcode=2003Geo....31..431A
}}</ref> or with a massive warming resulting from the release of [[Chemistry:Methane clathrate|methane ice]].<ref name="Marshall2006Explaining">{{cite journal |author=Marshall, C. R. |year=2006 |title=Explaining the Cambrian "Explosion" of Animals |journal=[[Earth:Annual Review of Earth and Planetary Sciences|Annual Review of Earth and Planetary Sciences]] |type=abstract |volume=34 |pages=355–384 |bibcode=2006AREPS..34..355M |doi=10.1146/annurev.earth.33.031504.103001 |s2cid=85623607}}</ref>
Such changes may reflect a cause of the Cambrian explosion, although they may also have resulted from an increased level of biological activity&nbsp;– 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===
[[Biology:Cladistics|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 related to each other than either is to A. Characteristics that are compared may be [[Biology:Anatomy|anatomical]], such as the presence of a notochord, or molecular, by comparing sequences of [[Biology:DNA|DNA]] or [[Biology:Protein|protein]]. The result of a successful analysis is a hierarchy of [[Biology:Clade|clade]]s&nbsp;– groups whose members are believed to share a common ancestor. The cladistic technique is sometimes problematic, as some features, such as wings or [[Biology:Evolution of the eye|camera eyes]], evolved more than once, [[Biology:Convergent evolution|convergently]]&nbsp;– 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&nbsp;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&nbsp;million years ago.

It is also possible to estimate how long ago two living clades diverged&nbsp;– i.e. about how long ago their last common ancestor must have lived &nbsp;– by assuming that DNA [[Biology:Mutation|mutation]]s accumulate at a constant rate. These "[[Biology:Molecular clock|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>{{cite journal
| author1=Hug, L.A. |author2=Roger, A.J.
| title=The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses
| journal=Molecular Biology and Evolution |date=August 2007 | volume=24 | issue=8 | pages=889–1897
| doi=10.1093/molbev/msm115
| pmid=17556757
| issn=0737-4038
| format=Free full text
| doi-access=free
}}</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 |author1=Peterson, Kevin J. |author2=Butterfield, N.J.
| journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 27
| pages = 9547–9552 | year = 2005
| title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record
| pmc = 1172262 | bibcode=2005PNAS..102.9547P
| doi-access = free }}</ref> However, the clocks can give an indication of branching rate, and when combined with the constraints of the fossil record, recent clocks suggest a sustained period of diversification through the Ediacaran and Cambrian.<ref>{{cite journal | author = Peterson, Kevin J.|date=April 2008| doi = 10.1098/rstb.2007.2233 | title = The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records | journal = Philosophical Transactions of the Royal Society B: Biological Sciences | volume = 363 | pages = 1435–1443 | pmid = 18192191  | issue = 1496 | issn = 0962-8436 | last2 = Cotton | first2 = JA | last3 = Gehling | first3 = JG | last4 = Pisani | first4 = D | pmc = 2614224 }}</ref>

==Explanation of key scientific terms==
[[File:Crown n Stem Groups.svg|thumb|upright=1.2|[[Stem group]]s<ref name="CraskeJefferies1989" />{{div col}}{{unbulleted list
| {{legend2|text={{resize|120%|'''—'''}}||{{nowrap|{{=}} Lines of descent}}}}
| {{legend2|black|border=1px solid gray|{{=}} Basal node}}
| {{legend2|white|border=1px solid gray|{{=}} Crown node}}
| {{legend2|#8080ff|border=1px solid gray|{{=}} Total group}}
| {{legend2|#faada7|border=1px solid gray|{{=}} Crown group}}
| {{legend2|#fdefa4|border=1px solid gray|{{=}} Stem group}}
}}{{div col end}}]]

===Phylum===
A [[Biology:Phylum|phylum]] is the highest level in the [[Linnaean taxonomy|Linnaean system for classifying organisms]]. 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 | isbn = 978-0-226-84548-7 | page = 7
}}"<cite>Classifications of organisms in hierarchical systems were in use by the 17th and 18th centuries. Usually, organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their 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
| isbn = 978-0-7432-5733-6
| pages = 1–4
}}"<cite>Evolutionary biologists often make sense of the conflicting diversity of form&nbsp;– not always does 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, [[Biology:Spider|spider]]s and [[Biology:Barnacle|barnacle]]s both belong to the phylum Arthropoda, but [[Biology:Earthworm|earthworm]]s and tapeworms, although similar in shape, belong to different phyla.  As chemical and genetic testing becomes more accurate, previously hypothesised phyla are often entirely reworked.

A phylum is not a fundamental division of nature, such as the difference between [[Physics:Electron|electron]]s and [[Physics:Proton|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">{{Cite book | author=Jefferies, R.P.S. | year=1979 | title=The origin of chordates&nbsp;– a methodological essay | editor=House, M.R. | series=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 | volume=43 | issue=1 | pages=157–165 | doi=10.1093/icb/43.1.157 | year=2003 | pmid=21680420| doi-access=free }}</ref>

===Stem group===
The concept of [[Biology:Stem group|stem group]]s was introduced to cover evolutionary "aunts" and "cousins" of living groups, and have been hypothesized based on this scientific theory. A [[Biology:Crown group|crown group]] is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group is a set of offshoots from the lineage at a point earlier than the last common ancestor of the crown group; it is a relative concept, for example [[Biology:Tardigrade|tardigrade]]s are living animals that form a crown group in their own right, but Budd (1996) regarded them as also being a stem group relative to the arthropods.<ref name="CraskeJefferies1989">{{cite journal | author1=Craske, A.J. | author2=Jefferies, R.P.S. | year=1989 | title=A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion | journal=Palaeontology | volume=32 | pages=69–99 | url=http://palaeontology.palass-pubs.org/pdf/Vol%2032/Pages%2069-99.pdf | access-date=2010-12-25 | archive-url=https://web.archive.org/web/20110824010939/http://palaeontology.palass-pubs.org/pdf/Vol%2032/Pages%2069-99.pdf | archive-date=2011-08-24 }}</ref><ref name="Budd1996">{{cite journal
| author=Budd, G.E. | year=1996
| title=The morphology of ''Opabinia regalis'' and the reconstruction of the arthropod stem-group
| journal=Lethaia | volume=29 | issue=1 | pages=1–14 | doi=10.1111/j.1502-3931.1996.tb01831.x
| bibcode=1996Letha..29....1B
}}</ref>
<!--
Groups that cannot easily be placed in an existing phylum are considered to be [[Biology:Stem group|stem group]]s. A stem group annelid, for instance, is any group that 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.
-->
{{clear}}
{{Annotated image | caption={{{caption|{{{1|A coelomate animal is basically a set of concentric tubes, with a gap between the gut and the outer tubes.}}}}}} | float={{{float|right}}} | image=Coelomate.svg | width=243| height=254 | image-width = 250 | image-left=0 | image-top=0 | annot-font-size=15
| annotations =
{{Annotation |30|5|[[Biology:Ectoderm|Skin<br />(ectoderm)]]}}
{{Annotation |10|60|[[Biology:Mesoderm|Muscle<br />(mesoderm)]]}}
{{Annotation |10|110|[[Biology:Coelom|Coelom]]}}
{{Annotation |10|150|[[Biology:Organ (anatomy)|Internal<br />organ]]}}
{{Annotation |10|200|Membrane<br />(peritoneum)}}
{{Annotation |160|210|[[Biology:Endoderm|Gut<br />(endoderm)]]}}
}}

===Triploblastic===
The term ''Triploblastic'' means consisting of three layers, which are formed in the [[Biology:Embryo|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&nbsp;– the best-known exceptions are [[Biology:Porifera|Porifera]] (sponges) and [[Biology:Cnidaria|Cnidaria]] (jellyfish, sea anemones, etc.).

===Bilaterian===
The [[Biology:Bilateria|bilateria]]ns are animals that have right and left sides at some point in their life histories. 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 [[Biology:Echinoderm|echinoderm]]s (sea stars, [[Biology:Sea urchin|sea urchin]]s, [[Biology:Holothuroidea|sea cucumbers]], 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 | title=Paired gill slits in a fossil with a calcite skeleton | journal=Nature | issue=6891 | pages=841–844 |date=June 2002 | doi=10.1038/nature00805 |author1=Dominguez, P. |author2=Jacobson, A.G. |author3=Jefferies, R.P.S. | volume=417 | pmid=12075349 | issn=0028-0836 |bibcode = 2002Natur.417..841D | s2cid=4388653 }}</ref> Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic (but the common Bilateria-Cnidaria ancestor's [[Biology:Planula|planula]] larva is suspected to be bilaterally symmetric).

===Coelomate===
The term ''Coelomate'' means having a body cavity (coelom) containing the internal organs. Most of the phyla featured in the debate about the Cambrian explosion{{Clarify|date=January 2019}} are coelomates: arthropods, [[Biology:Annelid|annelid]] worms, molluscs, echinoderms, and [[Biology:Chordate|chordate]]s&nbsp;– the noncoelomate priapulids are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have a coelom&nbsp;– for example [[Biology:Flatworm|flatworm]]s, whose organs are surrounded by [[Biology:Parenchyma|unspecialized tissues]].

==Precambrian life==
=== Evidence of animals around 1 billion years ago ===
{{further|Biology:Acritarch|Earth:Stromatolite}}
[[File:CambrianStromatolites.jpg|right|thumb|[[Earth:Stromatolite|Stromatolite]]s (Pika Formation, Middle Cambrian) near Helen Lake, Banff National Park, Canada]]
[[File:Stromatolites in Sharkbay.jpg|right|thumb|Modern stromatolites in Hamelin Pool Marine Nature Reserve, 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. Stromatolites, stubby pillars built by colonies of [[Biology:Microorganism|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
 | issue= 5295
 | pages = 1993–1997
 | date = 20 December 1996
 | doi = 10.1126/science.274.5295.1993f
 | bibcode=1996Sci...274.1993M
| doi-access = free
 }}
</ref><ref name="AwramikStromatoliteDiversityMetazoanAppearance">
{{cite journal
 | author = Awramik, S.M.
 | title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
 | journal = Science
 | volume = 174
 | pages = 825–827
 | date = 19 November 1971
 | doi=10.1126/science.174.4011.825
 | type = abstract
 | pmid = 17759393
 | issue = 4011
 | bibcode = 1971Sci...174..825A
| s2cid = 2302113
 }}
</ref><ref name="Bengtson2002OriginsOfPredation">
{{Cite book
 | author=Bengtson, S.
 | year=2002
 | contribution=Origins and early evolution of predation
 | title=The fossil record of predation. The Paleontological Society Papers 8
 | editor=Kowalewski, M. |editor2=Kelley, P.H.
 | pages=289–317
 | publisher=The Paleontological Society
 | url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
 | format = Free full text
 | access-date=2007-12-01
}}
</ref>

Precambrian marine diversity was dominated by small fossils known as [[Biology:Acritarch|acritarch]]s. This term describes almost any small organic walled fossil – from the egg cases of small [[Biology:Animal|metazoans]] to resting cysts of many different kinds of [[Biology:Chlorophyta|green algae]]. After appearing around {{Ma|2000}}, acritarchs 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 [[Earth:Neoproterozoic|Neoproterozoic]] era also show signs of antipredator defenses.<ref name="Bengtson2002OriginsOfPredation" /> 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 = 1–21
 | url = http://paleobiol.geoscienceworld.org/cgi/content/extract/34/1/1
 | format = extract
 | doi = 10.1666/07026.1
 | issue=1
| bibcode = 2008Pbio...34....1S
 | s2cid = 83713101
 }}
</ref>
In general, the fossil record shows a very slow appearance of these lifeforms in the Precambrian, with many cyanobacterial species making up much of the underlying sediment.<ref name="Butterfield2007" />[[File:Ediacaran trace fossil.jpg|thumb|An Ediacaran trace fossil, made when an organism burrowed below a [[Biology:Microbial mat|microbial mat]].]]
===Ediacaran organisms===
[[File:DickinsoniaCostata.jpg|thumb|right|''[[Biology:Dickinsonia|Dickinsonia costata]]'', an Ediacaran organism of unknown affinity, with a quilted appearance]]
{{main|Earth:Ediacaran biota|Biology:Cloudinidae|Biology:Kimberella|Biology:Spriggina}}
At the start of the Ediacaran period, much of the [[Biology:Acritarch|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
| author=Shen, B. |author2=Dong, L. |author3=Xiao, S. |author4=Kowalewski, M.
| journal=Science | type=abstract | pmid=18174439
| bibcode = 2008Sci...319...81S | s2cid=206509488 }}</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 | volume = 30 | pages = 203–221 | title = Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution | journal = Paleobiology | issn = 0094-8373 | issue = 2| s2cid = 129376371 }}</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 progressed.<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/ | format=free full text | doi-access=free }}</ref>

Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses&nbsp;– one paleontologist proposed that the strangest organisms should be classified as a separate [[Biology:Kingdom|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
| access-date= 2007-06-21
| type = abstract
| bibcode = 1992JGSoc.149..607S
| s2cid = 128681462
}}</ref>
[[File:Kimberella blue.jpg|thumb|right|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,{{Clarify|date=January 2019}} having been interpreted as early molluscs (''[[Biology:Kimberella|Kimberella]]''),<ref name="FedonkinWaggoner1997KimberellaMollusc">{{cite journal
| last1 = Fedonkin | first1 = M. A.
| last2 = Waggoner | first2 = B. M.
| title =The late Precambrian fossil ''Kimberella'' is a mollusc-like bilaterian organism
| journal =Nature
| volume =388
| pages =868–871
| date=November 1997
| doi = 10.1038/42242
| bibcode = 1997Natur.388..868F
| type =abstract
| issue = 6645
| s2cid = 4395089
| issn =0372-9311
| doi-access = free
}}</ref><ref name="Martin2000">{{cite journal
| author = Martin, M.W.
| author2=Grazhdankin, D.V. |author3=Bowring, S.A. |author4=Evans, D.A.D. |author5=Fedonkin, M.A. |author6= Kirschvink, J.L.
| 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 = 841–845
| doi = 10.1126/science.288.5467.841
| type = abstract
| pmid = 10797002
| bibcode = 2000Sci...288..841M }}</ref> echinoderms (''[[Biology:Arkarua|Arkarua]]'');<ref>{{cite journal |author1=Mooi, R.  |author2=Bruno, D. | title=Evolution within a bizarre phylum: Homologies of the first echinoderms | journal=American Zoologist | volume=38 | pages=965–974 | year=1999 | issue=6 | doi=10.1093/icb/38.6.965| doi-access=free }}</ref> and arthropods (''[[Biology:Spriggina|Spriggina]]'',<ref>{{cite journal | author =McMenamin, M.A.S | title =''Spriggina'' is a trilobitoid ecdysozoan | journal =Abstracts with Programs | volume =35 | issue =6 | page =105 | year =2003 | url =http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm | type =abstract | access-date =2007-08-21 | archive-date =2008-08-30 | archive-url =https://web.archive.org/web/20080830080220/http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm | url-status =dead }}</ref> ''[[Biology:Parvancorina|Parvancorina]]'',<ref>{{cite journal| first1 = J. P.| last2 = Gon | first2 = S. M.| last3 = Gehling | first3 = J. G.| last4 = Babcock | first4 = L. E.| last5 = Zhao | first5 = Y. L.| last6 = Zhang | first6 = X. L.| last7 = Hu | first7 = S. X.| last8 = Yuan | first8 = J. L.| last9 = Yu | first9 = M. Y.| last10 = Peng | first10 = J. | title = A ''Parvancorina''-like arthropod from the Cambrian of South China | journal = Historical Biology | volume = 18 | issue = 1| last1 = Lin | pages = 33–45 | year = 2006 | doi = 10.1080/08912960500508689| bibcode = 2006HBio...18...33L | s2cid = 85821717 }}</ref> ''[[Biology:Yilingia|Yilingia]]''). Still, debate exists about the classification of these specimens, mainly because the diagnostic features that 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.
| date=December 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
| pmid = 17120226
| s2cid=29130876
| issn = 0265-9247
}}</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.{{citation needed|date=January 2019}} If some were early members of the animal phyla seen today, the "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40M years is "soon" by evolutionary and geological standards).

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}}.{{efn|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
| pages=80–83
| year=1998
| doi = 10.1126/science.282.5386.80
| author=Seilacher, A. |author2=Bose, P.K. |author3=Pflüger, F.
| type=abstract
| pmid=9756480
| issue=5386
| bibcode = 1998Sci...282...80S }}
</ref> have since been recognised as nonbiogenic.<ref name="Budd2000" /><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| doi=10.1093/icb/43.1.219| year =2003| type=abstract| pmid =21680425| doi-access =free }}</ref>}} They were probably made by organisms resembling [[Biology:Earthworm|earthworm]]s 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>{{Cite book |author=Fedonkin, M.A. |title=Origin and early evolution of the Metazoa |publisher=Springer |year=1992 |isbn=978-0-306-44067-0 |editor=Lipps, J. |location=New York |pages=87–129 |chapter=Vendian faunas and the early evolution of Metazoa |oclc=231467647 |access-date=2007-03-08 |editor2=Signor, P. W. |chapter-url=https://books.google.com/books?id=gUQMKiJOj64C&pg=PP1}}</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|author=Dzik, J|chapter=The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|pages=405–414|doi=10.1144/SP286.30}}</ref>

==Cambrian life==
=== Trace fossils ===
Trace fossils (burrows, etc.) are a reliable indicator of what life was around, and indicate a diversification of life around the start of the Cambrian, with the freshwater realm colonized by animals almost as quickly as the oceans.<ref name="Kennedy2011">{{Cite journal | last1 = Kennedy | first1 = M. J. | last2 = Droser | first2 = M. L. | title = Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation | doi = 10.1130/G32002.1 | journal = Geology | volume = 39 | issue = 6 | pages = 583–586 | year = 2011 |bibcode = 2011Geo....39..583K }}</ref>

===Small shelly fauna===
{{Main|Biology:Small shelly fauna}}
Fossils known as "[[Biology:Small shelly fauna|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). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals), and small shells very like those of [[Biology:Brachiopod|brachiopod]]s and snail-like molluscs&nbsp;– but all tiny, mostly 1 to 2&nbsp;mm long.<ref name="Matthews1975">{{cite journal
| author = Matthews, S.C.
| author2=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 the Geological Society
| volume = 131
| issue = 3
| pages = 289–303
| doi = 10.1144/gsjgs.131.3.0289
| bibcode = 1975JGSoc.131..289M
| s2cid=140660306
}}</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 lagerstätten: a period of time otherwise lacking in fossils. Hence, they supplement the conventional fossil record and allow the fossil ranges of many groups to be extended.

=== Cnidarians ===
The first cnidarian larvae, represented by the genus ''Eolarva'', appeared in the Cambrian, although the identity of ''Eolarva'' as such is controversial. If it does represent a cnidarian larva, ''Eolarva'' would represent the first fossil evidence of indirect development in metazoans in the earliest Cambrian.<ref>{{Cite journal |last1=Zhang |first1=Huaqiao |last2=Dong |first2=Xi-Ping |date=1 November 2015 |title=The oldest known larva and its implications for the plesiomorphy of metazoan development |url=https://www.sciencedirect.com/science/article/pii/S2095927316302730 |journal=Science Bulletin |volume=60 |issue=22 |pages=1947–1953 |doi=10.1007/s11434-015-0886-9 |bibcode=2015SciBu..60.1947Z |s2cid=85853812 |issn=2095-9273 |access-date=10 December 2023}}</ref>

Medusozoans developed complex life cycles with a medusa stage during the Cambrian explosion, as evidenced by the discovery of ''[[Biology:Burgessomedusa|Burgessomedusa phasmiformis]].''<ref>{{Cite journal |last1=Moon |first1=Justin |last2=Caron |first2=Jean-Bernard |last3=Moysiuk |first3=Joseph |date=9 August 2023 |title=A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=290 |issue=2004 |doi=10.1098/rspb.2022.2490 |issn=0962-8452 |pmc=10394413 |pmid=37528711 |pmc-embargo-date=August 9, 2024 }}</ref>

=== Trilobites ===
[[File:Olenoides serratus oblique with antennas.jpg|thumb|A fossilized trilobite, an ancient type of arthropod: This specimen, from the Burgess Shale, preserves "soft parts"&nbsp;– the antennae and legs. ]]
The earliest [[Biology:Trilobite|trilobite]] fossils are about 530 million years old, but the class was already quite diverse and [[Biology:Cosmopolitan distribution|cosmopolitan]], suggesting 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 = March 1, 1999 | url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176
| type =abstract
| page =176
| doi =10.1017/S0022336000027700
| bibcode =1999JPal...73..176L
| s2cid =88588171
}}</ref>
The fossil record of trilobites began with the appearance of trilobites with mineral exoskeletons – not from the time of their origin.

===Crustaceans===
{{Details|Orsten}}
Crustaceans, one of the four great modern groups of arthropods, are very rare throughout the Cambrian. Convincing [[Biology:Crustacean|crustacean]]s 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
| doi = 10.1038/nature06724
| journal = Nature
| date=April 2008
| title = Sophisticated particle-feeding in a large Early Cambrian crustacean
| volume = 452
| issue = 7189
| pmid = 18337723
| issn = 0028-0836
| author1 = Harvey, T.H
| author2 = Butterfield, N.J
| bibcode=2008Natur.452..868H
| pages = 868–71
| s2cid = 4373816
}}</ref> The Cambrian record of crown-group crustaceans comes from microfossils. The Swedish Orsten horizons contain later Cambrian crustaceans, but only organisms smaller than 2&nbsp;mm are preserved. This restricts the data set to juveniles and miniaturised adults.

A more informative data source is the organic microfossils of the [[Earth:Mount Cap formation|Mount Cap formation]], Mackenzie Mountains, 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 [[Chemistry:Hydrofluoric acid|hydrofluoric acid]]. The diversity of this assemblage is similar to that of modern crustacean faunas. 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 (about 30&nbsp;cm)<ref>{{PalAss2007|title=The ecology and phylogeny of Cambrian pancrustaceans|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 and development, and creates a number of different approaches to avoid being eaten.<ref name="Harvey2008" />

=== Echinoderms ===
The earliest generally accepted echinoderm fossils appeared in the Late Atdabanian; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.<ref name="DornbosBottjer2000Helicoplacoids">{{cite journal |author1=Dornbos, S.Q. |author2=Bottjer, D.J. |year=2000 |title=Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution |journal=Geology |volume=28 |issue=9 |pages=839–842 |bibcode=2000Geo....28..839D |doi=10.1130/0091-7613(2000)28<839:EPOTEE>2.0.CO;2 |issn=0091-7613}}</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.

=== Burrowing ===

Around the start of the Cambrian (about {{Ma|539}}), many new types of traces first appear, including well-known vertical burrows such as ''[[Biology:Diplocraterion|Diplocraterion]]'' and ''Skolithos'', and traces normally attributed to arthropods, 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 [[Biology:Exoskeleton|exoskeleton]]s, although they were not necessarily mineralised.<ref name="Jensen2003" /> Meiofaunal as well as macrofaunal bilaterians participated in this invasion of infaunal niches.<ref>{{Cite journal |last1=Parry |first1=Luke A. |last2=Boggiani |first2=Paulo C. |last3=Condon |first3=Daniel J. |last4=Garwood |first4=Russell J. |last5=Leme |first5=Juliana de M. |last6=McIlroy |first6=Duncan |last7=Brasier |first7=Martin D. |last8=Trindade |first8=Ricardo |last9=Campanha |first9=Ginaldo A. C. |last10=Pacheco |first10=Mírian L. A. F. |last11=Diniz |first11=Cleber Q. C. |last12=Liu |first12=Alexander G. |date=11 September 2017 |title=Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil |url=https://www.nature.com/articles/s41559-017-0301-9 |journal=Nature Ecology & Evolution |language=en |volume=1 |issue=10 |pages=1455–1464 |doi=10.1038/s41559-017-0301-9 |pmid=29185521 |bibcode=2017NatEE...1.1455P |s2cid=40497407 |issn=2397-334X |access-date=16 December 2023}}</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.{{Citation needed|reason=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 artifact.<ref name="Seilacher2005">{{cite journal |author=Seilacher, Adolf |author2=Luis A. Buatoisb |author3=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 |bibcode=2005PPP...227..323S |doi=10.1016/j.palaeo.2005.06.003}}</ref>

=== Skeletonisation ===
The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules.<ref name="Li1998">{{Cite journal |last1=Li |first1=C. |last2=Chen |first2=J. Y. |last3=Hua |first3=T. E. |year=1998 |title=Precambrian Sponges with Cellular Structures |journal=Science |volume=279 |issue=5352 |pages=879–882 |bibcode=1998Sci...279..879L |doi=10.1126/science.279.5352.879 |pmid=9452391}}</ref> The oldest sponge spicules are monaxon siliceous, aged around {{ma|580}}, known from the Doushantou Formation in China and from deposits of the same age in Mongolia, although the interpretation of these fossils as spicules has been challenged.<ref name="Yin2001">{{Cite journal |last1=Yin |first1=L. |last2=Xiao |first2=S. |last3=Yuan |first3=X. |year=2001 |title=New observations on spiculelike structures from Doushantuo phosphorites at Weng'an, Guizhou Province |journal=Chinese Science Bulletin |volume=46 |issue=21 |pages=1828–1832 |bibcode=2001ChSBu..46.1828Y |doi=10.1007/BF02900561 |s2cid=140612813}}</ref> In the late Ediacaran-lowest Cambrian, numerous tube dwellings of enigmatic organisms appeared. It was organic-walled tubes (e.g. ''[[Biology:Saarina|Saarina]]'') and chitinous tubes of the sabelliditids (e.g. ''Sokoloviina'', ''Sabellidites'', ''Paleolina'')<ref name="Gnilovskaya1996">{{cite journal |last=Gnilovskaya |first=M. B. |year=1996 |title=New saarinids from the Vendian of the Russian Platform |journal=Dokl. Ross. Akad. Nauk |volume=348 |pages=89–93}}</ref><ref name="Fedonkin2003">{{cite journal |author=Fedonkin, M. A. |year=2003 |title=The origin of the Metazoa in the light of the Proterozoic fossil record |url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |journal=Paleontological Research |volume=7 |issue=1 |pages=9–41 |doi=10.2517/prpsj.7.9 |s2cid=55178329 |archive-url=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |archive-date=2009-02-26 |access-date=2009-11-16}}</ref> that prospered up to the beginning of the Tommotian. The mineralized tubes of ''[[Biology:Cloudina|Cloudina]]'', ''[[Biology:Namacalathus|Namacalathus]]'', ''[[Biology:Sinotubulites|Sinotubulites]]'', and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from {{ma|549|542}}, as well as the triradially symmetrical mineralized tubes of anabaritids (e.g. ''[[Biology:Anabarites|Anabarites]]'', ''Cambrotubulus'') from uppermost Ediacaran and lower Cambrian.<ref name="Zhuravlev 2009">{{cite journal |author=Andrey Yu. Zhuravlev |display-authors=0 |date=September 2009 |title=First finds of problematic Ediacaran fossil ''Gaojiashania'' in Siberia and its origin |url=http://geolmag.geoscienceworld.org/cgi/content/abstract/146/5/775 |journal=Geological Magazine |volume=146 |issue=5 |pages=775–780 |bibcode=2009GeoM..146..775Z |doi=10.1017/S0016756809990185 |s2cid=140569611}}</ref> Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and [[Biology:Thrombolite|thrombolite]]s,<ref name="Hofmann2001">{{cite journal |author=Hofmann, H.J. |author2=Mountjoy, E.W. |year=2001 |title=''Namacalathus-Cloudina'' assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldest shelly fossils |journal=Geology |volume=29 |issue=12 |pages=1091–1094 |bibcode=2001Geo....29.1091H |doi=10.1130/0091-7613(2001)029<1091:NCAINM>2.0.CO;2 |issn=0091-7613}}</ref><ref name="Grotzinger2000">{{cite journal |author=Grotzinger, J.P. |author2=Watters, W.A. |author3=Knoll, A.H. |year=2000 |title=Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia |url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/334 |journal=Paleobiology |volume=26 |issue=3 |pages=334–359 |doi=10.1666/0094-8373(2000)026<0334:CMITSR>2.0.CO;2 |issn=0094-8373 |s2cid=52231115}}</ref> i.e. they could live in an environment adverse to the majority of animals.

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 from [[Chemistry:Calcium carbonate|calcium carbonate]]).<ref name="Grotzinger2000" /><ref name="Hua2005">{{cite journal |author=Hua, H. |author2=Chen, Z. |author3=Yuan, X. |author4=Zhang, L. |author5=Xiao, S. |year=2005 |title=Skeletogenesis and asexual reproduction in the earliest biomineralizing animal ''Cloudina'' |journal=Geology |volume=33 |issue=4 |pages=277–280 |bibcode=2005Geo....33..277H |doi=10.1130/G21198.1}}</ref><ref name="Miller2004">{{cite web |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 |access-date=2007-04-24}}</ref> Secondly, these tubes are a device to rise over a substrate and competitors for effective feeding and, to a lesser degree, they serve as armor for protection against predators and adverse conditions of environment. Some ''Cloudina'' fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells.<ref name="Hua2003">{{Cite journal |author=Hua, H. |author2=Pratt, B.R. |author3=Zhang, L.U.Y.I. |year=2003 |title=Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic |journal=PALAIOS |volume=18 |issue=4–5 |pages=454–459 |bibcode=2003Palai..18..454H |doi=10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2 |issn=0883-1351 |s2cid=131590949}}</ref> A possible "[[Biology:Evolutionary arms race|evolutionary arms race]]" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion.<ref name="Bengtson2002OriginsOfPredation" />

In the lowest Cambrian, the stromatolites were decimated. This allowed animals to begin colonization of warm-water pools with carbonate sedimentation. At first, it was anabaritids and ''[[Biology:Protohertzina|Protohertzina]]'' (the fossilized grasping spines of [[Biology:Chaetognatha|chaetognaths]]) fossils. Such mineral skeletons as shells, sclerites, thorns, and plates appeared in uppermost Nemakit-Daldynian; they were the earliest species of [[Biology:Halkieria|halkierids]], [[Biology:Helcionelloida|gastropods]], [[Biology:Hyolitha|hyoliths]] and other rare organisms. The beginning of the Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs, hyoliths, and [[Biology:Sponge|sponge]]s, along with a rich complex of skeletal elements of unknown animals, the first archaeocyathids, [[Biology:Brachiopod|brachiopod]]s, [[Biology:Tommotiid|tommotiid]]s, and others.<ref name="Steiner2007">{{Cite journal |last1=Steiner |first1=M. |last2=Li |first2=G. |last3=Qian |first3=Y. |last4=Zhu |first4=M. |last5=Erdtmann |first5=B. D. |year=2007 |title=Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=254 |issue=1–2 |pages=67–99 |bibcode=2007PPP...254...67S |doi=10.1016/j.palaeo.2007.03.046}}</ref><ref name="Rozanov2008">{{cite journal |last1=Rozanov |first1=A. Yu |last2=Khomentovsky |first2=V. V. |last3=Shabanov |first3=Yu. Ya. |last4=Karlova |first4=G. A. |last5=Varlamov |first5=A. I. |last6=Luchinina |first6=V. A. |last7=Pegel' |first7=T. V. |last8=Demidenko |first8=Yu. E. |last9=Parkhaev |first9=P. Yu. |last10=Korovnikov |first10=I. V. |last11=Skorlotova |first11=N. A. |display-authors=0 |name-list-style=vanc |year=2008 |title=To the problem of stage subdivision of the Lower Cambrian |journal=Stratigraphy and Geological Correlation |volume=16 |issue=1 |pages=1–19 |bibcode=2008SGC....16....1R |doi=10.1007/s11506-008-1001-3 |s2cid=128128572}}</ref><ref name="Khomentovsky2005">{{cite journal |author1=V. V. Khomentovsky |author2=G. A. Karlova |year=2005 |title=The Tommotian Stage Base as the Cambrian Lower Boundary in Siberia |url=http://www.maikonline.com/maik/showArticle.do?auid=VAE43XYML4 |journal=Stratigraphy and Geological Correlation |volume=13 |issue=1 |pages=21–34 |archive-url=https://web.archive.org/web/20110714022431/http://www.maikonline.com/maik/showArticle.do?auid=VAE43XYML4 |archive-date=2011-07-14 |access-date=2010-06-04}}</ref><ref name="Khomentovsky2002">{{cite journal |author1=V. V. Khomentovsky |author2=G. A. Karlova |year=2002 |title=The Boundary between Nemakit-Daldynian and Tommotian Stages (Vendian-Cambrian Systems) of Siberia |url=https://elibrary.ru/item.asp?id=11900587 |journal=Stratigraphy and Geological Correlation |volume=10 |issue=3 |pages=13–34}}</ref> Also [[Biology:Soft-bodied organism|soft-bodied]] extant phyla such as [[Biology:Ctenophora|comb jellies]], [[Biology:Scalidophora|scalidophora]]ns, [[Biology:Entoprocta|entoprocta]]ns, [[Biology:Phoronid|horseshoe worms]] and [[Biology:Lobopodia|lobopodia]]ns had armored forms.<ref>[https://www.science.org/doi/10.1126/sciadv.1500092 A vanished history of skeletonization in Cambrian comb jellies – Science Advances]</ref> This sudden increase is partially an artefact of missing strata at the Tommotian type section, and most of this fauna in fact began to diversify in a series of pulses through the Nemakit-Daldynian and into the Tommotian.<ref name="Maloof2010">{{Cite journal |last1=Maloof |first1=A. C. |last2=Porter |first2=S. M. |last3=Moore |first3=J. L. |last4=Dudas |first4=F. O. |last5=Bowring |first5=S. A. |last6=Higgins |first6=J. A. |last7=Fike |first7=D. A. |last8=Eddy |first8=M. P. |year=2010 |title=The earliest Cambrian record of animals and ocean geochemical change |journal=Geological Society of America Bulletin |volume=122 |issue=11–12 |pages=1731–1774 |bibcode=2010GSAB..122.1731M |doi=10.1130/B30346.1}}</ref>

Some animals may already have had sclerites, thorns, and plates in the Ediacaran (e.g. ''Kimberella'' had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized in [[Earth:Siliciclastic|siliciclastic]] deposits.<ref name="Ivantsov2009">{{cite journal |last=Ivantsov |first=A. Y. |year=2009 |title=A New Reconstruction of ''Kimberella'', a Problematic Vendian Metazoan |journal=Paleontological Journal |volume=43 |issue=6 |pages=601–611 |bibcode=2009PalJ...43..601I |doi=10.1134/S003103010906001X |s2cid=85676210}}</ref> Older (~750&nbsp;Ma) fossils indicate that mineralization long preceded the Cambrian, probably defending small photosynthetic algae from single-celled eukaryotic predators.<ref name="Porter2011">{{Cite journal |last1=Porter |first1=S. |year=2011 |title=The rise of predators |journal=Geology |volume=39 |issue=6 |pages=607–608 |bibcode=2011Geo....39..607P |doi=10.1130/focus062011.1 |doi-access=free}}</ref><ref name="Cohen2011">{{Cite journal |last1=Cohen |first1=P. A. |last2=Schopf |first2=J. W. |last3=Butterfield |first3=N. J. |last4=Kudryavtsev |first4=A. B. |last5=MacDonald |first5=F. A. |year=2011 |title=Phosphate biomineralization in mid-Neoproterozoic protists |journal=Geology |volume=39 |issue=6 |pages=539–542 |bibcode=2011Geo....39..539C |doi=10.1130/G31833.1}}</ref>

===Burgess Shale type faunas===
{{main|Biology:Burgess Shale-type preservation}}

The Burgess Shale and similar lagerstätten preserve the soft parts of organisms, which provide 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 |author=Butterfield, Nicholas 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 |pmid=21680421 |doi-access=free}}</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&nbsp;– see talk page|date=July 2008}}
However, the assemblages may represent a "museum": a deep-water ecosystem that is evolutionarily "behind" the rapidly diversifying fauna 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 |issue=2 |pages=424–430 |bibcode=2008JPal...82..424M |doi=10.1666/06-130.1 |s2cid=85619898}}</ref>

Because the lagerstätten provide a mode and quality of preservation that is virtually absent outside of the Cambrian, many organisms appear completely different from 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 later workers to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged from [[Biology:Stem group|lineages]] before they established the phyla known today{{Clarify|date=August 2008}}&nbsp;– 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&nbsp;– perhaps implying a genuine absence of macroscopic metazoans.<ref name="Xiao2002">{{cite journal |author1=Xiao, Shuhai |author2=Steiner, M |author3=Knoll, A. H |last4=Knoll |first4=Andrew H. |year=2002 |title=A Reassessment of the Neoproterozoic Miaohe Carbonaceous Biota in South China |journal=Journal of Paleontology |volume=76 |issue=2 |pages=345–374 |doi=10.1666/0022-3360(2002)076<0347:MCCIAT>2.0.CO;2 |issn=0022-3360}}</ref>

== Stages ==

The early Cambrian interval of diversification lasted for about the next 20<ref name="dev.biologists.org"/><ref name=Budd/>–25<ref name="The Cambrian conundrum: early diver"/><ref>{{cite journal | last1 = Kouchinsky | first1 = A. | last2 = Bengtson | first2 = S. | last3 = Runnegar | first3 = B. N. | last4 = Skovsted | first4 = C. B. | last5 = Steiner | first5 = M. | last6 = Vendrasco | first6 = M. J. | year = 2012 | title = Chronology of early Cambrian biomineralization | journal = Geological Magazine | volume = 149 | issue = 2| pages = 221–251 | doi=10.1017/s0016756811000720| bibcode = 2012GeoM..149..221K | doi-access = free }}</ref> million years, and its elevated rates of evolution had ended by the base of Cambrian Series 2, {{Ma|521}}, coincident with the first trilobites in the fossil record.<ref>{{Cite journal | doi=10.1073/pnas.1819366116 |pmid = 30782836|pmc = 6410820|title = Trilobite evolutionary rates constrain the duration of the Cambrian explosion|journal = Proceedings of the National Academy of Sciences|volume = 116|issue = 10|pages = 4394–4399|year = 2019|last1 = Paterson|first1 = John R.|last2 = Edgecombe|first2 = Gregory D.|last3 = Lee|first3 = Michael S. Y.|bibcode = 2019PNAS..116.4394P|doi-access = free}}</ref> 
Different authors define intervals of diversification during the early Cambrian different ways:

Ed Landing recognizes three stages: Stage 1, spanning the Ediacaran-Cambrian boundary, corresponds to a diversification of biomineralizing animals and of deep and complex burrows; Stage 2, corresponding to the radiation of molluscs and stem-group [[Biology:Brachiopod|Brachiopod]]s (hyoliths and [[Biology:Tommotiid|tommotiid]]s), which apparently arose in intertidal waters; and Stage 3, seeing the Atdabanian diversification of trilobites in deeper waters, but little change in the intertidal realm.<ref>{{cite journal | last1=Landing | first1=E. | last2=Kouchinsky | first2=A. V. | year=2016 | title=Correlation of the Cambrian Evolutionary Radiation: geochronology, evolutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic significance | journal=Geol. Mag. | volume=153 | issue=4 | pages=750–756 | doi=10.1017/s0016756815001089| bibcode=2016GeoM..153..750L | s2cid=132867265 }}</ref>

Graham Budd synthesises various schemes to produce a compatible view of the SSF record of the Cambrian explosion, divided slightly differently into four intervals: a "Tube world", lasting from {{Ma|550|536}}, spanning the Ediacaran-Cambrian boundary, dominated by Cloudina, Namacalathus and pseudoconodont-type elements; a "Sclerite world", seeing the rise of halkieriids, tommotiids, and hyoliths, lasting to the end of the Fortunian (c. 525 Ma); a brachiopod world, perhaps corresponding to the as yet unratified Cambrian Stage 2; and Trilobite World, kicking off in Stage 3.<ref name="Budd, G.E. 2016">{{cite journal | last1=Budd | first1=G.E. | last2=Jackson | first2=I.S.C. | year=2016 | title=Ecological innovations in the Cambrian and the origins of the crown group phyla | journal=Phil. Trans. R. Soc. B | volume=371 | issue=1685 |  page=20150287 | doi=10.1098/rstb.2015.0287| pmid=26598735 | pmc=4685591 }}</ref>

Complementary to the shelly fossil record, trace fossils can be divided into five subdivisions: "Flat world" (late Ediacaran), with traces restricted to the sediment surface; Protreozoic III (after Jensen), with increasing complexity; ''pedum'' world, initiated at the base of the Cambrian with the base of the ''T.pedum'' zone (see [[Earth:Cambrian#Dating the Cambrian|Cambrian]]); ''Rusophycus'' world, spanning {{Ma|536|521}} and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm; and ''Cruziana'' world, with an obvious correspondence to Trilobite World.
<ref name="Budd, G.E. 2016"/>

==Validity==

There is strong evidence for species of [[Biology:Cnidaria|Cnidaria]] and [[Biology:Sponge|Porifera]] existing in the [[Earth:Ediacaran|Ediacaran]]<ref>{{Cite journal |last1=Xiao |first1=Shuhai |last2=Yuan |first2=Xunlai |last3=Knoll |first3=Andrew H. |date=2000-12-05 |title=Eumetazoan fossils in terminal Proterozoic phosphorites? |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=97 |issue=25 |pages=13684–13689 |doi=10.1073/pnas.250491697 |pmc=17636 |pmid=11095754 |bibcode=2000PNAS...9713684X|doi-access=free }}</ref> and possible members of Porifera even before that during the [[Earth:Cryogenian|Cryogenian]].<ref>{{Cite journal |last1=Yin |first1=Zongjun |last2=Zhu |first2=Maoyan |last3=Davidson |first3=Eric H. |last4=Bottjer |first4=David J. |last5=Zhao |first5=Fangchen |last6=Tafforeau |first6=Paul |date=2015-03-24 |title=Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=12 |pages=E1453–E1460 |doi=10.1073/pnas.1414577112 |issn=0027-8424 |pmc=4378401 |pmid=25775601 |bibcode=2015PNAS..112E1453Y|doi-access=free }}</ref> [[Biology:Bryozoa|Bryozoans, once thought to]] not appear in the fossil record until after the Cambrian, are now known from strata of Cambrian Age 3 from Australia and South China.<ref name="Taylor2013">{{Cite journal|last1=Zhang|first1=Z.|last2=Ma|first2=J.|year=2021|title=Fossil evidence unveils an early Cambrian origin for Bryozoa|journal=Nature|volume=599|issue=7884|pages=251–255|doi=10.1038/s41586-021-04033-w|pmid=34707285|pmc=8580826|bibcode=2021Natur.599..251Z|s2cid=}}</ref>

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 [[Biology:Protostome|protostome]] (one of the two main groups of coelomates),<ref name="FedonkinWaggoner1997KimberellaMollusc"/><ref name="Martin2000"/> the protostome and [[Biology:Deuterostome|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="Butterfield2006"/><ref name="ErwinDavidson2002LastCommonBilaterianAncestor"/> Since fossils of rather modern-looking [[Biology:Cnidaria|cnidaria]]ns ([[Biology:Jellyfish|jellyfish]]-like organisms) have been found in the 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 |author1=Erwin, D.H. |author2=Davidson, E.H. | journal=Development |volume=129 | pages=3021–3032 | date= July 1, 2002 | url=http://dev.biologists.org/content/129/13/3021.full | access-date=2008-07-10 | pmid=12070079 | issue=13 |doi=10.1242/dev.129.13.3021 }}</ref>

Trace fossils<ref name="Erwin1999OriginOfBodyplans"/> and predatory borings in ''Cloudina'' shells provide further evidence of Ediacaran animals.<ref name="BengtsonZhao1992PredatorialBorings">{{cite journal | date = 17 July 1992 | title = Predatorial Borings in Late Precambrian Mineralized Exoskeletons | journal = Science | volume = 257 | issue = 5068 | doi = 10.1126/science.257.5068.367 |author1=Bengtson, S. |author2=Zhao, Y. | pmid = 17832833 | pages = 367–9 |bibcode = 1992Sci...257..367B | s2cid = 6710335 }}</ref> Some fossils from the Doushantuo formation have been interpreted as embryos and one (''[[Biology:Vernanimalcula|Vernanimalcula]]'') as a bilaterian coelomate, although these interpretations are not universally accepted.<ref name="Chen2004">
{{cite journal |author1=Chen, J. Y. |author2=Bottjer, D. J. |author3=Oliveri, P. |author4=Dornbos, S. Q. |author5=Gao, F. |author6=Ruffins, S. |author7=Chi, H. |author8=Li, C. W. |author9=Davidson, E. H. |display-authors=5 |date=2004-07-09 |title=Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian |url=https://resolver.caltech.edu/CaltechAUTHORS:20141120-070215516 |journal=Science |volume=305 |issue=5681 |pages=218–222 |bibcode=2004Sci...305..218C |doi=10.1126/science.1099213 |pmid=15178752 |s2cid=1811682|doi-access=free }}
</ref><ref name="Bengtson2004">
{{cite journal |last1=Bengtson |first1=Stefan |last2=Budd |first2=G. |year=2004 |title=Comment on ''Small bilaterian fossils from 40 to 55 million years before the Cambrian'' |journal=Science |volume=306 |issue=5700 |pages=1291a |doi=10.1126/science.1101338 |pmid=15550644 |doi-access=free}}
</ref><ref name="ChenDefendVernanimacula">{{cite journal | year=2004 | title=Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian" | doi = 10.1126/science.1102328 | author=Chen, J.Y. |author2=Oliveri, P. |author3=Davidson, E. |author4=Bottjer, D.J. | journal=Science | volume=306 | page=1291 | issue=5700| pmid=15550644 | doi-access=free }}</ref> Earlier still, predatory pressure has acted on stromatolites and acritarchs since around {{ma |1250}}.<ref name="Bengtson2002OriginsOfPredation"/>

Some say that the evolutionary change was accelerated by an [[Order of magnitude|order of magnitude]],{{efn|As defined in terms of the extinction and origination rate of species.<ref name="Butterfield2007"/>}} but 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.{{efn|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 |pmid=21680426 |doi-access=free }}</ref>}} However, it does seem that some innovations linked to the explosion&nbsp;– such as resistant armour – only evolved once in the animal lineage; this makes a lengthy Precambrian animal lineage harder to defend.<ref name=Jacobs2000>{{Cite journal | last2 = Wray | last3 = Wedeen | doi = 10.1046/j.1525-142x.2000.00077.x | year = 2000 | pages = 340–347 | last4 = Kostriken | last5 = Desalle | pmid=11256378 | last8 = Lindberg | last7 = Gates | last6 = Staton | issue = 6 | volume = 2 | first5 = R. | first4 = R. | first3 = C. J. | first2 = C. G. | last1 = Jacobs | first6 = J. L. | journal = Evolution & Development | title = Molluscan engrailed expression, serial organization, and shell evolution | first8 = D. R. | first7 = R. D. | first1 = D. K.| s2cid = 25274057 }}</ref> Further, the conventional view that all the phyla arose in the Cambrian is flawed; while the phyla may have diversified in this time period, representatives of the crown groups of many phyla do not appear until much later in the Phanerozoic.<ref name=Budd2000>{{cite journal |last1=Budd |first1=G. E. |last2=Jensen |first2=S. |year=2000 |title=A critical reappraisal of the fossil record of the bilaterian phyla |volume=75 |issue=2 |pages=253–95 |journal=Biological Reviews of the Cambridge Philosophical Society |doi=10.1111/j.1469-185X.1999.tb00046.x |pmid=10881389|s2cid=39772232 }}</ref> Further, the mineralised phyla that form the basis of the fossil record may not be representative of other phyla, since most mineralised phyla originated in a [[Earth:Benthic zone|benthic]] setting. The fossil record is consistent with a Cambrian explosion that was limited to the benthos, with pelagic phyla evolving much later.<ref name=Budd2000/>

Ecological complexity among marine animals increased in the Cambrian, as well later in the Ordovician.<ref name="Bambach2007" /> 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 | issue = 1 | pages = 57–73 | bibcode = 2007Palgy..50...57E | doi-access = free }}</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>{{Cite journal |doi=10.1666/0094-8373(2003)029<0349:MDOCAA>2.0.CO;2 |year=2003 |volume=29 |pages=349–368 |title=Morphological diversity of Carboniferous arthropods and insights on disparity patterns through the Phanerozoic |author=Stockmeyer Lofgren, Andrea |journal=Paleobiology |last2=Plotnick |first2=Roy E. |last3=Wagner |first3=Peter J. |issn=0094-8373 |issue=3|s2cid=86264787 }}</ref><ref name="Harvey2008"/> and at a high (class/phylum) level, diversity is thought by some to have risen relatively smoothly through the Cambrian, stabilizing somewhat in the Ordovician.<ref name="Erwin2011">{{Cite journal | last1 = Erwin | first1 = D. H. | doi = 10.1016/j.ydbio.2011.01.020 | title = Evolutionary uniformitarianism | journal = Developmental Biology | year = 2011 | pmid = 21276788 | volume=357 | issue=1 | pages=27–34}}</ref> This interpretation, however, glosses over the astonishing and fundamental pattern of basal [[Biology:Polytomy|polytomy]] and phylogenetic telescoping at or near the Cambrian boundary, as seen in most major animal lineages.<ref>{{cite journal |year=2011 |author1=Mounce, R. C. P. |author2=Wills, M. A. |title=Phylogenetic position of ''Diania'' challenged |journal=Nature |volume=476 |issue=E1 |doi=10.1038/nature10266 |pages=E1; discussion E3–4 |bibcode = 2011Natur.476E...1M |pmid=21833044|s2cid=4417903 |url=http://opus.bath.ac.uk/25606/1/nature10266_proof1.pdf }}</ref> Thus Harry Blackmore Whittington's questions regarding the abrupt nature of the Cambrian explosion remain, and have yet to be satisfactorily answered.<ref>{{cite journal |year=2010 |author=McMenamin, M. |title=Harry Blackmore Whittington 1916–2010 |journal=Geoscientist |volume=20 |issue=11 |pages=5 |url=http://www.geolsoc.org.uk/gsl/site/GSL/lang/en/page8379.html |access-date=2011-08-12 |archive-url=https://archive.today/20120803053420/http://www.geolsoc.org.uk/gsl/site/GSL/lang/en/page8379.html |archive-date=2012-08-03 }}</ref>

===The Cambrian explosion as survivorship bias===

Budd and Mann<ref name="POTPa">{{cite journal |last1=Budd |first1=G. E. |last2=Mann |first2=R. P. |title=History is written by the victors: the effect of the push of the past on the fossil record |journal=Evolution |date=2018 |volume=72 |issue=11 |pages=2276–229 |doi=10.1111/evo.13593 |pmid=30257040 |pmc=6282550 |url=http://eprints.whiterose.ac.uk/135239/8/Budd_et_al-2018-Evolution.pdf }}</ref> suggested that the Cambrian explosion was the result of a type of [[Survivorship bias|survivorship bias]] called the "[[Biology:Push of the past|Push of the past]]". As groups at their origin tend to go extinct, it follows that any long-lived group would have experienced an unusually rapid rate of diversification early on, creating the illusion of a general speed-up in diversification rates. However, rates of diversification could remain at background levels and still generate this sort of effect in the surviving lineages.

==Possible causes==
Despite the evidence that moderately complex animals (triploblastic [[Biology:Bilateria|bilateria]]ns) 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 both the timing and magnitude of the explosion.

===Changes in the environment===
====Increase in oxygen levels====
[[Earth:Atmosphere of Earth#Earliest atmosphere|Earth's earliest atmosphere]] contained no free [[Chemistry:Oxygen|oxygen]] (O<sub>2</sub>); the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of [[Earth:Photosynthesis|photosynthesis]]. Cyanobacteria were the first organisms to evolve the ability to photosynthesize, introducing a steady supply of oxygen into the environment.<ref>{{cite journal|last1=Schirrmeister|first1=B.E.|last2=deVos|first2=J.M|last3=Antonelli|first3=A.|last4=Bagherri|first4=H.C.|title=Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event|journal=PNAS|date=2013|volume=110|issue=5|pages=1791–1796|doi=10.1073/pnas.1209927110|bibcode=2013PNAS..110.1791S|pmid=23319632|pmc=3562814|doi-access=free}}</ref> Initially, oxygen levels did not increase substantially in the atmosphere.<ref name="ReferenceA">{{cite journal|last1=Canfield|first1=D.E.|last2=Poulton|first2=S.W.|last3=Narbonne|first3=G.M.|title=Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life|journal=Science|date=2007|volume=315|doi=10.1126/science.1135013|pmid=17158290|issue=5808|pages=92–5|bibcode=2007Sci...315...92C|s2cid=24761414|doi-access=free}}</ref> The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water. Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in its diatomic form. Oxygen levels in the atmosphere increased substantially afterward.<ref>{{cite journal|last1=Bekker|first1=A.|last2=Holland|first2=H.D.|last3=Wang|first3=P.I.|last4=Rumble, III|first4=D.|last5=Stein|first5=H.J.|last6=Hannah|first6=J.L.|last7=Coetzee|first7=L.L.|last8=Beukes|first8=H.J.|title=Dating the rise of atmospheric oxygen|journal=Nature|date=2003|volume=427|issue=6970|pages=117–120|doi=10.1038/nature02260|pmid=14712267|bibcode=2004Natur.427..117B|s2cid=4329872}}</ref> As a general trend, the [[Chemistry:Concentration|concentration]] of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.<ref name="CowenHistLife"/>

Oxygen levels seem to have a positive correlation with diversity in eukaryotes well before the Cambrian period.<ref name="ReferenceB">{{cite journal|last1=Parfrey|first1=Laura Wegener|last2=Lahr|first2=Daniel J. G.|last3=Knoll|first3=Andrew H.|last4=Katz|first4=Laura A.|author-link4=Laura A. Katz|title=Estimating the timing of early eukaryotic diversification with multigene molecular clocks|journal=Proceedings of the National Academy of Sciences of the United States of America|date=August 16, 2011|volume=108|issue=33|pages=13624&ndash;13629|doi=10.1073/pnas.1110633108|pmid=21810989|pmc=3158185|bibcode=2011PNAS..10813624P |doi-access=free}}</ref> The last common ancestor of all extant eukaryotes is thought to have lived around 1.8 billion years ago. Around 800 million years ago, there was a notable increase in the complexity and number of eukaryotes species in the fossil record.<ref name="ReferenceB"/> Before the spike in diversity, eukaryotes are thought to have lived in highly sulfuric environments. Sulfide interferes with mitochondrial function in aerobic organisms, limiting the amount of oxygen that could be used to drive metabolism. Oceanic sulfide levels decreased around 800 million years ago, which supports the importance of oxygen in eukaryotic diversity.<ref name="ReferenceB"/> The increased ventilation of the oceans by sponges, which had already evolved and diversified during the Neoproterozoic, has been proposed to have increased the availability of oxygen and powered the Cambrian's rapid diversification of multicellular life.<ref name="ChangEtAl2019">{{cite journal |last1=Chang |first1=Shan |last2=Zhang |first2=Lei |last3=Clausen |first3=Sébastien |last4=Bottjer |first4=David J. |last5=Feng |first5=Qinglai |date=1 October 2019 |title=The Ediacaran-Cambrian rise of siliceous sponges and development of modern oceanic ecosystems |journal=Precambrian Research |volume=333 |page=105438 |doi=10.1016/j.precamres.2019.105438 |bibcode=2019PreR..33305438C |s2cid=202174665 |doi-access=free }}</ref><ref name="WangXieWen2022Precambrian">{{cite journal |last1=Wang |first1=Zhanghu |last2=Xie |first2=Xiaomin |last3=Wen |first3=Zhigang |date=December 2022 |title=Formation conditions of Ediacaran–Cambrian cherts in South China: Implications for marine redox conditions and paleoecology |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926822003114 |journal=Precambrian Research |volume=383 |page=106867 |doi=10.1016/j.precamres.2022.106867 |bibcode=2022PreR..38306867W |s2cid=253872322 |access-date=20 December 2022}}</ref> Molybdenum isotopes show that increases in biodiversity were strongly correlated with expansion of oxygenated bottom waters in the Early Cambrian, lending support for oxygen as a driver of the Cambrian evolutionary radiation.<ref>{{cite journal |last1=Xi |first1=Chen |last2=Hong-Fei  |first2=Ling |last3=Vance |first3=Derek |last4=Shields-Zhou |first4=Graham A. |last5=Zhu |first5=Maoyan |last6=Poulton |first6=Simon W. |last7=Och |first7=Lawrence M. |last8=Jiang |first8=Shao-Yong |last9=Li |first9=Da |last10=Cremonese |first10=Lorenzo |last11=Archer |first11=Corey |date=18 May 2015 |title=Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals |journal=Nature Communications |volume=6 |issue=1 |page=7142 |doi=10.1038/ncomms8142 |pmid=25980960 |pmc=4479002 |bibcode=2015NatCo...6.7142C |doi-access=free}}</ref>

The 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), while 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 tens of millions of years before the Cambrian explosion.<ref name="Knoll1999">e.g. {{cite journal
| author = Knoll, A.H.
| author2=Carroll, S.B.
| s2cid=8908451
| date = 1999-06-25
| title = Early Animal Evolution: Emerging Views from Comparative Biology and Geology
| journal = Science
| volume = 284
| issue = 5423
| doi = 10.1126/science.284.5423.2129
| pmid = 10381872
| pages = 2129–37
}}</ref> Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as [[Biology:Collagen|collagen]], which is 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 of the United States of America
| volume = 65
| issue = 4
| pages = 781–788
| doi = 10.1073/pnas.65.4.781
| type = abstract
| pmid = 5266150
| pmc = 282983
| bibcode = 1970PNAS...65..781T | doi-access = free
}}</ref> or the biosynthesis of molecules for the construction of a hard exoskeleton.<ref name="CatlingEtAl2005">{{cite journal
| doi = 10.1089/ast.2005.5.415
| journal = Astrobiology
| volume = 5
| issue = 3
| pages = 415–438
| date=June 2005
| title = Why O<sub>2</sub> Is Required by Complex Life on Habitable Planets and the Concept of Planetary "Oxygenation Time"
| pmid = 15941384
| issn = 1531-1074
| author1 = Catling, D.C
| author2 = Glein, C.R
| author3 = Zahnle, K.J
| author4 = McKay, C.P
| bibcode=2005AsBio...5..415C
}}</ref> However, animals were not affected when similar oceanographic conditions occurred in the Phanerozoic; therefore, some see no forcing role of the oxygen level on evolution.<ref name=Butterfield2009>{{Cite journal
| last1 = Butterfield | first1 = N. J.
| title = Oxygen, animals and oceanic ventilation: An alternative view
| doi = 10.1111/j.1472-4669.2009.00188.x
| journal = Geobiology
| volume = 7
| issue = 1
| pages = 1–7
| year = 2009
| pmid = 19200141
| bibcode = 2009Gbio....7....1B
| s2cid = 31074331
}}</ref>

====Ozone formation====
The amount of ozone (O<sub>3</sub>) required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence around the Cambrian explosion.<ref>{{cite web|last1=Keese| first1=Bob|title=Ozone| url=http://www.albany.edu/faculty/rgk/atm101/ozone.htm|website=www.albany.edu |publisher=University at Albany| access-date=22 November 2014}}</ref> The presence of the [[Physics:Ozone layer|ozone layer]] may have enabled the development of complex life and life on land, as opposed to life being restricted to the water.

====Snowball Earth====
{{main|Earth:Snowball Earth}}
In the late [[Earth:Neoproterozoic|Neoproterozoic]] (extending into the early [[Earth:Ediacaran|Ediacaran]] period), the Earth suffered [[Earth: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
| author1=Hoffman, P.F. |author2=Kaufman, A.J. |author3=Halverson, G.P. |author4=Schrag, D.P.  |name-list-style=amp | type=abstract
| pmid=9721097
| bibcode=1998Sci...281.1342H
| s2cid=13046760
}}</ref>
However, the snowball episodes occurred a long time before the start of the Cambrian, and it is difficult 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 organisms.<ref name="Bengtson2002OriginsOfPredation"/> Massive rock erosion caused by glaciers during the "Snowball Earth" may have deposited nutrient-rich sediments into the oceans, setting the stage for the Cambrian explosion.<ref>{{cite news |last1=Hirsch |first1=David |title=New Research Strengthens Link Between Glaciers and Earth's Puzzling 'Great Unconformity' |url=https://home.dartmouth.edu/news/2022/01/study-links-glaciers-earths-great-unconformity |access-date=January 27, 2022 |publisher=Dartmouth College |date=January 26, 2022}}</ref>

====Increase in the calcium concentration of the Cambrian seawater====
Newer research suggests that volcanically active midocean ridges caused a massive and sudden surge of the calcium concentration in the oceans, making it possible for marine organisms to build skeletons and hard body parts.<ref>[http://www.ibecbarcelona.eu/novel-evolutionary-theory-for-the-explosion-of-life/ Novel Evolutionary Theory For The Explosion Of Life]</ref>
Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell's [[Earth:Great Unconformity|Great Unconformity]].<ref name="Peters2012">{{Cite journal | last1 = Peters | first1 = S. E. | last2 = Gaines | first2 = R. R. | doi = 10.1038/nature10969 | title = Formation of the 'Great Unconformity' as a trigger for the Cambrian explosion | journal = Nature | volume = 484 | issue = 7394 | pages = 363–366 | year = 2012 | pmid =  22517163|bibcode = 2012Natur.484..363P | s2cid = 4423007 }}</ref>

An increase of calcium may also have been caused by erosion of the Transgondwanan Supermountain that existed at the time of the explosion. The roots of the mountain are preserved in present-day [[Place:East Africa|East Africa]] as an [[Physics:Orogeny|orogen]].<ref>{{Cite journal | last1 = Squire | first1 = R. J. | last2 = Campbell | first2 = I. H. | last3 = Allen | first3 = C. M. | last4 = Wilson | first4 = C. J. | title = Did the Transgondwanan Supermountain trigger the explosive radiation of animals on Earth? | year = 2006 | journal = Earth and Planetary Science Letters | volume = 250 | issue = 1 | pages = 116–133 | url = http://www.blc.arizona.edu/courses/schaffer/182h/Climate/Transgondwanan%20Supermountain.pdf | access-date = 11 September 2017 | doi = 10.1016/j.epsl.2006.07.032 | bibcode = 2006E&PSL.250..116S | archive-date = 13 July 2019 | archive-url = https://web.archive.org/web/20190713030808/http://www.blc.arizona.edu/courses/schaffer/182h/Climate/Transgondwanan%20Supermountain.pdf | url-status = dead }}</ref>

===Developmental explanations===
{{further|Biology:Evolutionary developmental biology}}

A range of theories are based on the concept that minor [[Biology:Evolutionary developmental biology|modifications to animals' development]] as they grow from [[Biology:Embryo|embryo]] to adult may have been able to cause very large changes in the final adult form. The [[Biology:Hox gene|Hox gene]]s, 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 = de Rosa, R. |author2=Grenier, J.K. |author3=Andreeva, T. |author4=Cook, C.E. |author5=Adoutte, A. |author6=Akam, M. |author7=Carroll, S.B. |author8=Balavoine, G. |date=June 1999 | doi = 10.1038/21631 | journal = Nature | volume = 399 |title=Hox genes in brachiopods and priapulids and protostome evolution | pmid = 10391241  | issue = 6738 | issn = 0028-0836 | pages = 772–6|bibcode = 1999Natur.399..772D |s2cid=4413694 }}</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.

This apparent paradox is addressed in a theory that focuses on the [[Physics:Physics|physics]] of development. It is proposed that the emergence of simple multicellular forms provided a changed context and spatial scale in which novel physical processes and effects were mobilized by the products of genes that had previously evolved to serve unicellular functions. Morphological complexity (layers, segments, [[Biology:Lumen (anatomy)|lumen]]s, appendages) arose, in this view, by [[Physics:Self-organization|self-organization]].<ref name="Newman2008">{{cite journal |author1=Newman, S.A.  |author2=Bhat, R. |date=April 2008 | doi = 10.1088/1478-3975/5/1/015008| journal = Physical Biology | volume = 5 | page = 0150580|title=Dynamical patterning modules: physico-genetic determinants of morphological development and evolution | pmid = 18403826  | issue = 1 | issn = 1478-3967|bibcode = 2008PhBio...5a5008N |s2cid=24617565 }}</ref>

[[Biology:Horizontal gene transfer|Horizontal gene transfer]] has also been identified as a possible factor in the rapid acquisition of the biochemical capability of biomineralization among organisms during this period, based on evidence that the gene for a critical protein in the process was originally transferred from a bacterium into sponges.<ref>{{Cite Q|Q21284056|doi-access=free}}</ref>

===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 [[Biology:Evolutionary arms race|arms races]] between predators and prey, and others focus on the more general mechanisms of [[Biology:Coevolution|coevolution]]. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they do not explain why the "explosion" happened when it did.<ref name="Marshall2006Explaining"/>

====End-Ediacaran mass extinction====
{{main|Earth: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 {{delta|13|C|link}} record. It is suspected that several global [[Earth:Anoxic event|anoxic event]]s were responsible for the extinction.<ref>[https://phys.org/news/2018-06-extreme-fluctuations-oxygen-gradual-cambrian.html Did extreme fluctuations in oxygen, not a gradual rise, spark the Cambrian explosion?]</ref><ref>[https://www.economist.com/science-and-technology/2018/06/07/the-cambrian-explosion-was-caused-by-a-lack-of-oxygen-not-an-abundance The Cambrian explosion was caused by a lack of oxygen, not an abundance]</ref>

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" />

====Anoxia====
The late Ediacaran oceans appears to have suffered from an [[Earth:Anoxic waters|anoxia]] that covered much of the seafloor, which would have given mobile animals with the ability to seek out more oxygen-rich environments an advantage over sessile forms of life.<ref>{{Cite web |url=https://uwaterloo.ca/science/news/early-ocean-anoxia-may-have-led-first-mass-extinction-event |title=Early ocean anoxia may have led to first mass extinction event {{!}} Science {{!}} University of Waterloo |access-date=2019-04-23 |archive-date=2023-04-22 |archive-url=https://web.archive.org/web/20230422224953/https://uwaterloo.ca/science/news/early-ocean-anoxia-may-have-led-first-mass-extinction-event |url-status=dead }}</ref>

====Increase in sensory and cognitive abilities====
{{main|Biology:Evolution of the eye}}
Andrew Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time, hunting and evading were both close-range affairs&nbsp;– smell, vibration, and touch were the only senses used. When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines, and similar defenses may also have evolved in response to vision. He further observed that, where animals lose vision in unlighted environments such as caves, diversity of animal forms tends to decrease.<ref>{{cite book | author=Parker, Andrew | title=In the Blink of an Eye | publisher=Perseus Books | location=Cambridge, Massachusetts | year=2003 | isbn=978-0-7382-0607-3 | oclc=52074044 | url=https://archive.org/details/inblinkofeye00park }}</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:Earth-Science Reviews|Earth-Science Reviews]] | volume = 77 | issue = 1 | pages = 1–229 |bibcode = 2006ESRv...77....1M }}</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 at a greater distance in the sea than sight can, but the appearance of these other senses apparently did not cause an evolutionary explosion.<ref name="Marshall2006Explaining"/>

One hypothesis posits that the development of increased cognitive abilities during the Cambrian drove diversity increase. This is evidenced by the fact that the novel ecological lifestyles created during the Cambrian required rapid, regular movement, a feature associated with brain-bearing organisms. The increasing complexity of brains, positively correlated with a greater range of motion and sensory abilities, enabled a wider range of novel ecological modes of life to come into being.<ref>{{cite journal |last1=Hsieh |first1=Shannon |last2=Plotnick |first2=Roy E. |last3=Bush |first3=Andrew M. |date=28 January 2022 |title=The Phanerozoic aftermath of the Cambrian information revolution: sensory and cognitive complexity in marine faunas |journal=Paleobiology |volume=48 |issue=3 |pages=397–419 |doi=10.1017/pab.2021.46 |bibcode=2022Pbio...48..397H |s2cid=246399509 |doi-access=free }}</ref>

====Arms races between predators and prey====
The ability to avoid or recover from [[Earth:Predation|predation]] often makes the difference between life and death, and is therefore one of the strongest components of [[Biology:Natural selection|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 a meal; 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| jstor=77442 |author1=Dawkins, R. |author2=Krebs, R.J.| doi=10.1098/rspb.1979.0081
| journal=Proceedings of the Royal Society B| pmid=42057| bibcode=1979RSPSB.205..489D | s2cid=9695900}}</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, the holes drilled in ''[[Biology:Cloudina|Cloudina]]'' shells, and traces of burrowing to avoid predators. 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"/> However, the intensity of predation does appear to have increased dramatically during the Cambrian<ref>{{cite journal |year=1988 |author=McMenamin, M. A. S. |title=Palaeocological feedback and the Vendian-Cambrian transition |journal=Trends in Ecology and Evolution |volume=3 |pages=205–208 |doi=10.1016/0169-5347(88)90008-0 |pmid=21227202 |issue=8 }}</ref> as new predatory "tactics" (such as shell-crushing) emerged.<ref name="Zhang2011">{{Cite journal | last1=Zhang | first1=Z. | last2=Holmer | first2=L. E. | last3=Robson | first3=S. P. | last4=Hu | first4=S. | last5=Wang | first5=X. | last6=Wang | first6=H. | title=First record of repaired durophagous shell damages in Early Cambrian lingulate brachiopods with preserved pedicles | doi=10.1016/j.palaeo.2011.01.010 | journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] | volume=302 | issue=3–4 | pages=206–212 | year=2011 | bibcode=2011PPP...302..206Z }}</ref> This rise of predation during the Cambrian was confirmed by the temporal pattern of the median predator ratio at the scale of genus, in fossil communities covering the Cambrian and Ordovician periods, but this pattern is not correlated to diversification rate.<ref name=":0">{{Cite journal |last=López-Villalta|first=Julián Simón |date=2016 |title=Testing the Predation-Diversification Hypothesis for the Cambrian—Ordovician Radiation|journal=Paleontological Research |volume=20 |issue=4 |pages=312–321 |doi=10.2517/2016PR022 |s2cid=131794128 |issn=1342-8144}}</ref><!--apparent COI--> This lack of correlation between predator ratio and diversification over the Cambrian and Ordovician suggests that predators did not trigger the large evolutionary radiation of animals during this interval. Thus the role of predators as triggerers of diversification may have been limited to the very beginning of the "Cambrian explosion".<ref name=":0" />

====Increase in size and diversity of planktonic animals====
Geochemical evidence strongly indicates that the total mass of [[Earth:Plankton|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 seabed, since their [[Physics:Drag|drag]] was about the same as their weight. This meant they were destroyed by [[Earth:Scavenger|scavenger]]s or by chemical processes before they reached the sea floor.<ref name="Butterfield2001">{{cite book
| author = Butterfield, N.J.
| year = 2001
| title = Ecology and evolution of Cambrian plankton
| publisher = The Ecology of the Cambrian Radiation. Columbia University Press, New York
| pages = 200–216
| isbn = 978-0-231-10613-9
| url = https://books.google.com/books?id=lbZUxpj4i6UC&q=%20Ecology%20and%20evolution%20of%20Cambrian%20plankton&pg=PA200
| access-date= 2007-08-19
}}</ref>

Mesozooplankton are plankton of a larger size. Early Cambrian specimens [[Biology:Filter feeder|filtered]] microscopic plankton from the seawater. These larger organisms would have produced droppings and ultimately corpses 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 new range of possible ways of life. If any of these remains sank uneaten to the sea floor they could be buried; this would have taken some [[Chemistry:Carbon|carbon]] out of [[Earth:Carbon cycle|circulation]], resulting in an increase in the [[Chemistry:Concentration|concentration]] of breathable oxygen in the seas (carbon readily [[Chemistry:Chemical compound|combines]] with oxygen).<ref name="Butterfield2001"/>

The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during the [[Earth:Ediacaran|Ediacaran]] period.<ref name="Butterfield2001ECR">{{Cite book
| author = Butterfield, N.J.
| year = 2001
| chapter = Ecology and evolution of Cambrian plankton
| title = The Ecology of the Cambrian Radiation
| publisher = Columbia University Press
| location = New York
| pages = 200–216
| chapter-url = https://books.google.com/books?id=oWcepB3Q-rIC&q=%20The%20Ecology%20of%20the%20Cambrian%20Radiation&pg=PR7
| chapter-format=PDF| access-date= 2007-08-19
| isbn = 978-0-231-10613-9
}}</ref><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
| url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/31/2_Suppl/36
| author1=Peterson, K.J. |author2=McPeek, M.A. |author3=Evans, D.A.D.  | s2cid=30787918
|name-list-style=amp | type=abstract
| issn=0094-8373
}}</ref>

Metazoans have an amazing ability to increase diversity through [[Biology:Coevolution|coevolution]].<ref name="Butterfield2007"/> This means that an organism's traits can lead to traits evolving in other organisms; a number of responses are possible, and a different species can potentially emerge from each one. As a simple example, the evolution of predation may have caused one organism to develop a defence, while another developed motion to flee. This would cause the predator lineage to diverge into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual coevolution 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 coevolution with animals.<ref name="Butterfield2007"/>

===Ecosystem engineering===
Evolving organisms inevitably change the environment they evolve in. The [[Earth:Devonian|Devonian]] colonization of land had planet-wide consequences for sediment cycling and ocean nutrients, and was likely linked to the Devonian mass extinction. A similar process may have occurred on smaller scales in the oceans, with, for example, the sponges filtering particles from the water and depositing them in the mud in a more digestible form; or burrowing organisms making previously unavailable resources available for other organisms.<ref name="Erwin2011a">{{Cite journal | last1 = Erwin | first1 = D. H. | last2 = Tweedt | first2 = S. | doi = 10.1007/s10682-011-9505-7 | title = Ecological drivers of the Ediacaran-Cambrian diversification of Metazoa | journal = Evolutionary Ecology | volume = 26 | issue = 2 | pages = 417–433 | year = 2011 | s2cid = 7277610 }}</ref>

==== Burrowing ====
Increases in burrowing changed the seafloor's geochemistry, and led to decreased oxygen in the ocean and increased CO<sub>2</sub> levels in the seas and the atmosphere, resulting in global warming for tens of millions years, and could be responsible for mass extinctions.<ref>[https://www.inverse.com/article/46647-global-warming-animals-cambrian-carbon-dioxide Early Global Warming Was Unexpectedly Caused by a Burst of Tiny Life Forms – Inverse]</ref> But 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&nbsp;– please take this into account when rewording!--> 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&nbsp;– creating new niches and the scope for higher diversity.<ref name="Seilacher2005" />

===Complexity threshold===
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.<ref name="SoléFernándezKauffman2003AdaptiveWalks">{{cite journal
| title=Adaptive walks in a gene network model of morphogenesis: insights into the Cambrian explosion
| year=2003
| journal=Int. J. Dev. Biol.
| volume=47
| issue=7
| pages=685–693
| pmid=14756344
| author1=Solé, R.V. |author2=Fernández, P. |author3=Kauffman, S.A.  |name-list-style=amp | bibcode=2003q.bio....11013S
| arxiv=q-bio/0311013
}}</ref> This genetic threshold may have a correlation to the amount of oxygen available to organisms. Using oxygen for metabolism produces much more energy than anaerobic processes. Organisms that use more oxygen have the opportunity to produce more complex proteins, providing a template for further evolution.<ref name="ReferenceA"/> These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments.<ref name="ReferenceC">{{cite journal|last1=Sperling|first1=E.A.|last2=Frieder|first2=C.A.|last3=Raman|first3=A.V.|last4=Girguis|first4=P.R.|last5=Levin|first5=L.A.|last6=Knoll|first6=A.H.|title=Oxygen, ecology, and the Cambrian radiation of animals|journal=PNAS|date=2013|volume=110|issue=33|pages=13446–13451|doi=10.1073/pnas.1312778110|pmid=23898193|bibcode=2013PNAS..11013446S|pmc=3746845|doi-access=free}}</ref> With the help of oxygen, genes that code for these proteins could contribute to the expression of [[Biology:Complex traits|complex traits]] more efficiently. Access to a wider range of structures and functions would allow organisms to evolve in different directions, increasing the number of niches that could be inhabited. Furthermore, organisms had the opportunity to become more specialized in their own niches.<ref name="ReferenceC"/>

==Relationship with the Great Ordovician Biodiversification Event ==
{{Main|Earth:Great Ordovician Biodiversification Event}}

After an [[Earth:Cambrian–Ordovician extinction event|extinction]] at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic. This event, known as the Great Ordovician Biodiversification Event (GOBE), has been considered a "follow-up" to the Cambrian explosion.<ref name="Droser2003">{{cite journal |author1=Droser, Mary L |author2=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=178–184 |doi=10.1093/icb/43.1.178 |pmid=21680422 |doi-access=free}}</ref> Recent studies have suggested that the Cambrian explosion were not two discrete events but one long evolutionary radiation.<ref>{{cite journal |last1=Harper |first1=David A. T. |last2=Cascales-Miñana |first2=Borja |last3=Servais |first3=Thomas |date=3 December 2019 |title=Early Palaeozoic diversifications and extinctions in the marine biosphere: A continuum of change |journal=Geological Magazine |volume=157 |issue=1 |pages=5–21 |doi=10.1017/S0016756819001298 |doi-access=free}}</ref> Analytical study of the Geobiodiversity Database (GBDB) and [[Biology:Paleobiology Database|Paleobiology Database]] (PBDB) failed to find a statistical basis for separating the two radiations.<ref>{{cite journal |last1=Servais |first1=Thomas |last2=Cascales-Miñana |first2=Borja |last3=Harper |first3=David A. T. |last4=Lefebvre |first4=Bertrand |last5=Munnecke |first5=Axel |last6=Wang |first6=Wenhui |last7=Zhang |first7=Yuandong |date=1 August 2023 |title=No (Cambrian) explosion and no (Ordovician) event: A single long-term radiation in the early Palaeozoic |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=623 |doi=10.1016/j.palaeo.2023.111592 |bibcode=2023PPP...62311592S |doi-access=free}}</ref>

Some researchers have proposed the existence of a biodiversity gap during the Furongian separating the Cambrian explosion and GOBE known as the Furongian Gap.<ref>{{cite journal |last1=Harper |first1=David A. T. |last2=Topper |first2=Timothy P. |last3=Cascales-Miñana |first3=Borja |last4=Servais |first4=Thomas |last5=Zhang |first5=Yuan-Dong |last6=Ahlberg |first6=Per |date=March–June 2019 |title=The Furongian (late Cambrian) Biodiversity Gap: Real or apparent? |url=https://www.sciencedirect.com/science/article/abs/pii/S1871174X18301628 |journal=Palaeoworld |volume=28 |issue=1–2 |pages=4–12 |doi=10.1016/j.palwor.2019.01.007 |s2cid=134062318 |access-date=4 July 2023|hdl=20.500.12210/34395 |hdl-access=free }}</ref> Studies of the Guole Konservat-Lagerstätte and similar fossil sites in South China have instead found the Furongian to instead be a time of rapid biological turnovers though, making the existence of the Furongian Gap highly controversial.<ref>{{cite journal |last1=Deng |first1=Yiying |last2=Fan |first2=Junxuan |last3=Yang |first3=Shengchao |last4=Shi |first4=Yukun |last5=Lu |first5=Zhengbo |last6=Xu |first6=Huiqing |last7=Sun |first7=Zongyuan |last8=Zhao |first8=Fangqi |last9=Hou |first9=Zhangshuai |date=15 May 2023 |title=No Furongian Biodiversity Gap: Evidence from South China |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=618 |doi=10.1016/j.palaeo.2023.111492 |bibcode=2023PPP...61811492D |doi-access=free}}</ref>

==Uniqueness of the early Cambrian biodiversification ==
The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a [[Biology:Coevolution|coevolution]]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| last1 = Butterfield| first1 = N. J.| year = 2007| title = Macroevolution and macroecology through deep time| journal = Palaeontology| volume = 50| issue = 1| pages = 41–55| doi = 10.1111/j.1475-4983.2006.00613.x| bibcode = 2007Palgy..50...41B| s2cid = 59436643| url = http://eprints.esc.cam.ac.uk/174/1/Butterfield__Palaeontolgy_50_Pt_1_2007_.pdf| access-date = 2019-09-19| archive-date = 2022-07-21| archive-url = https://web.archive.org/web/20220721114458/http://eprints.esc.cam.ac.uk/174/1/Butterfield__Palaeontolgy_50_Pt_1_2007_.pdf| url-status = dead}}</ref> The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoan [[Biology:Clade|clade]]s of which Cambrian fossils have been found. Later radiations, such as those of [[Biology:Fish|fish]] in the [[Earth:Silurian|Silurian]] and [[Earth:Devonian|Devonian]] periods, involved fewer [[Biology:Taxon|taxa]], mainly with very similar body plans.<ref name="CowenHistLife"/> Although the recovery from the [[Earth:Permian–Triassic extinction event|Permian-Triassic extinction]] started with about as few animal species as the Cambrian explosion, the recovery produced far fewer significantly new types of animals.<ref>{{cite journal |last=Erwin |first=D.H. |author2=Valentine, J.W. |author3=Sepkoski, J.J. |date=November 1987| title=A Comparative Study of Diversification Events: The Early Paleozoic Versus the Mesozoic |journal=Evolution |volume=41 |issue=6 |pages=1177–1186 |doi=10.2307/2409086 |pmid=11542112|jstor=2409086}}</ref>

Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailable [[Biology:Ecological niche|ecological niche]]s. When these were all occupied, limited space existed for such wide-ranging diversifications to occur again, because strong competition existed in all niches and incumbents usually had the advantage. If a wide range of empty niches had continued, clades would be able to continue diversifying and become disparate enough for us to recognise them as different [[Biology:Phylum|phyla]]; when niches are filled, lineages will continue to resemble one another long after they diverge, as limited opportunity exists 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 | issue=2 |date=April 1995 | pages=190–194 | doi=10.2307/3515182 | volume=10 | type=abstract | jstor=3515182
| bibcode=1995Palai..10..190V }}</ref>

There were two similar explosions in the [[Biology:Evolutionary history of plants#Colonization 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"/> Furthermore, angiosperms ([[Biology:Flowering plant|flowering plant]]s) originated and rapidly diversified during the [[Earth:Cretaceous|Cretaceous]] period.

==See also==
*[[Earth:Avalon explosion|Avalon explosion]]

== Footnotes ==
{{notelist|1}}

==References==
{{reflist|25em}}

==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. |author2=Jensen, J. |doi=10.1111/j.1469-185X.1999.tb00046.x |pmid=10881389 |issue=2 |s2cid=39772232 }}
* Collins, Allen G. [http://www.ucmp.berkeley.edu/phyla/metazoafr.html "Metazoa: Fossil record"]. Retrieved Dec. 14, 2005.
* {{cite journal
| author = Conway Morris, S.
| date=June 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
| doi =10.1098/rstb.2006.1846
| pmid = 16754615
| issn = 0962-8436
| pmc = 1578734}} An enjoyable account.
* {{cite book| title=Wonderful Life: The Burgess Shale and the Nature of History| author=Gould, S.J.|  year=1989| publisher=W.W. Norton & Company| title-link=Wonderful Life (book)| bibcode=1989wlbs.book.....G}}
* {{cite journal|author1=Kennedy, M. |author2=M. Droser |author3=L. Mayer. |author4=D. Pevear |author5=D. Mrofka  |name-list-style=amp |year=2006 |doi=10.1126/science.311.5766.1341c |journal=Science |volume=311 |issue=5766 |page=1341 |title=Clay and Atmospheric Oxygen|s2cid=220101640 }}
* {{cite journal
| author = Knoll, A.H.
| author2=Carroll, S.B.
| s2cid=8908451
| date = 1999-06-25
| title = Early Animal Evolution: Emerging Views from Comparative Biology and Geology
| journal = Science
| volume = 284
| issue = 5423
| doi = 10.1126/science.284.5423.2129
| pmid = 10381872
| pages = 2129–37
}}
* {{cite journal | last1 = Markov | first1 = Alexander V. | last2 = Korotayev | first2 = Andrey V. | year = 2007 | title = Phanerozoic marine biodiversity follows a hyperbolic trend | journal = Palaeoworld | volume = 16 | issue = 4| pages = 311–318 | doi=10.1016/j.palwor.2007.01.002}}
* {{cite journal | author=Montenari, M. |author2=Leppig, U. |title= The Acritarcha: their classification morphology, ultrastructure and palaeoecological/palaeogeographical distribution | journal=Paläontologische Zeitschrift | year=2003 | volume=77 | pages=173–194 | doi=10.1007/bf03004567 |title-link=Acritarch |issue=1 |bibcode=2003PalZ...77..173M |s2cid=127238427 }}
* {{cite journal |author=Wang, D. Y.-C. |author2=S. Kumar |author3=S. B. Hedges |date=January 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 B |volume=266 |pages=163–71 |doi=10.1098/rspb.1999.0617 |issue=1415 |pmid=10097391  |issn=0962-8452 |pmc=1689654}}
* Wood, Rachel A., "The Rise of Animals:  New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion", ''Scientific American'', vol. 320, no. 6 (June 2019), pp.&nbsp;24–31.
* {{cite journal |author1=Xiao, S. |author2=Y. Zhang |author3=A. Knoll  |name-list-style=amp |date=January 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 |bibcode = 1998Natur.391..553X  |issue=1 |s2cid=4350507 |issn=0090-9556}}
Timeline References:
* {{cite journal | 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 | author1=Martin, M.W | author2=Grazhdankin, D.V | author3=Bowring, S.A | author4=Evans, D.A.D | author5=Fedonkin, M.A | author6=Kirschvink, J.L | issue=5467 | bibcode = 2000Sci...288..841M }}

==External links==
* [https://web.archive.org/web/20120213225957/http://www.ijdb.ehu.es/web/contents.php?vol=47 The Cambrian "explosion" of metazoans and molecular biology: would Darwin be satisfied?]
* [https://web.archive.org/web/20080407005007/http://genome6.cu-genome.org/andrey/GouldComment.pdf On embryos and ancestors] by [[Biography:Stephen Jay Gould|Stephen Jay Gould]]
* {{cite journal|title=The Cambrian "explosion": Slow-fuse or megatonnage?| doi=10.1073/pnas.97.9.4426|volume=97|issue=9|journal=Proceedings of the National Academy of Sciences |pages=4426–4429 |pmid=10781036 |pmc=34314 |date=April 2000 | last1=Conway Morris | first1=S.|bibcode=2000PNAS...97.4426C| doi-access=free}}
* [http://www.bbc.co.uk/programmes/p003k9bg The Cambrian Explosion]&nbsp;– [http://www.bbc.co.uk/programmes/b006qykl ''In Our Time''], BBC Radio 4 broadcast, 17 February 2005
* {{Cite web | url = http://burgess-shale.rom.on.ca/en | title = Burgess Shale | publisher = Virtual Museum of Canada | year = 2011}}, exhaustive details about the Burgess Shale, its fossils, and its significance for the Cambrian explosion
* [http://www.kumip.ku.edu/cambrianlife/ Utah's Cambrian life] {{Webarchive|url=https://web.archive.org/web/20120723072953/http://www.kumip.ku.edu/cambrianlife/ |date=2012-07-23 }}&nbsp;– new (2008) website with good images of a range of Burgess-shale-type and other Cambrian fossils
* [http://paleobiology.si.edu/geotime/main/htmlversion/cambrian2.html Smithsonian National Museum]

[[Category:Cambrian first appearances|*]]
[[Category:Evolution]]
[[Category:Evolution of the biosphere]]
[[Category:Unsolved problems in biology]]

{{Sourceattribution|Cambrian explosion}}