Small shelly fauna
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The '''small shelly fauna''' (alternatively '''skeletal fossils''', abbreviated to '''SSF''') constitute an assemblage of millimetre-scale calcareous [[fossil]]s, with a nearly continuous record from the terminal [[Ediacaran]], throughout the lower Cambrian period, until more familiar organisms such as the [[trilobite]]s become established.
While some of the fossils represent the entire skeletons of small organisms, the bulk of the fauna comprises disarticulated remains of larger organisms.
==Census of small shelly fossils==
Small shelly fossils are typically, although not always, preserved in phosphate (which may or may not have been their original mineralogy). They are usually extracted from limestone by placing the limestone in a weak acid (typically [[acetic acid]]); the phosphatised fossils remain, untouched.<ref name=Dzik2007/>
The fauna includes a variety of tubes, some of which were the tests of calcifying organisms such as the [[cloudinid]]s.<ref name=Germs1972>{{cite journal
| author = Germs, G. J.; ,
| year = 1972
| title = New shelly fossils from Nama Group, South West Africa
| journal = American Journal of Science
| volume = 272
| pages = 752–761
| month = October
}}</ref> Also present are an array of cap-shaped shells, which are interpreted as the only plate protecting a small animal.<ref name=Budd2003/> But most of the SSF represents "scleritomes": skeletal elements or other fragments which once made up the external armour of early animals, such as ''[[Halkeria]]''<ref name=ConwayMorris1995>{{cite journal
| author = Conway Morris, S.; Peel, J.S
| year = 1995
| title = Articulated Halkieriids from the Lower Cambrian of North Greenland and Their Role in Early Protostome Evolution
| journal = Philosophical Transactions of the Royal Society of London, Ser. B Biol. Sci
| volume = 347
| pages = 305–358
| doi = 10.1098/rstb.1995.0029
}}</ref> or "scale worms".<ref name=ConwayMorris1997>{{cite journal
| author = Conway Morris, S
| year = 1997
| title = The Cuticular Structure of the 495-Myr-Old Type Species of the Fossil Worm ''Palaeoscolex, P. Piscatorum'' (?Priapulida)
| journal = Zoological Journal of the Linnaean Society
| volume = 495
| issue = 119
| pages = 69–82
| doi = 10.1006/zjls.1996.0065
}}</ref> The "coat of mail" disintegrated once the animal died, and its fragments became dispersed and, in cases, fossilised.<ref name=Budd2003/> Reconstructing these elements usually relies upon a fully articulate fossil being found in an exceptionally preserved [[lagerstatte]].<ref name=Budd2003/>
Small shelly fossils are composed of a number of minerals, the most important being [[silica]], calcium phosphate and calcium carbonate.<ref name=Bengtson2004>{{cite journal
| author = Bengtson, S
| year = 2004
| journal = PALEONTOLOGICAL SOCIETY PAPERS
| volume = 10
| pages = 67
| url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf
}}</ref> The mineral used probably reflects the chemistry of the oceans in which the fossil producer first evolved.<ref name=Porter2007>{{cite journal
| author = Porter, S.
| year = 2007
| title = Seawater Chemistry and Early Carbonate Biomineralization
| journal = Science
| volume = 316
| issue = 5829
| pages = 1302
| doi = 10.1126/science.1137284
| url = http://www.sciencemag.org/cgi/content/abstract/sci;316/5829/1302
}}</ref>
==Evolutionary significance==
The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a far more useful insight into the [[Cambrian explosion]] than the exceptional fossil record.<ref name=Budd2003>{{cite journal
| author = Budd, Graham E
| year = 2003
| 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
| url = http://intl-icb.oxfordjournals.org/cgi/content/abstract/43/1/157
| format = Free full text
}}</ref>
Although most elements in the fauna are difficult to identify, those which have been assigned positions in, or in the [[stem group]]s of, modern taxa permit an assessment of animal evolution to be made on the strength of the small shelly evidence.
Such an assessment shows that the earliest small shellies are the most basal. As time goes on, they can be placed in the stem group to an ever smaller clade. In other words, the earliest (Ediacaran) fossils can be tentatively considered diploblastic. Later fossils are more convincingly diploblastic. Subsequently, the [[Helcionellid]]s are the first shelly fossils that can be placed in the stem group to a phylum ([[mollusca]]). Again, as time progresses, so the arguments for stem group placements become stronger, and by the Atdabanian, some elements of the fauna can be assigned to the crown group of a phylum ([[echinoderm]]ata).<ref name=Budd2003/>
This gives the impression that the first (late Ediacaran) animals were basal members of later clades, with the phyla subsequently appearing in a "rapid, but nevertheless resolvable and orderly" fashion, rather than as a "sudden jumble",<ref name=Budd2003/>{{rp|163}} and thus reveals the true pace of the Cambrian explosion.<ref name=Budd2003/>
==Taphonomy==
The small shelly fossils are preserved by virtue of early phosphatisation, a taphonomic pathway that only seems to have been common during the early Cambrian, fading out with time as a result of increased bioturbation.<ref name=Dzik2007>{{The Rise and Fall of the Ediacaran Biota|Dzik, J|The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|405|414|30|http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf}}</ref>
The abrupt appearance of the small shelly fauna at the bast of the Cambrian may thus represent no more than a preservational artefact; the shells may have existed before hand, but not been phosphatised and thus dissolved in the acids used in their extraction.<ref name=Dzik1994>{{cite journal
| author = Dzik, J.
| title = Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic
| year = 1994
| journal = Acta Palaeontologica Polonica
| volume = 39
| issue = 3
| pages = 27–313
| url = http://www.paleo.pan.pl/people/Dzik/Dzik1994d.htm
}}</ref>
==References==
{{reflist}}
{{inline}}
*{{cite journal
| author = Bengtson, S
| year = 1992
| title = The Cap-Shaped Cambrian Fossil Maikhanella and the Relationship Between Coeloscleritophorans and Molluscs
| journal = Lethaia
| volume = 25
| pages = 401–420
| doi = 10.1111/j.1502-3931.1992.tb01644.x
}}
*{{cite journal
| author = Haas, W
| year = 1981
| title = Evolution of Calcareous Hardparts in Primitive Molluscs
| journal = Malacologia
| volume = 21
| pages = 403–418
}}
*{{cite journal
| author = Jacobs, David K.; Wray, Charles G.; Wedeen, Cathy J.; Kostriken, Richard; DeSalle, Rob; Staton, Joseph L.; Gates, Ruth D.; Lindberg, David R
| year = 2000
| title = Molluscan Engrailed Expression, Serial Organization, and Shell Evolution
| journal = Evolution and Development
| volume = 2
| issue = 6
| pages = 340–347
| doi = 10.1046/j.1525-142x.2000.00077.x
}}
*{{cite journal
| author = Matthews, S.C.; Missarzhevsky, V.V
| year = 1975
| 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 = 289–304
| doi = 10.1144/gsjgs.131.3.0289
}}
*Runnegar, Bruce 1992: Chapter 3 – ''Evolution of the Earliest Animals.'' <u>In</u>: Schopf, J. William (ed) ''Major Events in the History of Life.'' Jones and Bartlett.
*Valentine, James W. 1995: ''Late Precambrian bilaterians: Grades and clades.'' <u>In</u>: Fitch, Walter M.; Ayala, Francisco J. 1995: ''Tempo and mode in evolution: Genetics and paleontology 50 years after Simpson.'' National Academy of Sciences, pp. 87-107.
==External links==
*[http://www.peripatus.gen.nz/Paleontology/SmaSheFau.html Small shelly fauna]
*[http://www.peripatus.gen.nz/paleontology/CamExp.html Cambrian explosion]
*[http://hoopermuseum.earthsci.carleton.ca/camex/1jtommo.html Tommotian fauna, images]
[[Category:Fossils]]
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