View source for Small shelly fauna

{{Short description|Fossils from the Ediacaran and Cambrian periods}}
{{good article}}
{{CEXNAV|expanded=Key organisms}}
The '''small shelly fauna''', '''small shelly fossils''' ('''SSF'''), or '''early skeletal fossils''' ('''ESF''')<ref>{{Cite journal|doi=10.1144/SP448.10|title=The Precambrian–Phanerozoic and Ediacaran–Cambrian boundaries: A historical approach to a dilemma|journal=Geological Society, London, Special Publications|volume=448|issue=1|pages=311–349|year=2016|last1=Geyer|first1=Gerd|last2=Landing|first2=Ed|bibcode=2017GSLSP.448..311G|s2cid=133538050}}</ref> are [[biomineralization|mineralized]] [[fossil]]s, many only a few millimetres long, with a nearly continuous record from the latest stages of the [[Ediacaran]] to the end of the Early [[Cambrian]] [[period (geology)|Period]]. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as [[trilobite]]s. Since most SSFs were preserved by being covered quickly with [[phosphate]] and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

Some of the fossils represent the entire [[skeleton]]s of small organisms, including the mysterious ''[[Cloudina]]'' and some snail-like [[mollusc]]s. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including [[sponge]]s, molluscs, slug-like [[halkieriid]]s, [[brachiopod]]s, [[echinoderm]]s, and [[onychophora]]n-like organisms that may have been close to the ancestors of [[arthropod]]s.

One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeletons – suggested a sudden increase in the ocean's concentration of [[calcium]]. However, many SSFs are constructed of other minerals, such as [[silica]]. Because the first SSFs appear around the same time as organisms first started burrowing to avoid [[predation]], it is more likely that they represent early steps in an [[evolutionary arms race]] between predators and increasingly well-defended prey. On the other hand, mineralized skeletons may have evolved simply because they are stronger and take less energy to produce than all-organic skeletons like those of insects. Nevertheless, it is still true that the animals used minerals that were most easily accessible.

Although the small size and often fragmentary nature of SSFs makes it difficult to identify and classify them, they provide very important evidence for how the main groups of marine [[invertebrate]]s evolved, and particularly for the pace and pattern of evolution in the [[Cambrian explosion]]. Besides including the earliest known representatives of some modern [[phylum|phyla]], they have the great advantage of presenting a nearly continuous record of early Cambrian organisms whose bodies include hard parts.

==History of discovery==
{{Small shelly fauna graphical timeline}}
The term "small shelly fossils" was coined by Samuel Matthews and V. V. Missarzhevsky in 1975.<ref>{{cite journal |author1=Matthews, S. C. |author2=Missarzhevsky, V. V.
 | name-list-style=amp | 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 
 | url=http://jgs.geoscienceworld.org/cgi/content/abstract/131/3/289 | access-date=2008-07-18
| bibcode=1975JGSoc.131..289M|s2cid=140660306
 | url-access=subscription}}</ref> The term is often abbreviated to "small shellies" or "SSF".<ref name="Bengtson2004"/> It is quite a [[misnomer]] since, as Stefan Bengtson says, "they are not always small, they are commonly not shelly – and the term might equally well apply to [[Pleistocene]] [[Periwinkle (mollusc)|periwinkle]]s." Paleontologists have been unable to invent a better term, and have vented their frustration in parodies such as "small silly fossils" and "small smellies".<ref name="Bengtson2004"/>

The great majority of all the morphological features of later shelled organisms appear among the SSFs.<ref name="Bengtson2004"/><ref name="Dzik1994"/> No-one has attempted a formal definition of "small shelly fauna", "small shelly fossils" or other similar phrases.<ref>{{cite journal 
| author1=Steiner, M. |author2=Li, G. |author3=Qian, Y. |author4=Zhu, M. |author5=Erdtmann, B-D. |date=October 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 
| doi=10.1016/j.palaeo.2007.03.046
|bibcode=2007PPP...254...67S }}</ref>

Specimens and sometimes quite rich collections of these fossils were discovered between 1872 and 1967, but no-one drew the conclusion that the Early Cambrian contained a diverse range of animals in addition to the traditionally recognized [[trilobite]]s, [[archaeocyatha]]ns, etc. In the late 1960s [[Soviet Union|Soviet]] paleontologists discovered even richer collections of SSFs in beds below and therefore earlier than those containing Cambrian trilobites. Unfortunately the papers that described these discoveries were in Russian, and the 1975 paper by Matthews and Missarzhevsky first brought the SSFs to the serious attention of the non-Russian-reading world.<ref name="Bengtson2004"/>

There was already a vigorous debate about the early evolution of animals. [[Preston Cloud]] argued in 1948 and 1968 that the process was "explosive",<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}} and {{Cite book 
| author=Cloud, P. E. | year= 1968 | contribution=Pre-metazoan evolution and the origins of the Metazoa.
| pages=1–72 | editor=Drake, E. T. | title=Evolution and Environment | publisher=Yale University Press
| location=New Haven, Conn.
}}</ref> and in the early 1970s [[Niles Eldredge]] and [[Stephen Jay Gould]] developed their theory of [[punctuated equilibrium]], which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.<ref>{{Cite book
|author1=Eldredge, N. |author2=Gould, S. J.
 |name-list-style=amp | contribution=Punctuated equilibria: An alternative to phyletic gradualism. | pages=82–115
| editor=Schopf, T. J. M. | title=Models in Paleobiology | publisher=Freeman, Cooper & Co.
| location=San Francisco, CA.
}}</ref> On the other hand, around the same time Wyatt Durham and [[Martin Glaessner]] both argued that the animal kingdom had a long [[Proterozoic]] history that was hidden by the lack of fossils.<ref name="Bengtson2004"/><ref>{{cite journal 
| author=Durham, J. W. | year=1971 | title=The fossil record and the origin of the Deuterostomata | journal=Proceedings of the North American Paleontological Convention, Part H | pages=1104–1132 
}} and {{Cite book | author=Glaessner, M.F. | year=1972 | contribution=Precambrian palaeozoology | pages=43–52 
| editor=Jones, J. B. |editor2=McGowran, B. | title=Stratigraphic Problems of the Later Precambrian and Early Cambrian | volume=1 | publisher=University of Adelaide 
}}</ref>

==Occurrence==
Rich collections have been found in China, Russia, [[Mongolia]], [[Kazakhstan]], Australia, and Antarctica; and moderately [[Biodiversity|diverse]] ones in India, [[Pakistan]], [[Iran]], Europe and North America.<ref name="Bengtson2004"/> There are different views about the time range of the SSFs. The Russian discoveries of the late 1960s were assigned to the [[Tommotian]] [[age (geology)|age]] of the [[Cambrian]] [[period (geology)|period]], and for some time the term "small shelly fauna" was applied only to that age. On the other hand, Bengston includes in "SSF" [[Ediacaran]] fossils like ''[[Cloudina]]'' and post-Tommotian fossils like ''[[Microdictyon]]'' from the [[Maotianshan Shales]] [[lagerstätte]].<ref name="Bengtson2004"/> SSFs have been found in layers that also contain fossil [[trilobite]]s.<ref>{{cite journal | title=Small shelly fossils from the basal Emigrant Formation (Cambrian, uppermost Dyeran Stage) of Split Mountain, Nevada | author=Skovsted, C.B. | journal=Canadian Journal of Earth Sciences | volume=43 | pages=487–496 | date=April 2006 | doi=10.1139/E05-119 | url=https://cdnsciencepub.com/doi/10.1139/e05-119 | issue=4 | bibcode=2006CaJES..43..487S | url-access=subscription }}</ref> The [[mass extinction]] at the end of the Cambrian period's [[Botomian]] [[geologic timescale|age]] was thought to have wiped out most of the SSF, with the exception of the halkieriids, wiwaxiids and ''Pojetaia''.<ref>e.g. {{cite journal | author=Porter, S.M. |date=May 2004 | title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia | journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590 | doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2 |bibcode=2004JPal...78..574P |citeseerx=10.1.1.573.6134 |s2cid=131557288 }}</ref>

==Mode of preservation==
Small shelly fossils are typically, although not always, preserved in [[phosphate]]. Whilst some shellies were originally phosphatic, in most cases the phosphate represents a replacement of the original calcite.<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. | 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| year = 2010 | doi = 10.1130/B30346.1| bibcode = 2010GSAB..122.1731M }}</ref> They are usually extracted from limestone by placing the limestone in a weak acid, typically [[acetic acid]]; the phosphatized fossils remain after the rock is dissolved away.<ref name=Dzik2007/> Preservation of microfossils by phosphate seems to have become less common after the early Cambrian, perhaps as a result of increased [[bioturbation|disturbance of sea-floors by burrowing animals]].<ref name="Maloof2010"/> Without this fossil-forming mode, many small shelly fossils may not have been preserved &ndash; or been impossible to extract from the rock; hence the animals that produced these fossils may have lived beyond the early Cambrian – the apparent extinction of most SSFs by the end of the [[Cambrian]] may be an illusion.<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 |chapter-url=http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf | access-date=2008-08-01 }}</ref><ref name="Porter2004ClosingPhosphatizationWindow"/><ref name="Dzik1994"/> For decades it was thought that [[halkieriid]]s, whose "armor plates" are a common type of SSF, perished in the end-[[Botomian]] [[mass extinction]]; but in 2004 halkieriid armor plates were reported from Mid Cambrian rocks in Australia, a good 10&nbsp;million years more recent than that.<ref name="Porter2004HalkieriidsMidCambrianAustralia">{{cite journal
| author=Porter, S.M. |date=May 2004 
| title=Halkieriids in Middle Cambrian Phosphatic Limestones from Australia 
| journal=Journal of Paleontology | volume=78 |issue=3 | pages=574–590 
| doi=10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2
|bibcode=2004JPal...78..574P 
| issn=0022-3360 |citeseerx=10.1.1.573.6134 |s2cid=131557288 
}}</ref>

==Minerals used in shells==
Small shelly fossils are composed of a variety of minerals, the most important being [[silica]], [[calcium phosphate]] and [[calcium carbonate]].<ref name="Bengtson2004"/> The minerals used by each organism are influenced by the chemistry of the oceans the organism first evolved in, but then continue to be used even if the [[ocean chemistry]] changes. For example, in the [[Ediacaran]] period and the [[Nemakit–Daldynian]] [[geologic timescale|age]] of the [[Cambrian]], those animals that used [[calcium carbonate]] used the form called [[aragonite]]. On the other hand, animals that first appeared in the following [[Tommotian]] age used another form, [[calcite]].<ref name=Porter2007>{{cite journal
 | author = Porter, S.
 | year = 2007
 | title = Seawater Chemistry and Early Carbonate Biomineralization
 | journal = Science
 | volume = 316
 | issue = 5829
 | page = 1302
 | doi = 10.1126/science.1137284
 | pmid = 17540895
 | bibcode=2007Sci...316.1302P
| s2cid = 27418253
 }}</ref>

A recently discovered modern [[gastropod]] that lives near deep-sea [[hydrothermal vent]]s illustrates the influence of both earlier and contemporary local chemical environments: its shell is made of [[aragonite]], which is found in the earliest fossil [[mollusc]]s; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides [[pyrite]] and [[greigite]], which had never previously been found in any [[metazoan]] but whose ingredients are emitted in large quantities by the vents.<ref name="Bengtson2004"/>

Methods of constructing shells vary widely among the SSF, and in most cases the exact mechanisms are not known.<ref name="Bengtson2004"/>

==Evolution of skeletons and biomineralization==
{| class="wikitable" style="float:right; margin-left:2px;"
|- valign="top"
| rowspan="2" colspan="2" | &nbsp; 
| colspan="2" style="text-align:center;"| Biomineralized
|- style="vertical-align:top; text-align:center;"
|| No || style="text-align:center;"| Yes
|- valign="top"
| style="vertical-align:middle;" rowspan="2"| Skeleton 
| No 
| ''[[Dickinsonia]]''<ref name=Retallack2007>{{cite journal
  | author = Retallack, G.J. | year = 2007
  | title = Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil
  | journal = Alcheringa: An Australasian Journal of Palaeontology | volume = 31 | issue = 3 | pages = 215–240
  | url = http://www.informaworld.com/index/781217204.pdf | access-date = 2007-11-24 
  | doi = 10.1080/03115510701484705
  | bibcode = 2007Alch...31..215R
 | s2cid = 17181699
 }}</ref>
| ''[[Halkieria]]'' sclerites<ref name="ConwayMorrisPeel1990">{{cite journal 
  | title=Articulated halkieriids from the Lower Cambrian of north Greenland 
  |author1=Conway Morris, S. |author2=Peel, J.S. |name-list-style=amp | journal=Nature | volume=345 | pages=802–805 
  |date=June 1990 | doi=10.1038/345802a0 
  | issue=6278 | bibcode=1990Natur.345..802M
|s2cid=4324153 }}</ref>
|- valign="top"
| Yes 
| ''[[Kimberella]]''<ref name=Fedonkin2007>{{The Rise and Fall of the Ediacaran Biota |author=Fedonkin, M.A. |author2=Simonetta, A |author3=Ivantsov, A.Y. |chapter=New data on ''Kimberella'', the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications |pages=157–179 |doi=10.1144/SP286.12}}</ref>
| [[Helcionellid]]s<ref name="RunnegarPojeta1974" />
|}
{{biomineralization sidebar|exoskeletons}}

[[Biomineralization]] is the production of mineralized parts by organisms. Hypotheses to explain the evolution of biomineralization include physiological adaptation to changing chemistry of the oceans, defense against predators and the opportunity to grow larger. The functions of biomineralization in SSFs vary: some SSFs are not yet understood; some are components of armor; and some are [[skeleton]]s. A skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is [[biomineralization|biomineralized]]. Although some SSFs may not be skeletons, SSFs are biomineralized by definition, being shelly. Skeletons provide a wide range of possible advantages, including: protection, support, attachment to a surface, a platform or set of levers for muscles to act on, traction when moving on a surface, food handling, provision of [[filtration]] chambers and storage of essential substances.<ref name="Bengtson2004"/>

It has often been suggested that biomineralization evolved as a response to an increase in the concentration of [[calcium]] in the seas, which happened around the [[Ediacaran]]–[[Cambrian]] boundary, and that biomineralization's main benefit was to store harmlessly minerals that might have disrupted organisms' internal processes. For example, [[Mikhail A. Fedonkin]] suggested that an increase in the length of [[food chain]]s may have contributed, as animals higher up the food chain accumulate greater amounts of waste products and toxins relative to their size, and biomineralization may have been a way of isolating excess carbonates or silicates consumed with prey.<ref name="Fedonkin2003OriginOfMetazoa" /> However, biomineralizing a skeleton is a fairly expensive way to dispose safely of excess minerals, as the main construction cost is the organic matrix, mostly [[protein]]s and [[polysaccharide]]s, with which minerals are combined to form [[composite material]]s.<ref name="Bengtson2004"/> The idea that biomineralization was a response to changes in ocean chemistry is also undermined by the fact that small shelly fossils made of [[calcite]], [[aragonite]], [[calcium phosphate]] and [[silica]] appeared virtually simultaneously in a range of environments.<ref name="Dzik2007" />

Organisms started burrowing to avoid [[predation]] at around the same time. Jerzy Dzik suggested that biomineralization of skeletons was a defense against predators, marking the start of an [[evolutionary arms race]].<ref name="Dzik2007" /> He cited as another example of hardened defenses from this time the fact that the earliest protective "skeletons" included glued-together collections of inorganic objects — for example the early Cambrian worm ''Onuphionella'' built a tube covered with [[mica]] flakes.<ref>{{cite journal 
|author1=Signor, P.W. |author2=McMenamin, M.A.S. |name-list-style=amp |year=1988 
| title=The Early Cambrian worm tube ''Onuphionella'' from California and Nevada | journal =Journal of Paleontology | volume=62 | pages=233–240 
| issue=2 
| jstor=1305228 |doi=10.1017/S0022336000029863 |bibcode=1988JPal...62..233S |s2cid=133260258 }}</ref> Such a strategy required both anatomical adaptations that allowed organisms to collect and glue objects and also moderately sophisticated nervous systems to co-ordinate this behavior.<ref name="Dzik2007" />

On the other hand, Bernard Cohen argued that biomineralized skeletons arose for "engineering" reasons rather than as defenses. There are many other defensive strategies available to prey animals including mobility and acute senses, chemical defenses, and concealment. Mineral-organic composites are both stronger and take less energy to build than all-organic skeletons, and these two advantages would have made it possible for animals to grow larger and, in some cases, more muscular. In animals beyond a certain size, the larger muscles and their greater [[lever]]age produce forces all-organic skeletons are not rigid enough to withstand. The development of modern [[brachiopod]]s includes a progression from all-organic to mineral-organic composite shells, which may be a [[Ontogeny recapitulates phylogeny|clue to their evolutionary development]]. The evolution of rigid biomineralized exoskeletons may then have started an arms race in which predators developed drills or chemical weapons capable of penetrating shells, some prey animals developed heavier, tougher shells, etc.<ref>{{cite journal 
| author=Cohen, B.L. | year=2005 
| title=Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna 
| journal=Biological Journal of the Linnean Society | volume=85 | issue=4 | pages=483–490 
| url=http://eprints.gla.ac.uk/2933/01/Cohen_2933.pdf | access-date=2008-08-01 
| doi=10.1111/j.1095-8312.2005.00507.x 
| doi-access=free}}</ref>

Fedonkin suggested another explanation for the appearance of biomineralization around the start of the Cambrian: the [[Ediacara biota]] evolved and flourished in cold waters, which slowed their metabolisms and left them with insufficient spare energy for biomineralization; but there are signs of global warming around the start of the Cambrian, which would have made biomineralization easier. A similar pattern is visible in living marine animals, since biomineralized skeletons are rarer and more fragile in polar waters than in the tropics.<ref name="Fedonkin2003OriginOfMetazoa">{{cite journal | author=Fedonkin, M. A. | journal=Paleontological Research | volume=7 | issue=1 | pages=9–41 | title=The origin of the Metazoa in the light of the Proterozoic fossil record | year=2003 | url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf | access-date=2008-08-24 | doi=10.2517/prpsj.7.9 | bibcode=2003PalRe...7....9F | s2cid=55178329 | archive-url=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf | archive-date=2009-02-26 }}</ref>

==Evolutionary significance==
In some locations, up to 20% of ''[[Cloudina]]'' fossils show borings, holes that are thought to have been made by predators.<ref name="Hua2003BoringsInCloudinaShells">{{Cite journal |author = Hua, H. |author2=Pratt, B.R. |author3=Zhang, L.U.Y.I. | year = 2003 | journal = PALAIOS | title = Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic | doi = 10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2 | volume = 18 | issn = 0883-1351 | issue = 4–5 |page=454 |bibcode=2003Palai..18..454H |s2cid=131590949 }}</ref><ref name="Bengtson1992PredatorialBorings">{{cite journal | author = Bengtson, S. |author2=Zhao, Y. | date = 1992-07-17 | title = Predatorial Borings in Late Precambrian Mineralized Exoskeletons | journal = Science | volume = 257 | issue = 5068 | pages = 367–9 | doi = 10.1126/science.257.5068.367 | pmid = 17832833 |bibcode = 1992Sci...257..367B |s2cid=6710335 }}</ref> The very similar shelly fossil ''[[Sinotubulites]]'', which is often found in the same locations, was not affected by borings. In addition, the distribution of borings in ''Cloudina'' suggests selection for size – the largest holes appear in the largest shells. This evidence of ''selective'' attacks by predators suggests that new species may have arisen in response to predation, which is often presented as a potential cause of the rapid diversification of animals in the early Cambrian.<ref name="Bengtson1992PredatorialBorings" />
[[File:Crown n Stem Groups.svg|thumb|275px|[[Stem group]]s<ref>{{cite journal |author1=Craske, A.J. |author2=Jefferies, R.P.S. |name-list-style=amp | 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 }}</ref>{{colbegin}}
{{ubl |{{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}}
}}{{colend}}]]
The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a more useful insight into the [[Cambrian explosion]] than instances of [[lagerstätte|exceptional preservation]].<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 |pmid  = 21680420 |doi-access= free }}</ref> Although most of the SSFs are difficult to identify, those assigned positions in modern [[taxon|taxa]], or in their [[stem group]]s of evolutionary "aunts" or "cousins", enable scientists to assess the pattern and speed of animal evolution 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) small shellies can be tentatively considered [[diploblastic]], in other words made of two main tissue layers. Later shellies are more convincingly triploblastic, as all "higher" animals are. Subsequently, the [[Helcionellid]]s are the first shelly fossils that can be placed in the stem group to a phylum ([[mollusca]]). As one looks at more recent SSFs, the arguments for stem group placements become stronger, and by the [[Atdabanian]], some SSFs can be assigned to the crown group of a modern phylum, [[echinoderm]]s.<ref name=Budd2003/>
This gives the impression that the first SSF animals, from the late Ediacaran, 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/>

==Types of small shelly fossil==
{{Further | List of small shelly fossils}}

===Ediacaran forms===
{{multiple image
| image1 = Cloudina.svg| width1 = 98 | caption1 = ''[[Cloudina]]''
| image2 = Namacalathus.svg| width2 = 107 |caption2 = Computer model of ''[[Namacalathus]]''
}}
The few collections of SSF from the [[Ediacaran]] period have a limited range of forms<ref name="Bengtson2004"/> Fully and partially mineralized tubes are common and form a really mixed collection: the structures and compositions of their walls vary widely; specimens have been classified as members of a wide range of [[cladistics|clades]] including [[foraminifera]]ns, [[cnidarians]], [[polychaete]] and [[Siboglinidae|pogonophoran]] [[annelid]]s, [[sipunculid]]s and others.<ref name="Bengtson2004"/> ''[[Cloudina]]'''s "tube", which was {{convert|8|to|150|mm|in}} long, consisted of nested cones that were [[biomineralization|mineralized]] with [[calcium carbonate]] but left unmineralized gaps between the cones.<ref name="Grant1990Cloudina">{{cite journal | author = Grant, S.W. | year = 1990 | title = Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic | journal = American Journal of Science | issue = 290–A | pages = 261–294 | url = http://lib.bioinfo.pl/pmid:11538690 | access-date = 2008-07-19 | pmid = 11538690 | volume = 290-A | archive-url = https://web.archive.org/web/20110522041703/http://lib.bioinfo.pl/pmid:11538690 | archive-date = 2011-05-22 }}</ref><ref name=Germs1972>{{cite journal
| doi = 10.2475/ajs.272.8.752
| author = Germs, G. J. |date=October 1972 | title = New shelly fossils from Nama Group, South West Africa
| journal = American Journal of Science | volume = 272 | pages = 752–761 
| issue = 8
| bibcode = 1972AmJS..272..752G}}</ref> ''[[Sinotubulites]]'' built long thin tubes that were more flexible but probably had mineralized ridges.<ref name="ChenBengtsonEtAl2007TubeStructureOfSinotubulites">{{cite journal
| year=2007 |author1=Chen, Z. |author2=Bengtson, S. |author3=Zhou, C.-M. |author4=Hua, H. |author5=Yue, Z.
| title=Tube structure and original composition of Sinotubulites: shelly fossils from the late Neoproterozoic in southern Shaanxi, China
| journal=Lethaia |doi=10.1111/j.1502-3931.2007.00040.x
| volume=41 | page=37 
}}</ref>

''[[Namapoikia]]'' was probably either a [[sponge]] or a [[coral]]-like organism, and built dwellings up to {{convert|1|m|in}} across out of calcium carbonate.<ref name="Wood2002Namapoikia">{{cite journal 
| title=Proterozoic Modular Biomineralized Metazoan from the Nama Group, Namibia
| author1=Wood, R.A. |author2=Grotzinger, J.P. |author3=Dickson, J.A.D. | journal=Science | date=28 June 2002 
| pages=2383–6 | doi=10.1126/science.1071599
| volume=296 
| pmid=12089440 
| issue=5577 |bibcode = 2002Sci...296.2383W | s2cid=9515357 }}</ref>

[[Spicule (sponge)|Spicule]]s are spines or star-like combinations of spines, made of [[silica]], and are thought to be the remains of [[sponge]]s.<ref name="Bengtson2004"/><ref>{{cite journal 
 |author=Brasier, M.D. |author2=Green, O. |author3=Shields, G.
| title=Ediacarian sponge spicule clusters from southwestern Mongolia and the origins of the Cambrian fauna
| journal=Geology |date=April 1997 | volume=25 | issue=4 | pages=303–306 
| url=http://geology.geoscienceworld.org/cgi/content/abstract/25/4/303 | access-date=2008-07-19 
| doi=10.1130/0091-7613(1997)025<0303:ESSCFS>2.3.CO;2 
|bibcode = 1997Geo....25..303B | url-access=subscription }}</ref><ref>{{cite journal 
| title=Long-Expected Sponges from the Neoproterozoic Ediacara Fauna of South Australia
| author1=Gehling, J.G. |author2=Rigby, J.K. |name-list-style=amp | journal=Journal of Paleontology | volume=70 
| issue=2 |date=March 1996 | pages=185–195 
| jstor=1306383 
| doi=10.1017/S0022336000023283 | bibcode=1996JPal...70..185G | s2cid=130802211 }}</ref>

''[[Namacalathus]]'', which may have been a [[cnidarian]], closely related to [[jellyfish]] and [[coral]]s, built [[goblet]]-like dwellings with stalks up to {{convert|30|mm|in}} long. This type of shape is known as a "stalked test", since "test" in biology means a roughly spherical shell.<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
| journal = Paleobiology | volume = 26 | issue = 3 | pages = 334–359
| url = http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/334 | access-date=2008-07-19 
| doi = 10.1666/0094-8373(2000)026<0334:CMITSR>2.0.CO;2
|bibcode=2000Pbio...26..334G |s2cid=52231115 | issn = 0094-8373
| url-access = subscription }}</ref>

===Cambrian forms===
In finds from the early Cambrian, tubes and spicules become more abundant and diverse, and new types of SSF appear. Many have been attributed to well-known groups such as [[mollusc]]s, slug-like [[halkieriid]]s, [[brachiopods]], [[echinoderm]]s, and [[onychophora]]n-like organisms that may have been close to the ancestors of arthropods.<ref name="Bengtson2004"/> A multitude of problematic tubular fossils, such as [[anabaritid]]s, [[Hyolithellus]] or [[Torellella]] characterize the earliest Cambrian Small Shelly Fossil skeletal assemblages.

[[File:Halkieria2.jpg| thumb | right | 150px |Fossil of ''[[Halkieria]]'', showing numerous [[sclerite]]s on the sides and back, and the cap-like shells at both ends.]]
Most of the Cambrian SSF consists of [[sclerites]], fragments that once made up the external armor of early animals, such as ''[[Halkieria]]''<ref name=ConwayMorris1995>{{cite journal
|author1=Conway Morris, S. |author2=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 B]]
| volume = 347
| pages = 305–358
| doi = 10.1098/rstb.1995.0029
| issue = 1321
|bibcode=1995RSPTB.347..305C }}</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 Linnean Society
| volume = 495
| issue = 119
| pages = 69–82
| doi = 10.1006/zjls.1996.0065
| doi-access = free
}}</ref> Fairly complete and assembled sets, which are rare, are called "scleritomes". In many cases the body shapes of sclerites' creators and the distribution of sclerites on their bodies are not known.<ref name="Bengtson2004"/> The "coat of [[Mail (armour)|mail]]" generally disintegrated once the animal died, and its fragments became dispersed and sometimes fossilized. Reconstructing these elements usually relies upon a fully articulated fossil being found in an exceptionally preserved [[lagerstätte]].<ref name=Budd2003/> Such discoveries may in turn enable paleontologists to make sense of other similar fragments, such as those labelled ''[[Maikhanella]]''.<ref>{{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
| issue = 4
| bibcode = 1992Letha..25..401B
}}</ref>

Many sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue and which show no evidence of accretionary growth. It is not clear whether coelosclerites [[convergent evolution|evolved independently]] in different groups of animals or were inherited from a common ancestor.<ref name="Bengtson2004"/>
[[Halkieriid]]s produced scale- or spine-shaped coelosclerites, and complete specimens show that the animals were [[slug]]-shaped, and had cap-shaped shell plates at both ends in addition to the sclerites.<ref name="Bengtson2004"/><ref name="ConwayMorris1995" /> [[chancelloriidae|Chancelloriids]] produced star-shaped composite coelosclerites. They are known to have been animals that looked like [[cactus|cacti]] and have been described as internally like sponges,<ref name="Bengtson2004"/> although they may have been more closely related to halkieriids.<ref name="Porter2008ChancelloriidsHalkieriids">{{cite journal 
| title=Skeletal microstructure indicates Chancelloriids and Halkieriids are closely related 
| author=Porter, S.M | journal=Palaeontology | volume=51 | issue=4 | year=2008 | pages=865–879 
| doi=10.1111/j.1475-4983.2008.00792.x 
| bibcode=2008Palgy..51..865P | url=http://www.geol.ucsb.edu/faculty/porter/Papers/Coeloscleritophora.pdf | access-date=2008-08-07 
| doi-access=free }}</ref>
[[File:Water current Yochelcionella by geyer svg hariadhi.svg| thumb | right |The tiny [[Helcionellid]] fossil ''[[Yochelcionella]]'' is thought to be an early [[mollusc]]. This restoration shows water flowing in under the shell, over the gills and out through the "exhaust pipe".]]

Some sclerites are mineralized with [[calcium phosphate]] rather than [[calcium carbonate]].<ref name="Bengtson2004"/> Tommotiids have a wide range of sclerite shapes and internal structures, and may in fact represent a [[polyphyletic]] set of lineages, in other words they may have independently developed phosphatic scleritomes rather than inheriting them from a common ancestor. On the other hand, they may be closely related to the ancestors of modern [[brachiopod]]s, animals that at first sight look like [[bivalve]] [[mollusc]]s, but brachiopods stand on fleshy stalks and their internal anatomy is different.<ref>{{cite journal 
| title= The Early Cambrian tommotiid ''Micrina'', a sessile bivalved stem group brachiopod |author=Holmer, L.E. |author2=Skovsted, C.B |author3=Brock, G.A. |author4=Valentine, J.L. |author5=Paterson, J. R.
| journal=  Biology Letters|date=June 2008
| url= | doi=10.1098/rsbl.2008.0277 
| volume= 4 
| pages= 724–8 
| pmid= 18577500 
| issue= 6 
| pmc= 2614141
}} There is a reconstruction of ''Micrina'' at {{cite web | url=http://catalogue-of-organisms.blogspot.com/2008/06/back-to-scleritome-tommotiids-revealed.html 
| title=Catalogue of Organisms: Back to the Scleritome – Tommotiids Revealed! | access-date=2008-07-23
}}</ref><ref>{{cite journal 
| title=The scleritome of ''Eccentrotheca'' from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny
 |author=Skovsted, C. B |author2=Brock, G. A. |author3=Paterson, J. R. |author4=Holmer, L. E. |author5=Budd, G. E.
| journal=Geology |date=February 2008 | volume=36 | issue=2 | pages=171–174 
| url=http://geology.geoscienceworld.org/cgi/content/abstract/36/2/171 | access-date=2008-07-23
| doi=10.1130/G24385A.1
| bibcode=2008Geo....36..171S
| url-access=subscription }}</ref> Some sclerites and small pieces of "debris" are regarded as the remains of [[echinoderm]]s.<ref name="LandingEtAl2002LatestEarlyCambrianSSFsQuébec" /> Other phosphatic sclerites include tooth-shaped, hook-shaped and plate-like objects, mostly of unknown origin. However it is known that some, including ''[[Microdictyon]]'', were produced by [[lobopod]]s, animals that looked like worms with legs and are thought to be close to the ancestors of [[arthropod]]s.<ref name="Bengtson2004"/>

[[Univalve]]d and [[bivalve]]d shells are fairly common. Some cap-shaped shells are thought to be the only sclerite covering their creators,<ref name=Budd2003 /> while others are known to be parts of a more complex armor system like ''[[Halkieria]]'''s.<ref name="ConwayMorris1995" /> The [[Helcionellid]]s are thought to be early [[mollusc]]s with somewhat snail-like shells. Some have horizontal "exhaust pipes" on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards.<ref name="RunnegarPojeta1974">{{cite journal 
|author1=Runnegar, B. |author2=Pojeta, J. |name-list-style=amp | year=1974 
| title=Molluscan phylogeny: the paleontological viewpoint 
| journal=Science | volume=186 | issue=4161 | pages=311–7 | doi=10.1126/science.186.4161.311 
| pmid=17839855 | bibcode=1974Sci...186..311R
|s2cid=46429653 }}</ref><ref>{{Cite book 
| author=Peel, J.S. | year=1991 | contribution=Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca | editor=Simonetta, A.M. |editor2=Conway Morris, S 
| title=The Early Evolution of Metazoa and the Significance of Problematic Taxa | isbn=978-0-521-40242-2
| publisher=Cambridge University Press |pages=157–177 
}}</ref> [[Hyolith]]s left small conical shells.<ref name="LandingEtAl2002LatestEarlyCambrianSSFsQuébec" /> These animals may have been molluscs or worm-like [[Sipuncula]]. Other molluscan univalved shells have been found in Canada.<ref name="LandingEtAl2002LatestEarlyCambrianSSFsQuébec" /> Some bivalve shells have been found with both parts still joined, and include both [[brachiopod]]s and [[bivalve]] molluscs.<ref name="LandingEtAl2002LatestEarlyCambrianSSFsQuébec" /> Fossils have been found that resemble the [[Operculum (gastropod)|opercula]] ("lids") used by snails to close the openings in their armor, and are attributed to [[hyolith]]s, small animals that had conical shells and may have been molluscs or worm-like [[Sipuncula]].<ref name="LandingEtAl2002LatestEarlyCambrianSSFsQuébec">{{cite journal 
| title=Latest Early Cambrian Small Shelly Fossils, Trilobites, and Hatch Hill Dysaerobic Interval on the Québec Continental Slope
 |author=Landing, E. |author2=Geyer, G. |author3=Bartowski, K.E. | journal=Journal of Paleontology |date=March 2002 | issue=2 | pages=287–305| doi=10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2
| url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/76/2/287 | access-date=2008-07-30 
| volume=76 
|bibcode=2002JPal...76..287L | issn=0022-3360  | url-access=subscription }}</ref>

Small [[arthropod]]s with bivalve-like shells have been found in early Cambrian beds in China,<ref>{{cite journal|last=Yuan|first=X. |author2=Xiao, S. |author3=Parsley, R. L. |author4=Zhou, C. |author5=Chen, Z. |author6=Hu, J.|date=April 2002|title=Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition|journal=Geology|volume=30|issue=4|pages=363–366| doi=10.1130/0091-7613(2002)030<0363:TSIAEC>2.0.CO;2|issn=0091-7613|bibcode = 2002Geo....30..363Y  }}</ref> and other fossils (''[[Mongolitubulus]] henrikseni'') represent spines that snapped off bivalved arthropod carapaces.<ref name=Skovsted2005>{{Cite journal| last1 = Skovsted | first1 = C.| title = A carapace of the bradoriid arthropod ''Mongolitubulus'' from the Early Cambrian of Greenland| journal = GFF| volume = 127| issue = 3| page = 217| year = 2005| doi = 10.1080/11035890501273217| bibcode = 2005GFF...127..217S| s2cid = 129309039}}</ref>

=== Post-Cambrian forms ===

SSFs after the Cambrian start to pick up more recognizable and modern groups. By the mid-Ordovician, the majority of SSFs simply represent larval molluscs, mostly gastropods.<ref>Dzik, J. 1994. Evolution of "small shelly fossils" assemblages of the early Paleozoic. Acta Palaeontologica Polonica, 39, 247–313.</ref>

== See also ==
* [[Carinachitidae]]
* [[Minibeast]]
* ''[[Onychomicrodictyon]]''

== Notes ==

{{reflist|2}}

==External links==
*[https://web.archive.org/web/20020408001444/http://www.peripatus.gen.nz/Paleontology/CamExp.html Cambrian explosion]
*[https://web.archive.org/web/20020717131651/http://www.peripatus.gen.nz/Paleontology/SmaSheFau.html Small shelly fauna]
*[http://hoopermuseum.earthsci.carleton.ca/camex/1jtommo.html Tommotian fauna, images]

{{Cambrian footer}}

[[Category:Prehistoric marine animals]]
[[Category:Ediacaran life]]
[[Category:Fossil record of animals]]
[[Category:Index fossils]]
[[Category:Cambrian fossil record]]
[[Category:Ediacaran fossil record]]
[[Category:Cambrian invertebrates]]
[[Category:Ediacaran first appearances]]
[[Category:Cambrian extinctions]]