Editing Exoskeleton (section)

== Paleontological significance ==
[[File:BoredEncrustedShell.JPG|thumb|280px|Borings in exoskeletons can provide evidence of animal behaviour. In this case, boring [[sponge]]s attacked this [[hard clam]] shell after the death of the clam, producing the trace fossil ''[[Entobia]]''.]]
Exoskeletons, as hard parts of organisms, are greatly useful in assisting the preservation of organisms, whose soft parts usually rot before they can be fossilized. Mineralized exoskeletons can be preserved as shell fragments. The possession of an exoskeleton  permits a couple of other routes to [[fossil]]ization. For instance, the strong layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.<ref name=Fedonkin2007>{{cite book |author1=M. A. Fedonkin |author2=A. Simonetta |author3=A. Y. Ivantsov |year=2007 |chapter=New data on ''Kimberella'', the Vendian mollusk-like organism (White sea region, Russia): palaeoecological and evolutionary implications |editor=Patricia Vickers-Rich & Patricia |title=The Rise and Fall of the Ediacaran Biota |series=Geological Society of London, Special Publications |volume=286 |issue=1 |location=London |publisher=[[Geological Society]] |pages=157–179 |doi=10.1144/SP286.12 |isbn=978-1-86239-233-5 |oclc=191881597 |bibcode = 2007GSLSP.286..157F |s2cid=331187 }}</ref> Alternatively, [[Lagerstätte|exceptional preservation]] may result in chitin being mineralised, as in the [[Burgess Shale]],<ref name=Butterfield2003>{{cite journal | title = Exceptional fossil preservation and the Cambrian Explosion | year = 2003 | journal = [[Integrative and Comparative Biology]] | volume = 43 | issue = 1 | pages = 166–177 | doi = 10.1093/icb/43.1.166 | author = Nicholas J. Butterfield | pmid=21680421| doi-access = free | citeseerx = 10.1.1.597.6522 }}</ref> or transformed to the resistant polymer [[keratin]], which can resist decay and be recovered.

However, our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already [[Biomineralization|mineralised]] are usually preserved, such as the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone.<ref name=Fedonkin2007/> The most significant limitation is that, although there are 30-plus [[phylum|phyla]] of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilised.<ref name="CowenHistLife">{{cite book | author=Richard Cowen |year=2004 |edition=4th | title=History of Life | publisher=[[Wiley-Blackwell]] |isbn=978-1-4051-1756-2}}</ref>

Mineralized skeletons first appear in the fossil record shortly before the base of the [[Cambrian period]], {{Ma|550}}. The evolution of a mineralised exoskeleton is considered a possible driving force of the [[Cambrian explosion]] of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian ([[Ediacaran]]) [[Ediacara biota|organisms]] produced tough outer shells<ref name=Fedonkin2007/> while others, such as ''[[Cloudinid|Cloudina]]'', had a calcified exoskeleton.<ref name=Hua2003/> Some ''Cloudina'' shells even show evidence of predation, in the form of borings.<ref name=Hua2003>{{Cite journal |author1=Hong Hua |author2=Brian R. Pratt |author3=Lu-yi Zhang | 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 | issue = 4–5 | pages = 454–459|bibcode=2003Palai..18..454H |s2cid=131590949 }}</ref>