Editing Exoskeleton (section)

== Evolution ==
{{biomineralization sidebar|exoskeletons}}
{{Further|Small shelly fauna}}

The fossil record primarily contains mineralized exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started with a non-mineralized exoskeleton which they later mineralized, it is difficult to comment on the very early evolution of each lineage's exoskeleton. It is known, however, that in a very short course of time, just before the Cambrian period, exoskeletons made of various materials – silica, [[calcium phosphate]], [[calcite]], [[aragonite]], and even glued-together mineral flakes – sprang up in a range of different environments.<ref name=Dzik2007>{{cite book |author=J. Dzik |year=2007 |chapter=The Verdun Syndrome: simultaneous origin of protective armor and infaunal shelters at the Precambrian–Cambrian transition |editor=Patricia Vickers-Rich & Patricia |title=The Rise and Fall of the Ediacaran Biota |journal=Geological Society, London, Special Publications |volume=286 |issue=1 |location=London |publisher=[[Geological Society]] |pages=405–414 |doi=10.1144/SP286.30 |isbn=978-1-86239-233-5 |chapter-url=http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf |oclc=191881597 |bibcode=2007GSLSP.286..405D |url-status=live |archive-url=https://web.archive.org/web/20081003122817/http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf |archive-date=2008-10-03 |citeseerx=10.1.1.693.9187 |s2cid=33112819 }}</ref> Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralized, and did not change from this mineral morph - even when it became less favourable.<ref name="Porter2007"/>

Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells,<ref name=Fedonkin2007/> while others, such as ''Cloudina'', had a calcified exoskeleton,<ref name=Hua2003/> but mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the "[[small shelly fauna]]". Just after the base of the Cambrian, these miniature fossils become diverse and abundant – this abruptness may be an illusion since the chemical conditions which preserved the small shells appeared at the same time.<ref name=Dzik1994>{{cite journal| author = J. Dzik| 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| url-status = live| archive-url = https://web.archive.org/web/20081205034112/http://www.paleo.pan.pl/people/Dzik/Dzik1994d.htm| archive-date = 2008-12-05}}</ref> Most other shell-forming organisms appeared during the Cambrian period, with the [[Bryozoa]]ns being the only calcifying phylum to appear later, in the [[Ordovician]]. The sudden appearance of shells has been linked to a change in [[ocean chemistry]] which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However, this is unlikely to be a sufficient cause, as the main construction cost of shells is in creating the [[protein]]s and [[polysaccharide]]s required for the shell's [[composite material|composite structure]], not in the precipitation of the mineral components.<ref name="Bengtson2004" /> Skeletonization also appeared at almost  the same time that animals started [[burrow]]ing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators.<ref name=Dzik2007 />

Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms, the stable calcite and the [[Metastability|metastable]] aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outside this range. When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve.{{cn|date=May 2023}}

Except for the molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry – thus which form was more easily precipitated – at the time that the lineage first evolved a calcified skeleton, and does not change thereafter.<ref name=Porter2007/> However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry – the magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions.<ref name=Kiessling2008>{{cite journal|author1=Wolfgang Kiessling |author2=Martin Aberhan |author3=Loïc Villier | year = 2008| title = Phanerozoic trends in skeletal mineralogy driven by mass extinctions| journal = [[Nature Geoscience]]| doi = 10.1038/ngeo251| volume = 1| pages = 527–530| issue=8|bibcode = 2008NatGe...1..527K }}</ref> A recently discovered<ref name=Waren2003>{{Cite journal | doi = 10.1126/science.1087696 | pmid = 14605361 | year = 2003 |author1=Anders Warén |author2=Stefan Bengtson |author3=Shana K. Goffredi |author4=Cindy L. Van Dover | title = A hot-vent gastropod with iron sulfide dermal sclerites | volume = 302 | issue = 5647 | page = 1007 | journal = [[Science (journal)|Science]]| s2cid = 38386600 }}</ref> modern [[gastropod]] ''[[Chrysomallon squamiferum]]'' that lives near deep-sea [[hydrothermal vent]]s illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which is found in some of the earliest fossil molluscs; but it also has armour plates on the sides of its foot, and these are mineralised with the iron sulfides [[pyrite]] and [[greigite]], which had never previously been found in any [[Animal|metazoan]] but whose ingredients are emitted in large quantities by the vents.<ref name="Bengtson2004"/>

[[File:Cicada exoskeleton - tokyo area - aug 15 2021.webm|thumb|thumbtime=1|Exoskeleton of a cicada]]