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Chitons date as far back as either the Early Ordovician or the Cambrian, not the Devonian like the infobox claimed. |
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{{Automatic taxobox
|taxon=Polyplacophora
|fossil_range={{fossil range|earliest cambrian=|
|image=Tonicella-lineata.jpg
|image_caption=A live lined chiton, ''[[Tonicella lineata]]'' photographed ''[[in situ]]'': The anterior end of the animal is to the right.
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==Evolutionary origins==
Chitons have a relatively good fossil record, stretching back
[[File:Matthevia.JPG|thumb|right|Separate plates from ''Matthevia'', a Late [[Cambrian]] polyplacophoran from the Hellnmaria Member of the Notch Peak Limestone, Steamboat Pass, southern House Range, Utah are shown with a [[cent (United States coin)|US one-cent coin]] (19 mm in diameter).]]
Based on this and co-occurring fossils, one plausible hypothesis for the origin of polyplacophora has that they formed when an aberrant monoplacophoran was born with multiple centres of calcification, rather than the usual one. Selection quickly acted on the resultant conical shells to form them to overlap into protective armour; their original cones are homologous to the tips of the plates of modern chitons.<ref name=Runnegar1974/>
The chitons evolved from [[multiplacophora]] during the Palaeozoic, with their relatively conserved modern-day body plan being fixed by the Mesozoic.<ref name=Vendrasco/>
The earliest fossil evidence of [[aesthete (chiton)|aesthetes]] in chitons comes from around 400 Ma, during the [[Early Devonian]].<ref name=Serb2008/>
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