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===Volvation===
[[File:Flexicalymene meeki cephalon view.JPG|thumb|An enrolled phacopid trilobite ''[[Flexicalymene]] meeki'' from the Upper Ordovician of Ohio]]
Trilobite fossils are often found "enrolled" (curled up) like modern [[Armadillidiidae|pill bugs]] for protection; evidence suggests enrollment ("[[volvation]]") helped protect against the inherent weakness of the [[Arthropod cuticle#Mechanical properties|arthropod cuticle]] that was exploited by [[Anomalocarididae|anomalocarid]] predators.<ref name="Nedin1999">{{citation |author=Nedin, C. |year=1999 |title=''Anomalocaris'' predation on nonmineralized and mineralized trilobites |journal=[[Geology (journal)|Geology]] |volume=27 |issue=11 |pages=987–990 |doi=10.1130/0091-7613(1999)027<0987:APONAM>2.3.CO;2 |bibcode=1999Geo....27..987N}}</ref> The earliest evidence of volvation is a little over 510 million years old and has been found in [[Olenellidae]], but these forms didn't have any of the interlocking mechanisms found in later trilobites.<ref>[https://www.cam.ac.uk/research/news/early-rollers-scientists-pinpoint-very-first-enrolling-animal Early rollers: scientists pinpoint very first ‘enrolling’ animal]</ref>
Some trilobites achieved a fully closed capsule (e.g. ''[[Phacops]]''), while others with long pleural spines (e.g. ''[[Selenopeltis]]'') left a gap at the sides or those with a small pygidium (e.g. ''[[Paradoxides]]'') left a gap between the cephalon and pygidium.<ref name=Whittington97a /> In ''Phacops'', the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure.<ref name=Bruton03a /> The doublure carries a [[Heinz Christian Pander|Panderian]] notch or protuberance on each segment to prevent over rotation and achieve a good seal.<ref name=Bruton03a /> Even in an agnostid, with only 2 articulating thoracic segments, the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition.<ref name=Bruton&Nakrem05>{{citation |last1=Bruton |first1=D. L. |last2=Nakrem |first2=H. A. |year=2005 |title=Enrollment in a Middle Ordovician agnostoid trilobite |journal=Acta Palaeontologica Polonica |volume=50| edition=3 |pages=441–448 |url=http://app.pan.pl/archive/published/app50/app50-441.pdf| access-date=June 22, 2009}}</ref>
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