Editing Chelicerata (section)

===Fossil record===
There are large gaps in the chelicerates' [[fossil]] record because, like all [[arthropod]]s, their [[exoskeleton]]s are [[organic matter|organic]] and hence their fossils are rare except in a few [[lagerstätte]]n where conditions were exceptionally suited to preserving fairly soft tissues. The [[Burgess shale]] animals like ''[[Sidneyia]]'' from about {{ma|505}} have been classified as chelicerates, the latter because its appendages resemble those of the [[Xiphosura]] (horseshoe crabs). However, [[cladistic]] analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whether ''[[Fuxianhuia]]'' from earlier in the [[Cambrian]] period, about {{ma|525}}, was a chelicerate. Another Cambrian fossil, ''[[Kodymirus]]'', was originally classified as an [[aglaspid]] but may have been a [[eurypterid]] and therefore a chelicerate. If any of these was closely related to chelicerates, there is a gap of at least 43&nbsp;million years in the record between true chelicerates and their nearest not-quite chelicerate relatives.<ref name="old">{{citation
| author=Wills, M.A.
| title=How good is the fossil record of arthropods? An assessment using the stratigraphic congruence of cladograms
| journal=Geological Journal | volume=36 | issue=3–4 | pages=187–210 | year=2001
| doi=10.1002/gj.882
| s2cid=86279530
| doi-access=free
| bibcode=2001GeolJ..36..187W
}}</ref>
[[File:20191003 Mollisonia plenovenatrix side.png|thumb|Reconstruction of ''Mollisonia plenovenatrix'', the oldest known arthropod with confirmed chelicerae]]
''[[Sanctacaris]]'', member of the family [[Sanctacarididae]] from the Burgess Shale of [[Canada]], represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age.<ref name="san">{{cite journal|url=https://www.researchgate.net/publication/266683582|title=''Sanctacaris uncata'': the oldest chelicerate (Arthropoda)|first=David A.|last=Legg|journal=Naturwissenschaften|year=2014|volume=101|issue=12|pages=1065–1073|doi=10.1007/s00114-014-1245-4|pmid=25296691|bibcode=2014NW....101.1065L|s2cid=15290784}}</ref> Although its chelicerate nature has been doubted for its pattern of [[tagmosis]] (how the segments are grouped, especially in the head),<ref name="old" /> a restudy in 2014 confirmed its phylogenetic position as the oldest chelicerate.<ref name="san" /> Another fossil of the site, ''[[Mollisonia]]'', is considered a basal chelicerate and it has the oldest known chelicerae and proto-[[book gills]].<ref>{{Cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |date=2019 |title=A middle Cambrian arthropod with chelicerae and proto-book gills |url=https://www.nature.com/articles/s41586-019-1525-4 |journal=Nature |language=en |volume=573 |issue=7775 |pages=586–589 |doi=10.1038/s41586-019-1525-4 |pmid=31511691 |bibcode=2019Natur.573..586A |s2cid=256819092 |issn=1476-4687|url-access=subscription }}</ref>
[[File:Lunataspis aurora MM I-4000A.jpg|left|thumb|232x232px|Holotype of the [[xiphosura]]n ''Lunataspis aurora'']]
The eurypterids have left few good fossils and one of the earliest confirmed eurypterid, ''[[Pentecopterus|Pentecopterus decorahensis]]'', appears in the Middle [[Ordovician]] period {{ma|467.3}}, making it the oldest eurypterid.<ref name=":0">{{citation
|last1=Lamsdell
|first1=James C.
|last2=Briggs
|first2=Derek E. G.
|last3=Liu
|first3=Huaibao
|last4=Witzke
|first4=Brian J.
|last5=McKay
|first5=Robert M.
|year=2015
|title=The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa
|journal=[[BMC Evolutionary Biology]]
|volume=15|issue=1
|
pages=169
|doi=10.1186/s12862-015-0443-9
|pmc=4556007
|pmid=26324341
|doi-access=free
|bibcode=2015BMCEE..15..169L
}}</ref>
Until recently the earliest known [[xiphosura]]n fossil dated from the Late Llandovery stage of the [[Silurian]] {{ma|436|428}},<ref>{{citation
|author1=Moore, R.A. |author2=Briggs, D.E.G. |author3=Braddy, S.J. |author4=Anderson, L.I. |author5=Mikulic, D.G. |author6=Kluessendorf, J. |name-list-style=amp | title=A new synziphosurine (Chelicerata, Xiphosura) from the late Llandovery (Silurian) Waukesha Lagerstaette, Wisconsin, USA
| journal=Journal of Paleontology | date=March 2005| volume=79 | issue=2
| pages=242–250
| doi=10.1666/0022-3360(2005)079<0242:ANSCXF>2.0.CO;2
|bibcode=2005JPal...79..242M |s2cid=56570105 | issn=0022-3360
 }}</ref> but in 2008 an older specimen described as ''[[Lunataspis|Lunataspis aurora]]'' was reported from about {{ma|445}} in the Late [[Ordovician]].<ref>{{citation
|author1=Rudkin, D.M. |author2=Young, G.A. |author3=Nowlan, G.S. |s2cid=62891048 |name-list-style=amp | title=The Oldest Horseshoe Crab: a New Xiphosurid from Late Ordovician Konservat-Lagerstätten Deposits, Manitoba, Canada
| journal=Palaeontology | volume=51 | issue=1 | date=January 2008
| doi=10.1111/j.1475-4983.2007.00746.x
| pages=1–9
| doi-access=free |bibcode=2008Palgy..51....1R }}</ref>

The oldest known [[arachnid]] is the [[trigonotarbid]] ''[[Palaeotarbus]] jerami'', from about {{ma|420}} in the [[Silurian]] period, and had a triangular [[cephalothorax]] and segmented abdomen, as well as eight legs and a pair of [[pedipalp]]s.<ref>{{citation |author=Dunlop, J.A. |title=A trigonotarbid arachnid from the Upper Silurian of Shropshire |date=September 1996 |journal=Palaeontology |volume=39 |issue=3 |pages=605–614 |url=http://palaeontology.palass-pubs.org/pdf/Vol%2039/Pages%20605-614.pdf |access-date=2008-10-12 |archive-url=https://web.archive.org/web/20081216214632/http://palaeontology.palass-pubs.org/pdf/Vol%2039/Pages%20605-614.pdf |archive-date=2008-12-16 |url-status=usurped}} The fossil was originally named ''[[Eotarbus]]'' but was renamed when it was realized that a [[Carboniferous]] arachnid had already been named ''[[Eotarbus]]'': {{citation |author=Dunlop, J.A. |title=A replacement name for the trigonotarbid arachnid ''Eotarbus'' Dunlop |journal=Palaeontology |volume=42 |issue=1 |page=191 |year=1999 |bibcode=1999Palgy..42..191D |doi=10.1111/1475-4983.00068 |s2cid=83825904 |doi-access=free}}</ref>

''[[Attercopus]] fimbriunguis'', from {{ma|386}} in the [[Devonian]] period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider,<ref name="VollrathSelden2007BehaviorInEvolutionOfSpiders">{{citation 
 |author1=Vollrath, F. 
 |author2=Selden, P.A. 
 |title=The Role of Behavior in the Evolution of Spiders, Silks, and Webs 
 |journal=Annual Review of Ecology, Evolution, and Systematics 
 |date=December 2007 
 |volume=38 
 |pages=819–846 
 |doi=10.1146/annurev.ecolsys.37.091305.110221 
 |url=http://homepage.mac.com/paulselden/Sites/Website/ARES.pdf 
 |access-date=2008-10-12 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20081209102852/http://homepage.mac.com/paulselden/Sites/Website/ARES.pdf 
 |archive-date=2008-12-09 
 }}</ref> but it lacked [[spinneret (spider)|spinnerets]] and hence was not a true spider.<ref>{{citation
|author1=Selden, P.A. |author2=Shear, W.A. | title=Fossil evidence for the origin of spider spinnerets
| journal=PNAS | date=July 2008 | doi = 10.1073/pnas.0809174106 | pmid=19104044 | pmc=2634869 | volume=105 |issue=52 | pages=20781–5|bibcode=2008PNAS..10520781S |doi-access=free }}</ref> Rather, it was likely sister group to the spiders, a clade which has been named Serikodiastida.<ref name=Garw>{{cite journal|title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders|first1=Russell J.|last1=Garwood|first2=Jason A.|last2=Dunlop|year=2014|journal=PeerJ|volume=2|pages=e641|doi=10.7717/peerj.641|pmid=25405073|pmc=4232842 |doi-access=free }}</ref> Close relatives of the group survived through to the [[Cretaceous]] Period.<ref name="WangDunlop2018">{{cite journal|last1=Wang|first1=Bo|last2=Dunlop|first2=Jason A.|last3=Selden|first3=Paul A.|last4=Garwood|first4=Russell J.|last5=Shear|first5=William A.|last6=Müller|first6=Patrick|last7=Lei|first7=Xiaojie|title=Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins|journal=Nature Ecology & Evolution|volume=2|issue=4|year=2018|pages=614–622|issn=2397-334X|doi=10.1038/s41559-017-0449-3|pmid=29403075|bibcode=2018NatEE...2..614W |s2cid=4239867|url=https://www.research.manchester.ac.uk/portal/en/publications/cretaceous-arachnid-chimerarachne-yingi-gen-et-sp-nov-illuminates-spider-origins(b82d83bd-6081-4187-acfc-fb7358c33358).html}}</ref> Several [[Carboniferous]] spiders were members of the [[Mesothelae]], a basal group now represented only by the [[Liphistiidae]],<ref name="VollrathSelden2007BehaviorInEvolutionOfSpiders" /> and fossils suggest taxa closely related to the spiders, but which were not true members of the group were also present during this Period.<ref name="GarwoodDunlop2016">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason A.|last3=Selden|first3=Paul A.|last4=Spencer|first4=Alan R. T.|last5=Atwood|first5=Robert C.|last6=Vo|first6=Nghia T.|last7=Drakopoulos|first7=Michael|title=Almost a spider: a 305-million-year-old fossil arachnid and spider origins|journal=Proceedings of the Royal Society B: Biological Sciences|volume=283|issue=1827|year=2016|pages=20160125|issn=0962-8452|doi=10.1098/rspb.2016.0125|pmid=27030415|pmc=4822468|doi-access=free}}</ref>

The Late [[Silurian]] ''[[Proscorpius]]'' has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had [[gill]]s rather than [[book lung]]s or [[Invertebrate trachea|trachea]]e; its mouth was completely under its head and almost between the first pair of legs, as in the extinct [[eurypterid]]s and living [[horseshoe crab]]s.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> Fossils of terrestrial scorpions with [[book lung]]s have been found in Early [[Devonian]] rocks from about {{ma|402}}.<ref>{{citation
| author=Shear, W.A., Gensel, P.G. and Jeram, A.J.
| title=Fossils of large terrestrial arthropods from the Lower Devonian of Canada
| journal=Nature | volume=384 | pages=555–557 | date=December 1996| doi=10.1038/384555a0 | issue=6609
| bibcode=1996Natur.384..555S
| s2cid=4367636
}}</ref> The oldest species of scorpion found as of 2021 is ''[[Dolichophonus|Dolichophonus loudonensis]]'', which lived during the Silurian, in present-day Scotland.<ref>{{Cite journal|last1=Anderson|first1=Evan P.|last2=Schiffbauer|first2=James D.|last3=Jacquet|first3=Sarah M.|last4=Lamsdell|first4=James C.|last5=Kluessendorf|first5=Joanne|last6=Mikulic|first6=Donald G.|date=2021|title=Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12534|journal=Palaeontology|language=en|volume=64|issue=3|pages=429–474|doi=10.1111/pala.12534|bibcode=2021Palgy..64..429A |s2cid=234812878|issn=1475-4983|url-access=subscription}}</ref>