Editing Chelicerata (section)

==Evolutionary history==
===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>

===Relationships with other arthropods===
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Arthropoda]]
 |1={{clade
 |label1=[[Pancrustacea]]
 |1={{clade
 |1=[[Hexapoda]]
 |2=[[Crustacean|Crustacea]]
 }}
 |label2=[[Paradoxopoda]]
 |2={{clade
 |1=[[Myriapoda]]
 |2=Chelicerata
 }}
 }}
}}
</div>&nbsp;A recent view of chelicerate [[phylogeny]]<ref>{{cite book|vauthors=Giribet G, Edgecombe G|chapter= The Arthropoda: A Phylogenetic Framework|date= April 2013|title=Arthropod Biology and Evolution|pages= 17–40|doi=10.1007/978-3-642-36160-9_2|isbn= 978-3-642-36159-3}}</ref></div>
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px;margin-left:1em">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Arthropoda]]
 |1={{clade
 |1=Chelicerata
 |label2=[[Mandibulata]]
 |2={{clade
 |1=[[Crustacean|Crustacea]]
 |2={{clade
 |1=[[Trilobita]]
 |label2=[[Tracheata]]
 |2={{clade
 |1=[[Hexapoda]]
 |2=[[Myriapoda]]
 }}
 }}
 }}
 }}
}}
</div>&nbsp;A "traditional" view of chelicerate [[phylogeny]]<ref>{{Cite journal|vauthors=Turbeville J, Pfeifer D, Field K, Raff R|title= The phylogenetic status of arthropods, as inferred from 18S rRNA sequences|journal=Molecular Biology and Evolution| volume=8|issue=5|date=September 1991|pages=669–686|doi=10.1093/oxfordjournals.molbev.a040677|pmid= 1766363|doi-access=free}}</ref><ref>{{Cite journal|vauthors=Giribet G, Ribera C|date=2000|title=A Review of Arthropod Phylogeny: New Data Based on Ribosomal DNA Sequences and Direct Character Optimization|journal=Cladistics|volume=16|issue=2|pages=204–231|doi=10.1111/j.1096-0031.2000.tb00353.x|pmid=34902954 |s2cid=84370269}}</ref></div>
The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups ([[crustacea]]ns; [[hexapoda|hexapods]], which includes [[insect]]s; and [[myriapod]]s, which includes [[centipede]]s and [[millipede]]s) than these other groups are to each other. Recent research since 2001, using both [[molecular phylogenetic]]s (the application of cladistic analysis to [[biochemistry]], especially to organisms' [[DNA]] and [[RNA]]) and detailed examination of how various arthropods' [[nervous system]]s develop in the [[embryo]]s, suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives. However, these results are derived from analyzing only living arthropods, and including extinct ones such as [[trilobite]]s causes a swing back to the "traditional" view, placing trilobites as the sister-group of the [[Tracheata]] (hexapods plus myriapods) and chelicerates as least closely related to the other groups.<ref name="Jenner2006ChallengingReceivedWisdoms">{{citation
| author=Jenner, R.A.
| title=Challenging received wisdoms: Some contributions of the new microscopy to the new animal phylogeny
| journal=Integrative and Comparative Biology | volume=46| issue=2 | year=2006| pages=93–103 | doi=10.1093/icb/icj014 | pmid=21672726
| doi-access=free}}</ref>
{{Clear}}

===Major sub-groups===
<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:90%">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=Chelicerata
  |1={{clade
       |1=[[Xiphosura]] (horseshoe crabs) [[File:Limulus polyphemus (aquarium) (white background).jpg|70 px]]
       |2={{clade
            |1=[[Eurypterida]]'''†''' <span style="{{MirrorH}}">[[File:Eurypterus Paleoart (no background).png|70 px]]</span>
            |2=[[Chasmataspidida]]'''†''' [[File:20200606 Chasmataspis laurencii.png|80px]]
          }}
       |label3=[[Arachnida]]
       |3={{clade
           |1={{clade
                |1=[[Scorpiones]] [[File:Buthus_mariefranceae_(10.3897-zookeys.686.12206)_Figure_1.jpg|65px]] 
                |2=[[Opiliones]] (harvestmen) [[File:Phalangium opilio 2 (Nemo5576) (white background).jpg|120px]]
              }}
           |2={{clade
                |1=[[Pseudoscorpiones]] <span style="{{MirrorH}}">[[File:Neobisium sylvaticum 03.png|70px]]</span>
                |2=[[Solifugae]] (sun spiders) [[File:Ammotrecha itzaana 4414721993.png|80px]]
              }}
           |3=[[Palpigradi]] (microwhip scorpions) [[File:Live Eukoenenia spelaea in its cave habitat (no background).png|70px]]
           |4={{clade
                |1=[[Trigonotarbida]]'''†''' [[File:20201202 Trigonotarbus johnsoni.png|80px]]
                |2={{clade
                     |1=[[Araneae]] ([[spider]]s) [[File:Aptostichus simus Monterey County.jpg|70 px]]
                     |2={{clade
                          |1=[[Haptopoda]]'''†''' [[File:20200823 Plesiosiro madeleyi.png|80px]]
                          |2={{clade
                               |1=[[Amblypygi]] (whip spiders) [[File:Flickr - ggallice - Tailless whip-scorpion, La Muerta.png|120px]]
                               |2={{clade
                                    |1=[[Uropygi]] (whip scorpions) [[File:Whip Scorpion body (9672115742) (white background).png|80px]] 
                                    |2=[[Schizomida]] [[File:Brignolizomus woodwardi 175486060.jpg|80px]]
                                  }}
                             }}
                        }}
                   }}
              }}
           |5={{clade
                |1=[[Ricinulei]] (hooded tickspiders) [[File:Ricinulei from Fernandez & Giribet (2015).png|70px]]
                |2=[[Anactinotrichida]] [[File:Ixodes scapularis P1170301a (white background).png|70px]]
                |3=[[Acariformes]] ([[mite]]s) [[File:Rote Samtmilbe Namibia.png|70px]]
              }}
         }}
     }}
}}
</div>Shultz (2007)'s evolutionary family tree of [[arachnid]]s<ref name="Schultz2007ArachnidPhylogeny" /> – '''†''' marks extinct groups.</div>

It is generally agreed that the Chelicerata contain the [[Class (biology)|classes]] [[Arachnida]] ([[spider]]s, [[scorpion]]s, [[mite]]s, etc.), [[Xiphosura]] ([[horseshoe crab]]s) and [[Eurypterida]] (sea scorpions, extinct).<ref name="Schultz2007ArachnidPhylogeny">{{citation
| author=Schultz, J.W.
| title=A phylogenetic analysis of the arachnid orders based on morphological characters
| journal=Zoological Journal of the Linnean Society | year=2007 | volume=150 | pages=221–265
| doi=10.1111/j.1096-3642.2007.00284.x
| issue=2
| doi-access=free
}}</ref> The extinct [[Chasmataspidida]] may be a sub-group within Eurypterida.<ref name="Schultz2007ArachnidPhylogeny" /><ref>{{citation
|author1=O. Tetlie, E. |author2=Braddy, S.J.
|s2cid=73596575
| title=The first Silurian chasmataspid, ''Loganamaraspis dunlopi'' gen. et sp. nov. (Chelicerata: Chasmataspidida) from Lesmahagow, Scotland, and its implications for eurypterid phylogeny
| journal=Transactions of the Royal Society of Edinburgh: Earth Sciences | year=2003 | volume=94
| pages=227–234 | doi=10.1017/S0263593300000638
| issue=3
}}</ref> The [[Pycnogonida]] ([[sea spider]]s) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved.<ref name="PoschmannDunlop2006NewSeaSpider">{{citation
|author1=Poschmann, M. |author2=Dunlop, J.A.
| title=A New Sea Spider (Arthropoda: Pycnogonida) with a Flagelliform Telson from the Lower Devonian Hunsrück Slate, Germany
| journal=Palaeontology | volume=49 | issue=5 | pages=983–989 | year=2006
| doi=10.1111/j.1475-4983.2006.00583.x
| doi-access=free |bibcode=2006Palgy..49..983P
}}</ref>

However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis of [[DNA]] features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY).<ref>{{citation
| author=Gonzalo Giribet G., Edgecombe, G.D., Wheeler, W.C., and Babbitt, C.
| s2cid=16833833
| title=Phylogeny and Systematic Position of Opiliones: A Combined Analysis of Chelicerate Relationships Using Morphological and Molecular Data
| journal=Cladistics | volume=18
| issue=1 | pages=5–70 | year=2002
| pmid=14552352 | doi=10.1111/j.1096-0031.2002.tb00140.x
| doi-access=free
}}</ref> An analysis in 2007 using only anatomical features produced the [[cladogram]] on the right, but also noted that many uncertainties remain.<ref name="Shultz2007ArachnidPhylogeny">{{citation
| author=Shultz, J.W.
| title=A phylogenetic analysis of the arachnid orders based on morphological characters
| journal=Zoological Journal of the Linnean Society | year=2007 | volume=150 | pages=221–265
| doi=10.1111/j.1096-3642.2007.00284.x
| issue=2
| doi-access=free
}}</ref> In recent analyses the clade [[Tetrapulmonata]] is reliably recovered, but other ordinal relationships remain in flux.<ref name="WangDunlop2018"/><ref name="GarwoodDunlop2017">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason A.|last3=Knecht|first3=Brian J.|last4=Hegna|first4=Thomas A.|title=The phylogeny of fossil whip spiders|journal=BMC Evolutionary Biology|volume=17|issue=1|year=2017|issn=1471-2148|doi=10.1186/s12862-017-0931-1|pmid=28431496|doi-access=free|pmc=5399839|page=105|bibcode=2017BMCEE..17..105G }}</ref><ref name="GarwoodDunlop2016"/><ref name="GarwoodDunlop2014">{{cite journal|last1=Garwood|first1=Russell J.|last2=Dunlop|first2=Jason|title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders|journal=PeerJ|volume=2|year=2014|pages=e641|issn=2167-8359|doi=10.7717/peerj.641|doi-access=free|pmid=25405073|pmc=4232842}}</ref><ref name="Giribet2018">{{cite journal|last1=Giribet|first1=Gonzalo|title=Current views on chelicerate phylogeny—A tribute to Peter Weygoldt|journal=Zoologischer Anzeiger|volume=273|year=2018|pages=7–13|issn=0044-5231|doi=10.1016/j.jcz.2018.01.004|bibcode=2018ZooAn.273....7G |s2cid=90344977 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:37308630|url-access=subscription}}</ref><ref name="SharmaKaluziak2014">{{cite journal|last1=Sharma|first1=Prashant P.|last2=Kaluziak|first2=Stefan T.|last3=Pérez-Porro|first3=Alicia R.|last4=González|first4=Vanessa L.|last5=Hormiga|first5=Gustavo|last6=Wheeler|first6=Ward C.|last7=Giribet|first7=Gonzalo|title=Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal|journal=Molecular Biology and Evolution|volume=31|issue=11|year=2014|pages=2963–2984|issn=1537-1719|doi=10.1093/molbev/msu235|pmid=25107551|doi-access=free}}</ref><ref name="BallesterosSharma2019">{{cite journal|last1=Ballesteros|first1=Jesús A|last2=Sharma|first2=Prashant P|last3=Halanych|first3=Ken|title=A Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error|journal=Systematic Biology|volume=68|issue=6|year=2019|pages=896–917|issn=1063-5157|doi=10.1093/sysbio/syz011|pmid=30917194|doi-access=free}}</ref>

The position of scorpions is particularly controversial. Some early fossils such as the Late [[Silurian]] ''[[Proscorpius]]'' have been classified by paleontologists as scorpions, but described as wholly aquatic as they had [[gill]]s rather than [[book lung]]s or [[Invertebrate trachea|trachea]]e. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct [[eurypterid]]s and living [[horseshoe crab]]s.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> This presents a difficult choice: classify ''Proscorpius'' and other aquatic fossils as something other than scorpions, despite the similarities; accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups;<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata" /> or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and other [[arachnid]]s,<ref name="BraddyAldridgeEtAl1999LamellateBookGills" /> so that either scorpions are not arachnids or "arachnids" are not monophyletic.<ref name="Weygoldt1998EvolutionAndSystematicsOfChelicerata">{{citation
| author=Weygoldt, P. | title=Evolution and systematics of the Chelicerata
| journal=Experimental and Applied Acarology | volume=22 | issue=2 | date=February 1998 | pages=63–79
| doi=10.1023/A:1006037525704
| s2cid=35595726
}}</ref> [[Cladistic]] analyses have recovered ''[[Proscorpius]]'' within the scorpions,<ref name="Garw"/> based on reinterpretation of the species' breathing apparatus.<ref>{{cite journal |author1=Jason A. Dunlop |author2=O. Erik Tetlie |author3=Lorenzo Prendini |s2cid=53521811 |doi=10.1111/j.1475-4983.2007.00749.x |title=Reinterpretation of the Silurian scorpion ''Proscorpius osborni'' (Whitfield): integrating data from Palaeozoic and recent scorpions |year=2008 |journal=Palaeontology |volume=51 |issue=2 |pages=303–320|doi-access=free |bibcode=2008Palgy..51..303D }}</ref> This is reflected also in the reinterpretation of ''[[Palaeoscorpius]]'' as a terrestrial animal.<ref>{{cite journal |author1=G. Kühl |author2=A. Bergmann |author3=J. Dunlop |author4=R. J. Garwood |author5=J. Rust |doi=10.1111/j.1475-4983.2012.01152.x|title=Redescription and palaeobiology of ''Palaeoscorpius devonicus'' Lehmann, 1944 from the Lower Devonian Hunsrück Slate of Germany |year=2012 |journal=Palaeontology |volume=55 |issue=4 |pages=775–787|doi-access=free |bibcode=2012Palgy..55..775K }}</ref>

A 2013 phylogenetic analysis<ref>{{Cite journal|last=Lamsdell|first=James C.|date=2013-01-01|title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura|journal=Zoological Journal of the Linnean Society|language=en|volume=167|issue=1|pages=1–27|doi=10.1111/j.1096-3642.2012.00874.x|issn=0024-4082|doi-access=free}}</ref> (the results presented in a cladogram below) on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera ''Eurypterus'', ''Parastylonurus'', ''[[Rhenopterus]]'' and ''[[Stoermeropterus]]'') concluded that the Xiphosura, as presently understood, was [[paraphyletic]] (a group sharing a [[last common ancestor]] but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group [[Sclerophorata]] within the clade [[Dekatriata]] (composed of sclerophorates and [[Chasmataspidida|chasmataspidids]]). This work suggested it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is in turn part of the [[Prosomapoda]], a group including the [[Xiphosurida]] (the only monophyletic xiphosuran group) and other stem-genera. A recent phylogenetic analysis of the chelicerates places the Xiphosura within the Arachnida as the sister group of Ricinulei,<ref name="BallesterosSharma2019" /><ref>{{cite journal |last1=Sharma |first1=Prashant P. |last2=Gavish-Regev |first2=Efrat |title=The Evolutionary Biology of Chelicerata |journal=Annual Review of Entomology |date=28 January 2025 |volume=70 |issue=1 |pages=143–163 |doi=10.1146/annurev-ento-022024-011250 |pmid=39259983 |issn=1545-4487}}</ref> but others still retrieve a monophyletic arachnida.<ref name="Lozano-FernandezTanner2019">{{cite journal|last1=Lozano-Fernandez|first1=Jesus|last2=Tanner|first2=Alastair R.|last3=Giacomelli|first3=Mattia|last4=Carton|first4=Robert|last5=Vinther|first5=Jakob|last6=Edgecombe|first6=Gregory D.|last7=Pisani|first7=Davide|title=Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida|journal=Nature Communications|volume=10|issue=1|year=2019|page=2295|issn=2041-1723|doi=10.1038/s41467-019-10244-7|pmid=31127117|pmc=6534568|bibcode=2019NatCo..10.2295L|doi-access=free}}</ref>

{{clade|{{clade
 |1=†''[[Fuxianhuia]]''
 |2={{clade
 |label1=†[[Antennulata]]
 | 1={{clade
 | 1=†''[[Emeraldella]]''
 | 2=†[[Trilobitomorpha]]
 | 3=†''[[Sidneyia]]''
}} 
 | 2={{clade
 |label1=†[[Megacheira]]
 | 1={{clade
 | 1=†''[[Yohoia]]''
 | 2={{clade
 | 1=†''[[Alalcomenaeus]]''
 | 2=†''[[Leanchoilia]]''
}} }}
 |label2=Chelicerata
 | 2={{clade
 |label1=[[Pycnogonida]]
 | 1={{clade
 | 1=†''[[Palaeoisopus]]''
 | 2={{clade
 | 1=''[[Pycnogonum]]''
 | 2=†''[[Haliestes]]''
 }} }}
 |label2=Euchelicerata
 | 2={{clade
 | 1=†''[[Offacolus]]''
 |label2=[[Prosomapoda]]
 | 2={{clade
 | 1=†''[[Weinbergina]]''
 | 2={{clade
 | 1={{clade
 | 1=†''[[Venustulus]]''
 | 2=†''[[Camanchia]]''
}}
 | 2={{clade
 | 1=†''[[Legrandella]]''
 | 2={{clade
 |label1=[[Xiphosura]]
 | 1={{clade
 | 1=†''[[Kasibelinurus]]''
 | 2={{clade
 | 1=†''[[Willwerathia]]''
 |label2=[[Xiphosurida]]
 | 2={{clade
 | 1=†''[[Lunataspis]]''
 | 2={{clade
 | 1=†[[Belinurina]]
 | 2=[[Limulina]]
}} }} }} }}
 |label2=[[Planaterga]]
 | 2={{clade
 | 1={{clade
 | 1=†''[[Pseudoniscus]]''
 | 2=†''[[Cyamocephalus]]''
}}
 | 2=†''[[Pasternakevia]]''
 | 3={{clade
 | 1=†''[[Bunodes]]''
 | 2=†''[[Limuloides]]''
}}
 | 4=†''[[Bembicosoma]]''
 |label5=[[Dekatriata]]
 | 5={{clade
 | 1=†[[Chasmataspidida]]
 |label2=[[Sclerophorata]]
 | 2={{clade
 | 1=[[Arachnida]]
 | 2=†[[Eurypterida]]
}} }} }} }} }} }} }} }} }} }} }} }}|label1=[[Arachnomorpha]]|style=font-size:80%; line-height:80%}}