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Chelicerata
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===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%}}
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