Prokoenenia wheeleri, copyright Robert Deans.

Belongs within: Euarthropoda.
Contains: Koupichela, Parasitiformes, Pseudoscorpiones, Acariformes, Phalangiotarbida, Opiliones, Xiphosura, Ricinulei, Scorpiones, Trigonotarbida, Tetrapulmonata, Solifugae, Eukoeneniidae.

The Euchelicerata are a major clade of arthropods uniting the arachnids (spiders, mites, scorpions and related taxa) with their living and fossil marine relatives. A potential synapomorphy is the presence of multilobate exopods on post-cephalic limbs that are covered by a sclerite or operculum though this feature has been lost in most living groups (Aria & Caron 2019). Early fossil euchelicerates include Sarotrocercus from the Cambrian Burgess Shale, a small arthropod with large stalked eyes and two pairs of sturdy, spinose appendages on the head. Dibasterium durgae is an elongate-bodied species known from the mid-Silurian Herefordshire Lagerstätte in which prosomal limbs 2 to 5 are biramous (Briggs et al. 2012).

The monophyly of the arachnids has been subject to some debate with some authors suggesting that ‘arachnid’ characters may represent convergent adaptations to terrestriality. Recent molecular phylogenetic analyses have suggested that the horseshoe crabs, long thought to be the only surviving primarily aquatic euchelicerates, may be nested within the arachnid crown group.

Other relationships within the Euchelicerata are similarly contentious with a wide array of arrangements having been proposed at one time or another, few of them strongly supported. A relationship between the Tetrapulmonata (including spiders and whip scorpions) and the fossil Trigonotarbida is supported by features including cheliceral structure, presence of a megoperculum, and booklungs on the genital and first post-genital somites (Shultz 2007). Molecular analyses have also supported a relationship between Tetrapulmonata and Scorpiones in a clade that has been named Arachnopulmonata and may be characterised by the presence of book lungs.

Particularly hard to place are the Palpigradi, a group of minute, segmented arachnids with with an elongate abdomen terminating in a whip-like, seta-bearing flagellum. Palpigrades inhabit damp habitats such as soil or leaf litter with members of the genus Leptokoenenia inhabiting interstitial habitats along shorelines. The Jurassic fossil Sternarthron zitteli has been assigned to the Palpigradi but is considerably larger than any living species and may be misplaced (Harvey 2002).

The mites are a diverse assemblage of mostly very small arachnids commonly treated as a formal group Acari or Acarina. Mites have been divided between two major lineages: the Acariformes have birefringent setae with a core of actinopilin whereas the Parasitiformes (or Anactinotrichida) lack such setae. Whether the mites as a whole represent a monophyletic group remains a contested issue. Mite larvae have only six walking legs when they first emerge, with the fourth pair developing in later instars. This feature is shared with the Ricinulei, leading some authors to suggest a relationship between mites and ricinuleids, but other analyses have placed the groups separately.

<==Euchelicerata [Merostomata, Myliosomata, Synxiphosurina, Xiphosuridea]
    |  i. s.: Melbournopterus crossotus Caster & Kjellesvig-Waering 1953TB04
    |         Venustulus waukeshaensisBS12
    |         Lunatapsis auroraJB12
    |         Eolimulus ([Eolimulidae]S93
    |           `--E. alatus (Moberg 1892)S93
    |--+--Dibasterium Briggs, Siveter et al. 2012AC19, BS12
    |  |    `--*D. durgae Briggs, Siveter et al. 2012BS12
    |  `--+--Sarotrocercus Whittington 1981LSE13, CB04
    |     `--Offacolus [Offacolidae]AC17
    |          `--O. kingi Orr, Siveter et al. 2000SB02
    `--Arachnida (see below for synonymy)BSL19
         |  i. s.: EnyoE12
         |           |--E. algirica Lucas 1846E12
         |           `--E. amaranthina Lucas 1846E12
         |         Cyrtocephalus Lucas 1846E12
         |           |--C. terricola Lucas 1846E12
         |           `--C. walckenaerii Lucas 1846E12
         |         Monastes Lucas 1846E12
         |           |--M. lapidarius Lucas 1846E12
         |           `--M. paradoxus Lucas 1846E12
         |         HemialgesF54
         |         Falculifera echinopusF54
         |         Hygrochoreutes krameriBK91
         |         Tenuipalponychus citriG91
         |         Beerella depictaW99
         |         Sonotetranychus albifloraeW99
         |         Pachylichus Canestrini 1894 [=Pachylicus (l. c.) non Roewer 1923]KA-Z11
         |         Fonsecia ptyasi Rao & Hiregander 1959S69
         |         Hoshikadania Sasa & Asanuma 1951S94
         |         Trichadenus Rondani 1870CF77
         |         Crocidurobia michaeliGHG91
         |         Tapinattus melanognathus (see below for synonymy)S99
         |         Diacrotricha Oudem. 1906B28
         |         Gamasides Leach 1814B28
         |         GreeniellaR13
         |           |--G. alfkeniR13
         |           `--G. perkensiR13
         |         Chaetozythia Karst. 1888 (n. d.)KC01
         |         AlanopsBS12
         |         WeinberginidaeS93
         |           |--Weinbergina opitzi Richter & Richter 1929AC19, CB04
         |           `--Legrandella lombardii Eldredge 1974S93
         |         KoupichelaAC19
         |         Astoma parasiticumG20
         |         ‘Cephalodiscus’ Berlese 1916 non M’Intosh 1882M14
         |         Neumannia Trouessart 1888EH19
         |  `--+--PseudoscorpionesBSL19
         |     `--AcariformesBSL19
            |  `--OpilionesBSL19
               |  |  `--RicinuleiBSL19
               |  `--ArachnopulmonataBSL19
               |       |--ScorpionesBSL19
               |       `--PantetrapulmonataGD14
               |            |  i. s.: Ecchosis pulchribothrium Selden & Shear 1991S93
               |            |--TrigonotarbidaGD16
               |            |--TetrapulmonataBSL19
               |            `--MesotarbusLSE13
               |                 |--M. angustus (Pocock 1911)S93
               |                 |--M. eggintoni (Pocock 1911)S93
               |                 |--M. hindiS93
               |                 `--M. intermedius Petrunkevitch 1949S93
                  `--Palpigradi [Microteliphonida]BSL19
                       |  i. s.: TriadokoeneniaHST06
                       |         Allokoenenia afra Silvestri 1913HST06
                       |         LeptokoeneniaM86
                       |           |--L. gerlachi Condé 1965M86
                       |           `--L. scurra Monniot 1966M86
                       |         KoeneniodesHST06
                       |           |--K. berndi Condé 1988HST06
                       |           |--K. deharvengiHST06
                       |           |--K. frondiger Rémy 1950HST06
                       |           |--K. leclerciHST06
                       |           |--K. madecassus Rémy 1950HST06
                       |           |--K. malagasorum Rémy 1960HST06
                       |           |--K. notabilisHST06
                       |           `--K. spinigerHST06
                       |--Prokoenenia [Prokoeneniidae]HST06
                       |    `--P. wheeleriGE02
                       `--Sternarthron Haase 1890H02 [SternarthronidaeS93]
                            `--*S. zitteli Haase 1890H02

Arachnida [Acari, Acarida, Acaridiae, Acarina, Acarinomorpha, Acaromorpha, Actinoderma, Apatellata, Apulmonata, Arthrogastra, Chelodonta, Cryptognomae, Cryptoperculata, Didactyla, Dromopoda, Epimerata, Galeodides, Haplocnemata, Holacarina, Holotracheata, Lipoctena, Meridogastra, Microphtira, Microptirae, Micrura, Neosternata, Novogenuata, Obisides, Pedipalpata, Phalangiae, Phalangiata, Phalangita, Poecilophysidea, Riciniae, Rostrosomata, Scorpionides, Solenostoma, Solpugae, Sternocoxata, Stomothecata, Synziphosurina, Xiphosurida]BSL19

Tapinattus melanognathus [=Attus melanognathus; incl. Salticus convergens Doleschall 1859, Marpissa dissimilis Koch 1846, M. incerta Koch 1846, Attus muscivorus Vinson 1864, Salticus nigrolimbatus Cambr. 1869]S99

*Type species of generic name indicated


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