Euchelicerata

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). 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] RS10
    |  i. s.: Melbournopterus crossotus Caster & Kjellesvig-Waering 1953 TB04
    |         Venustulus waukeshaensis BS12
    |         Lunatapsis aurora JB12
    |         Eolimulus ([Eolimulidae] S93
    |           `--E. alatus (Moberg 1892) S93
    |--+--Dibasterium Briggs, Siveter et al. 2012 AC19, BS12
    |  |    `--*D. durgae Briggs, Siveter et al. 2012 BS12
    |  `--+--Sarotrocercus Whittington 1981 LSE13, CB04
    |     `--Offacolus [Offacolidae] AC17
    |          `--O. kingi Orr, Siveter et al. 2000 SB02
    `--Arachnida (see below for synonymy) BSL19
         |  i. s.: Enyo E12
         |           |--E. algirica Lucas 1846 E12
         |           `--E. amaranthina Lucas 1846 E12
         |         Cyrtocephalus Lucas 1846 E12
         |           |--C. terricola Lucas 1846 E12
         |           `--C. walckenaerii Lucas 1846 E12
         |         Monastes Lucas 1846 E12
         |           |--M. lapidarius Lucas 1846 E12
         |           `--M. paradoxus Lucas 1846 E12
         |         Hemialges F54
         |         Falculifera echinopus F54
         |         Hygrochoreutes krameri BK91
         |         Tenuipalponychus citri G91
         |         Beerella depicta W99
         |         Sonotetranychus albiflorae W99
         |         Pachylichus Canestrini 1894 [=Pachylicus (l. c.) non Roewer 1923] KA-Z11
         |         Fonsecia ptyasi Rao & Hiregander 1959 S69
         |         Hoshikadania Sasa & Asanuma 1951 S94
         |         Trichadenus Rondani 1870 CF77
         |         Crocidurobia michaeli GHG91
         |         Tapinattus melanognathus (see below for synonymy) S99
         |         Diacrotricha Oudem. 1906 B28
         |         Gamasides Leach 1814 B28
         |         Greeniella R13
         |           |--G. alfkeni R13
         |           `--G. perkensi R13
         |         Chaetozythia Karst. 1888 (n. d.) KC01
         |         Alanops LSE13
         |         Weinberginidae S93
         |           |--Weinbergina opitzi Richter & Richter 1929 AC19, CB04
         |           `--Legrandella lombardii Eldredge 1974 S93
         |         Koupichela AC19
         |         Astoma parasiticum G20
         |--+--Parasitiformes BSL19
         |  `--+--Pseudoscorpiones BSL19
         |     `--Acariformes BSL19
         `--+--+--Phalangiotarbida GD16
            |  `--Opiliones BSL19
            `--+--+--+--Xiphosura BSL19
               |  |  `--Ricinulei BSL19
               |  `--Arachnopulmonata BSL19
               |       |--Scorpiones BSL19
               |       `--Pantetrapulmonata GD14
               |            |  i. s.: Ecchosis pulchribothrium Selden & Shear 1991 S93
               |            |--Trigonotarbida GD16
               |            |--Tetrapulmonata BSL19
               |            `--Mesotarbus LSE13
               |                 |--M. angustus (Pocock 1911) S93
               |                 |--M. eggintoni (Pocock 1911) S93
               |                 |--M. hindi S93
               |                 `--M. intermedius Petrunkevitch 1949 S93
               `--+--Solifugae BSL19
                  `--Palpigradi [Microteliphonida] BSL19
                       |  i. s.: Triadokoenenia HST06
                       |         Allokoenenia afra Silvestri 1913 HST06
                       |         Leptokoenenia M86
                       |           |--L. gerlachi Condé 1965 M86
                       |           `--L. scurra Monniot 1966 M86
                       |         Koeneniodes HST06
                       |           |--K. berndi Condé 1988 HST06
                       |           |--K. deharvengi HST06
                       |           |--K. frondiger Rémy 1950 HST06
                       |           |--K. leclerci HST06
                       |           |--K. madecassus Rémy 1950 HST06
                       |           |--K. malagasorum Rémy 1960 HST06
                       |           |--K. notabilis HST06
                       |           `--K. spiniger HST06
                       |--Eukoeneniidae S93
                       |--Prokoenenia [Prokoeneniidae] HST06
                       |    `--P. wheeleri GE02
                       `--Sternarthron Haase 1890 H02 [Sternarthronidae S93]
                            `--*S. zitteli Haase 1890 H02

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

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

REFERENCES

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