Belongs within: Euarthropoda.
Contains: Koupichela, Parasitiformes, Pseudoscorpiones, Trombidiformes, Sarcoptiformes, 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 body is ancestrally divided between an anterior prosoma and posterior opisthosoma. The prosoma bears sensory and locomotory appendages whereas the opisthosoma bears the digestive, reproductive and respiratory organs. The anteriormost appendages on the prosoma, the chelicerae, are primitively pincer-like and are used in feeding. The majority of arachnids are predators on other invertebrates though herbivorous, omnivorous or detritivorous taxa are also included. Many are liquid feeders, often using saliva and/or toxins to pre-digest prey tissue before ingestion. The second pair of appendages, the pedipalps, are primarily sensory appendages though they may also be used in manipulating food and mating. Four pairs of walking legs are typically present. Basal fossil forms have the post-anal telson developed into an elongate spine that is also present in modern Xiphosura.
Early fossil euchelicerates include taxa commonly associated as the Synxiphosurina and considered forerunners of the modern xiphosurans. Overall appearance is similar to xiphosurans, with an elongate anal spine, but segments of the opisthosoma are unfused. However, the monophyly of such a group is uncertain. Sarotrocercus from the Cambrian Burgess Shale is 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. Offacolus kingi from the same deposit has a similar limb morphology but the posterior part of the opisthosoma is fused (Briggs et al. 2012). Venustulus waukeshaensis is known from the Silurian of Wisconsin. Weinbergina opitzi has been described from the Devonian of Hunsrück in Germany (Briggs et al. 2012). Legrandella lombardii is another Devonian species from Bolivia (Selden 1993). Eolimulus alatus from the Cambrian of Sweden had a short and moderately convex prosoma with large compound eye nodes. It is known from a carapace only so its euchelicerate affinities may be open to question (Selden 1993).
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. Crown group euchelicerates are united by their uniramous appendages and the loss of the seventh pair of appendages on the prosoma (Aria & Caron 2019).
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). Ecchosis pulchribothrium from the Devonian of New York may represent an early member of this clade but is only known from fragments of isolated limbs.
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. Alternative relationships for scorpions have been suggested with Opiliones and Pseudoscorpiones (Garwood & Dunlop 2014). Putative synapomorphies for Scorpiones and Opiliones in a clade Stomothecata include the presence of a preoral chamber formed by projections of the anterior leg coxae. Scorpiones and Pseudoscorpiones may be united by the modification of the pedipalps into a pair of large chelae. If these groups are not monophyletic then these characters presumably represent convergences. Conversely, Pseudoscorpiones have been united with the Solifugae on the basis of possessing legs with short femora and long patellae. Such a clade has been labelled the Haplocnemata or Apatellata. Solifugae have in turn been united with the acariform mites in a clade Poecilophysidea on the basis of male genital characters.
Particularly hard to place are the Palpigradi, a group of minute, segmented arachnids with with an elongate abdomen terminating in a whip-like flagellum. Chelicerae are three-segmented; pedipalps are nine-segmented. Pedipalps and walking legs all end in three claws. The first pair of legs are modified as sensory organs and may be held aloft when moving. The carapace is divided between the third and fourth pairs of legs. The flagellum comprise fifteen segments, each surrounded by whorls of setae. Palpigrades inhabit damp habitats such as soil or leaf litter with members of the genus Leptokoenenia inhabiting interstitial habitats along shorelines. Members of the family Prokoeneniidae have three pairs of eversible sacs on the opisthosoma, placed on sternites 4 to 6. The Madagascan genus Triadokoenenia lacks the median frontal organ that is usually present on the palpigrade prosoma. 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. The body lacks conspicuous primary segmentation though some lineages retain vestiges of segmental division. The chelicerae and pedipalps are associated on a distinct tagma, the gnathosoma or capitulum, from the remainder of the body, the idiosoma. The gnathosoma thus resembles a head though the eyes, if present, are located on the dorsum of the idiosoma. Mites have been divided between two major lineages with the Acariformes having birefringent setae with a core of actinopilin whereas the Parasitiformes (or Anactinotrichida) lack such setae. Acariformes are further characterised by the integration of the leg coxae into the venter of the podosoma, often forming coxisternal plates. 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. The Acariformes are in turn divided between the Sarcoptiformes, included the oribatid beetle mites and allies, and the Trombidiformes. The Trombidiformes lack a rutella on the subcapitulum and ancestrally have two pairs of setae in row c (the anteriormost row on the opisthosoma) whereas Sarcoptiformes usually possess a rutella or pseudorutella and most commonly have three or four pairs of setae in row c.
<==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
| 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
| Coleopterophagus Berlese 1882B82
| |--C. carabicola Berlese 1882B82
| `--C. megninii (Berlese 1882) [=Dermoglyphus megninii; incl. Tyroglyphus acaroides]B82
| Masolaelaps bandicootaG67
|--+--ParasitiformesBSL19
| `--+--PseudoscorpionesBSL19
| `--Acariformes (see below for synonymy)BSL19
| |--TrombidiformesGD14
| `--SarcoptiformesGD14
`--+--+--PhalangiotarbidaGD16
| `--OpilionesBSL19
`--+--+--+--XiphosuraBSL19
| | `--RicinuleiBSL19
| `--ArachnopulmonataBSL19
| |--ScorpionesBSL19
| `--PantetrapulmonataGD16
| | 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
`--+--SolifugaeBSL19
`--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
|--EukoeneniidaeS93
|--Prokoenenia [Prokoeneniidae]HST06
| `--P. wheeleriGE02
`--Sternarthron Haase 1890H02 [SternarthronidaeS93]
`--*S. zitteli Haase 1890H02
Acariformes [Actinochaeta, Actinochitinosi, Actinotrichida, Cursoria, Endeostigmatina, Neocotylea, Pachygnathoidea]BSL19
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]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
References
[AC17] Aria, C., & J.-B. Caron. 2017. Burgess Shale fossils illustrate the origin of the mandibulate body plan. Nature 545: 89–92.
[AC19] Aria, C., & J.-B. Caron. 2019. A middle Cambrian arthropod with chelicerae and proto-book gills. Nature 573: 586–589.
[BSL19] Ballesteros, J. A., C. E. Santibáñez López, Ľ. Kováč, E. Gavish-Regev & P. P. Sharma. 2019. Ordered phylogenomic subsampling enables diagnosis of systematic errors in the placement of the enigmatic arachnid order Palpigradi. Proceedings of the Royal Society of London Series B—Biological Sciences 286: 20192426.
[B82] Berlese, A. 1882. Acari, Myriopoda et Scorpiones hucusque in Italia reperta. Fascicle 1. Typis Fratelli Salmin: Patavii.
[B28] Betrem, J. G. 1928. Monographie der Indo-Australischen Scoliiden mit zoogeographischen Betrachtungen. H. Veenman & Zonen: Wageningen.
[BK91] Biesiadka, E., & W. Kowalik. 1991. Water mites (Hydracarina) as indicators of trophy and pollution in lakes. In: Dusbábek, F., & V. Bukva (eds) Modern Acarology: Proceedings of the VIII International Congress of Acarology, held in České Budĕjovice, Czechoslovakia, 6–11 August 1990 vol. 1 pp. 475–481. SPB Academic Publishing: The Hague.
[BS12] Briggs, D. E. G., D. J. Siveter, D. J. Siveter, M. D. Sutton, R. J. Garwood & D. Legg. 2012. Silurian horseshoe crab illuminates the evolution of arthropod limbs. Proceedings of the National Academy of Sciences of the USA 109 (39): 15702–15705.
[CF77] Canestrini, G., & F. Fanzago. 1877. Intorno agli Acari Italiani. Atti del Reale Istituto Veneto di Scienze, Lettere ed Arti, Serie 5, 4: 69–208, pls 2–7.
[CB04] Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169–193.
[EH19] Énay, R., & M. K. Howarth. 2019. Part L, revised, volume 3B, chapter 7: Systematic descriptions of the Perisphinctoidea. Treatise Online 120: 1–184.
[E12] Evenhuis, N. L. 2012. Publication and dating of the Exploration Scientifique de l’Algérie: Histoire Naturelle des Animaux Articulés (1846–1849) by Pierre Hippolyte Lucas. Zootaxa 3448: 1–61.
[F54] Forster, R. R. 1954. The New Zealand harvestmen (sub-order Laniatores). Canterbury Museum Bulletin 2: 1–329.
[GD14] Garwood, R. J., & J. Dunlop. 2014. Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders. PeerJ 2: e641.
[GD16] Garwood, R. J., J. A. Dunlop, P. A. Selden, A. R. T. Spencer, R. C. Atwood, N. T. Vo & M. Drakopoulos. 2016. Almost a spider: a 305-million-year-old fossil arachnid and spider origins. Proceedings of the Royal Society of London Series B—Biological Sciences 283: 20160125.
[GE02] Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt. 2002. Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5–70.
[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.
[G67] Green, R. H. 1967. The murids and small dasyurids in Tasmania. Parts 1 and 2. Records of the Queen Victoria Museum 28: 1–19.
[G91] Gupta, S. K. 1991. The mites of agricultural importance in India with remarks on their economic status. In: Dusbábek, F., & V. Bukva (eds) Modern Acarology: Proceedings of the VIII International Congress of Acarology, held in České Budĕjovice, Czechoslovakia, 6–11 August 1990 vol. 1 pp. 509–522. SPB Academic Publishing: The Hague.
[H02] Harvey, M. S. 2002. The neglected cousins: what do we know about the smaller arachnid orders? Journal of Arachnology 30 (2): 357–372.
[HST06] Harvey, M. S., F. Štáhlavský & P. D. Theron. 2006. The distribution of Eukoenenia mirabilis (Palpigradi: Eukoeneniidae): a widespread tramp. Records of the Western Australian Museum 23 (2): 199–203.
[JB12] Johnson, M. E., & B. G. Baarli. 2012. Development of intertidal biotas through Phanerozoic time. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 63–128. Springer.
[KC01] Kirk, P. M., P. F. Cannon, J. C. David & J. A. Stalpers. 2001. Ainsworth & Bisby’s Dictionary of the Fungi 9th ed. CAB International: Wallingford (UK).
[KA-Z11] Kury, A. B., & M. A. Alonso-Zarazaga. 2011. Addenda and corrigenda to the “Annotated catalogue of the Laniatores of the New World (Arachnida, Opiliones)”. Zootaxa 3034: 47–68.
[LSE13] Legg, D. A., M. D. Sutton & G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4 (2485): 1-7.
[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.
[M86] Monniot, F. 1986. Palpigradi. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) p. 697. E. J. Brill/Dr W. Backhuys: Leiden.
[R13] Reuter, O. M. 1913. Lebensgewohnheiten und Instinkte der Insekten bis zum Erwachen der sozialen Instinkte. R. Friedländer & Sohn: Berlin.
[S93] Selden, P. A. 1993. Arthropoda (Aglaspidida, Pycnogonida and Chelicerata). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 297–320. Chapman & Hall: London.
[S99] Simon, E. 1899. Ergebnisse einer Reise nach dem Pacific (Schauinsland 1896–1897). Arachnoideen. Zoologische Jahrbücher, Abteilung für Systematik, Geographie un Biologie der Thiere 12 (4): 411–437.
[S94] Southcott, R. V. 1994. Revision of the larvae of the Microtrombidiinae (Acarina, Microtrombidiidae), with notes on life histories. Zoologica 48 (2): 1–155.
[S69] Steyskal, G. C. 1969. The mistreatment of the Latin genitive case in forming names of parasites. Systematic Zoology 18 (3): 339–342.
[SB02] Sutton, M. D., D. E. G. Briggs, D. J. Siveter, D. J. Siveter & P. J. Orr. 2002. The arthropod Offacolus kingi (Chelicerata) from the Silurian of Herefordshire, England: Computer based morphological reconstructions and phylogenetic affinities. Proceedings of the Royal Society of London Series B—Biological Sciences 269: 1195–1203.
[TB04] Tetlie, O. E., S. J. Braddy, P. D. Butler & D. E. G. Briggs. 2004. A new eurypterid (Chelicerata: Eurypterida) from the Upper Devonian Gogo Formation of Western Australia, with a review of the Rhenopteridae. Palaeontology 47 (4): 801–809.
[W99] Walter, D. E. 1999. Cryptic inhabitants of a noxious weed: mites (Arachnida: Acari) on Lantana camara L. invading forests in Queensland. Australian Journal of Entomology 38: 197–200.