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Temporal range: Upper Carboniferous
Plesiosiro is an extinct arachnid genus known exclusively from only nine specimens from the Upper Carboniferous of Coseley, Staffordshire, United Kingdom. The genus is monotypic, represented only by the species Plesiosiro madeleyi described by Reginald Innes Pocock in his important 1911 monograph on British Carboniferous arachnids. It is the only known member of the order Haptopoda. The original locality from which these fossils originate is no longer available thus it is unclear whether any further examples will be found.
The original fossils have been redescribed in detail by Alexander Petrunkevitch in 1949 and Dunlop in 1999. A supposed example from the Coal Measures of Lancashire is a misidentification.
Relationships with other arachnids are obscure. Plesiosiro means "close to Siro", which is a genus of cyphophthalmid (Cyphophthalmi); the most primitive group of the living harvestmen (Opiliones). These harvestmen do, in some ways, resemble the reconstructed body plan of the haptopods.
Revisions have confirmed that Haptopoda should be treated as a separate and independent order. The most recent study tentatively recognised a group named Schizotarsata Shultz, 2007 comprising Haptopoda, Amblypygi (whip spiders), Uropygi (whip scorpions) and Schizomida (schizomids). All share the character of a subdivided tarsus (or foot) which gives the group its name. Some cladistic analyses recover the phalangiotarbids as sister group to the Opiliones, albeit with low support.
Haptopoda originates from Greek "haptos" (= tangible, subject to the sense of touch) + "pous, podos" (= foot) and refers to its quite long front pair of legs with their subdivided tips which look as though they might have been used to 'feel' their way around in front of the animal.
- Reginald Innes Pocock (1911). A Monograph of the Terrestrial Carboniferous Arachnida of Great Britain. Monographs of the Palaeontographical Society, London. pp. 1–84.
- Alexander Petrunkevitch (1949). "A study of Palaeozoic Arachnida". Transactions of the Connecticut Academy of Arts and Sciences. 37: 69–315.
- Jason A. Dunlop (1999). "A redescription of the Carboniferous arachnid Plesiosiro madeleyi Pocock, 1911 (Arachnida: Haptopoda)" (PDF). Transactions of the Royal Society of Edinburgh: Earth Sciences. 90: 29–47. doi:10.1017/s0263593300002492.
- Jeffrey W. Shultz (2007). "A phylogenetic analysis of the arachnid orders based on morphological characters" (PDF). Zoological Journal of the Linnean Society. 150 (2): 221–265. doi:10.1111/j.1096-3642.2007.00284.x.
- Garwood, Russell J.; Dunlop, Jason A. (2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ. 2: e641. doi:10.7717/peerj.641. PMC . PMID 25405073. Retrieved June 15, 2015.
- Alexander Petrunkevitch (1955). "Arachnida". In R. C. Moore. Part P, Arthropoda 2. Treatise on Invertebrate Paleontology. Lawrence: Geological Society of America and University of Kansas Press. pp. 42–162.