Xiphosura
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{{Taxobox
| name = Xiphosura
| image = Limulus_polyphemus.jpg
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| subphylum = [[Chelicerata]]
| classis = [[Xiphosura]]
| subdivision_ranks = Orders
| subdivision =
Synziphosurida <br>
Xiphosurida
}}
'''Xiphosura''' is a [[class (biology)|class]] of marine [[Chelicerata|chelicerates]] which includes a large number of extinct lineages and only four recent species in the family [[Limulidae]], which include the [[horseshoe crab]]s. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the [[Jurassic]] ''[[Mesolimulus]]'', and are considered to be [[living fossil]]s.
Xiphosura are typically placed the class [[Merostomata]], comprising also the [[Eurypterida|eurypterids]], but this traditional usage in the past reflected the acceptance of an unnatural ([[paraphyly|paraphyletic]]) group. Although the name Merostomata can be seen in some textbooks, without reference to the Eurypterida, some individuals have urged that this usage should be discouraged (e.g. Boudreaux, 1979).
==Size==
Minimum adult body size in [[Paleozoic]] species is 1-3 cm. Recent species may attain 60 cm.
<!-- Image with disputed fair-use status removed: [[Image:Limulus-appendages.jpg|left|thumb|400px|Diagram of appendages of ''Limulus polyphemus'', chelicerae in red, prosomal appendages I-V in shades of orange, opisthosomal appendages (opercles) in shades of yellow and the chilaria in purple.]] -->
==Characterization==
Body covered with heavy mineralized cuticle, divided into anterior [[prosoma]] and posterior [[opisthosoma]], this carrying a [[caudal spine]] (also referred to as a [[telson]], though this same term is also used for a different structure in crustaceans).
Prosoma dorsally covered by semicircular [[carapace]] which may bear eyes (median ocelli and lateral compound eyes) and ventrally the walking legs and [[chelicera]]e. Coxae of legs carry [[gnathobase]]s performing mastication. Reduced appendages of absorbed first opisthosomal segment can still be seen as a sixth pair of small legs called [[chilaria]].
Opisthosoma divided into mesosoma, with flattened appendages and metasoma (posterior) without appendages. Mesosoma may have all segments fused forming a [[thoracetron]].
==Ecology==
Recent Xiphosura are marine [[benthos|benthic]] living on soft substrate, coming ashore for reproduction. Based on the sediment associations, it was proposed that Paleozoic species were not marine, but lived in fresh water and brackish water.
==Taxa removed from Xiphosura==
There are two groups originally included in the Xiphosura, but which have been assigned to separate classes:
* [[Aglaspida]] Walcott 1911 (Cambrian to Ordovician).
* [[Chasmataspida]] Caster & Brooks, 1956 (Lower Ordovician).
==Classification of the Xiphosura==
Class Xiphosura Latreille, 1802
* †Order Synziphosurida
** Weinberginidae Richter and Richter, 1929 (Lower [[Devonian]])
** Bunodidae Packard 1886
*** Bunodinae Packard 1886 (Upper [[Silurian]] to Downtonian)
*** Limuloidinae Størmer, 1952 (Upper Silurian)
** Pseudoniscidae Packard 1886 (Upper Silurian)
** Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian)
* Order Xiphosurida
** †Suborder Bellinurina
*** Elleriidae Raymond, 1944 (Upper Devonian to Upper [[Carboniferous]])
*** Euproopidae Eller, 1938 (= Liomesaspidae Raymond 1944) (Upper Carboniferous to Lower Permian)
*** Bellinuridae Zittel and Eastman, 1913 (Middle Devonian to Upper Carboniferous)
** Suborder Limulina
*** †Rolfeiidae Selden and Siveter, 1987 (Early Carboniferous to Early Permian)
*** †Paleolimulidae Raymond, 1944 (Carboniferous to Permian)
*** †Moravuridae Pribyl, 1967 ([[Mississippian]])
*** †Austrolimulidae + †Heterolimulidae (Middle [[Triassic]])
*** Limulidae Zittel, 1885
**** †Mesolimulinae Størmer, 1952 (Lower Triassic to [[Cretaceous]])
**** Limulinae Zittel, 1885
***** Tachypleini Pocock, 1902 ([[Miocene]] to '''Recent''')
***** Limulini Zittel, 1885 ('''Recent''')
==References==
*Anderson, L. I. and P. A. Selden. 1997. Opisthosomal fusion and phylogeny of Palaeozoic Xiphosura. Lethaia 30:19-31.
* Boudreaux H. B., 1979. Arthropod phylogeny with special reference to insects. John Willey & sons, New York, Chichester, Brisbane, Toronto. 320 pp.
* Brusca, R. C. & Brusca, G. J. (2002) Invertebrates. Sinauer Associates, Massachusetts. 880 pages, 615 illustrations.
* Dunlop, J. A. 1997. Palaeozoic arachnids and their significance for arachnid phylogeny. Proceedings of the 16th European Colloquium of Arachnology, 65-82
* Dunlop, J.A.; Selden, P.A. 1997: The Early History and Phylogeny of the Chelicerates. In Fortey, R.A.; Thomas, R.H. (eds.) 1997: Arthropod Relationships. Systematics Association Special Volume Series 55, pp. 221-235.
* Rohdendorf B.B. (editor), Fundamentals of Paleontology, vol. 9, Arthropoda-Tracheata and Chelicerata: 894 pp. [1991 English translation of Russian original, Smithsonian Institution Libraries and National Science Foundation].
* Snodgrass, R.E., 1952. A textbook of arthropod anatomy. Hafner Publishing Company, New York.
==External links==
*[http://www.peripatus.gen.nz/Taxa/Arthropoda/Index.html Peripatus] - an overview of arthropod relationships.
*[http://www.palaeos.com/Invertebrates/Arthropods/Chelicerata/Merostomata/Xiphosura.html Paleos] - a site with a synoptic account of the Xiphosura, focused on fossils.
{{Arthropods}}
[[Category:Arthropods]]
[[Category:Xiphosura| ]]
[[ca:Xifosur]]
[[fa:شمشیردمان]]
[[ru:Мечехвосты]]