Xiphosura

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Xiphosura is a class of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs. 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 fossils.

Xiphosura are typically placed the class Merostomata, comprising also the eurypterids, but this traditional usage in the past reflected the acceptance of an unnatural (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.

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 chelicerae. Coxae of legs carry gnathobases 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 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

• Peripatus - an overview of arthropod relationships.
• Paleos - a site with a synoptic account of the Xiphosura, focused on fossils.