page_title: Xiphosura
section_title: Classification
hierarchical_section_title: Xiphosura / Classification
caption_attribution_description: Deutsch: Versteinerung eines Pfeilschwanzkrebses - Mesolimulus walchi - aus Solnhofen. Museum für Naturkunde (Berlin). English: Fossil of a Limulidae - Mesolimulus walchi, Solnhofen. Museum für Naturkunde (Berlin).
context_page_description: Xiphosura is an order of arthropods related to arachnids, or, according to one recent study, actual arachnids. They are sometimes called horseshoe crabs. They first appeared in the Hirnantian. Currently, there are only four living species. They are members of the order Xiphosura, which contains two suborders, Xiphosurida and Synziphosurina.
  The group has hardly changed in hundreds of 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. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.
  Xiphosura were traditionally placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural group. Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The Merostomata label originally did not include Eurypterida, although they were added in as a better understanding of the extinct group evolved.
context_section_description: Xiphosuran classification as of 2018:
  Order Xiphosura Latreille, 1802
  †Maldybulakia Tesakov & Alekseev, 1998  (Devonian)
  †Willwerathia Størmer, 1969 (Devonian)
  †Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian)
  Suborder Xiphosurida
  †Infraorder Bellinurina
  †Bellinuridae Zittel & Eastman, 1913 (Middle Devonian to Upper Carboniferous)
  Infraorder Limulina
  †Bellinuroopsis Chernyshev, 1933 (Carboniferous)
  †Rolfeiidae Selden & Siveter, 1987 (Early Carboniferous to Early Permian)
  Superfamily †Paleolimuloidea Anderson & Selden, 1997
  †Paleolimulidae Raymond, 1944 (Carboniferous to Permian)
  Superfamily Limuloidea
  †Valloisella Racheboeuf, 1992 (Carboniferous)
  †Austrolimulidae Riek, 1955 (Middle Triassic)
  Limulidae Zittel, 1885 (Triassic to recent)
  Limulinae Zittel, 1885 (recent)
  Tachypleinae Pocock, 1902 (recent)
caption: Mesolimulus from the Solnhofen limestone
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