order of marine chelicerates

Xiphosura is an order of marine chelicerates. It includes a large number of extinct lines and only four recent species in the family Limulidae, which include the horseshoe crabs.

Temporal range: Silurian – Recent
Horseshoe crab (Limulus polyphemus)
Scientific classification

Latreille, 1802

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.[1]

Xiphosura are traditionally placed in the class Merostomata. This term was originally made for the eurypterids; now, with the Xiphosura added, it is known to be paraphyletic.

Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged.[2] The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions (symplesiomorphies).[3]

The Xiphosura is basal to a clade comprising Eurypterida and Arachnida.[3] It is estimated that the Xiphosura diverged from the Arachnida 480 million years ago.[4]


  1. Mikko Haaramo (21 March 2003). "Xiphosura - Horseshoe crabs". Mikko's Phylogeny Archive.
  2. H.B. Boudreaux (1979). Arthropod phylogeny with special reference to Insects. Wiley. pp. 1–320. ISBN 9780471042907.
  3. 3.0 3.1 Colin Tudge (2002). "Spiders, scorpions, mites, eurypterids, horseshoe crabs, and sea spiders. Subphylum Chelicerata and Subphylum Pycnogonida". The variety of life: a survey and a celebration of all the creatures that have ever lived. Oxford University Press. pp. 309–326. ISBN 9780198604266.
  4. Davide Pisani, Laura L. Poling, Maureen Lyons-Weiler & S. Blair Hedges (2004). "The colonization of land by animals: molecular phylogeny and divergence times among arthropods". BMC Biology. 2: 1. doi:10.1186/1741-7007-2-1. PMC 333434. PMID 14731304.{{cite journal}}: CS1 maint: multiple names: authors list (link)