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| Xiphosura Temporal range: Silurian – Recent | |
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| Horseshoe crab (Limulus polyphemus) | |
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| Order: | Xiphosura Latreille, 1802 |
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.
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 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 This term was originally made for the eurypterids; now, with the Xiphosura added, it is known to be paraphyletic.
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