Xiphosura (4 pics)
Xiphosura is an order (biology) 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).