Definitions for xiphosura
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Xiphosura, order Xiphosura(noun)
horseshoe crabs and extinct forms
Xiphosura is an order of marine chelicerates that 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. 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. However, as the similarities, which were once thought to tie the group together, are now understood to be symplesiomorphies, it seems that abandoning the term altogether to avoid any confusion arising from these historical usages of the term is the best strategy.
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