Xiphosura

Xiphosura is an order of <a href="/facts/Arthropod/JKJLTFzp">arthropods</a> related to <a href="/facts/Arachnid/xPJbsLga">arachnids</a>. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, <a href="/facts/Horseshoe_crab/IuaBGa9T">Limulidae</a>). They first appeared in the <a href="/facts/Hirnantian/gnAHw02z">Hirnantian</a> (<a href="/facts/Late_Ordovician/BjMrLIEA">Late Ordovician</a>). Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.
The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be <a href="/facts/Living_fossil/EujFvU2D">living fossils</a>. 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 historically placed in the class Merostomata, although this term was intended to encompass also the <a href="/facts/Eurypterida/w1GHNTAB">eurypterids</a>, whence it denoted what is now thought to be an unnatural (<a href="/facts/Paraphyletic/tmTWjACz">paraphyletic</a>) group (although this is a grouping recovered in some recent cladistic analyses). 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. Now Eurypterida is classified within <a href="/facts/Sclerophorata/hK1v8yAf">Sclerophorata</a> together with the <a href="/facts/Arachnid/xPJbsLga">arachnids</a>, and therefore, Merostomata is now a synonym of Xiphosura. Several recent phylogenomic studies place Xiphosura within <a href="/facts/Arachnid/xPJbsLga">Arachnida</a>, often as the <a href="/facts/Sister_group/pNNuVex4">sister group</a> of <a href="/facts/Ricinulei/QodJckDz">Ricinulei</a>; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.

Xiphosura open-in-new

Xiphosura