Xiphosura

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 This term was originally made for the eurypterids; now, with the Xiphosura added, it is known to be paraphyletic. 
Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions (symplesiomorphies).
The Xiphosura is basal to a clade comprising Eurypterida and Arachnida. It is estimated that the Xiphosura diverged from the Arachnida 480 million years ago.