What is Chelicerata?
{{taxobox | name = Chelicerata | fossil_range = late [[Ordovician]] <br/>445 mya to present (see text) | image = Limule(dD).jpg | regnum = [[Animalia]] | phylum = [[Arthropoda]] | unranked subphylum = [[Arachnomorpha]] | subphylum = '''Chelicerata''' | image_caption = [[Horseshoe crab]] | authority = [[Richard Heymons|Heymons]], 1901 | subdivision_ranks = Classes | subdivision = * [[Arachnida]] * [[Merostomata]] * [[Pycnogonida]] (see text) }} [[File:Stream spider.jpg|thumb|230px|Spider]] The '''Chelicerata''' are a sub-[[phylum]] of the [[Arthropods]]. The sub-phylum consists of only two [[Class (biology)|classes]]: the [[Arachnid]]s and the [[Merostomata]]. The Merostomata are the extinct [[Eurypterids]] and the [[horseshoe crabs]] ([[Xiphosura]]). The horseshoe crabs are a small, ancient group which has survived relatively unchanged for millions of years. Groups like this are often called [[living fossil]]s. The Arachnida is the huge and successful terrestrial (air-breathing) group of [[spider]]s and their relatives. The chelicerates originated as [[Ocean|marine]] animals. The first confirmed chelicerate [[fossil]]s, the [[eurypterid]]s, date from 445 million years ago in the late [[Ordovician]] period. Although only four marine species survive, all of them [[horseshoe crab]]s, there are over 77,000 species of air-breathing chelicerates, and there may be about 500,000 unidentified species.<ref>Margulis, Lynn and Schwartz, Karlene 1998. ''Five Kingdoms, an illustrated guide to the phyla of life on Earth''. 3rd ed, Freeman. {{ISBN|0-7167-3027-8}}</ref><ref>Ruppert E.E. Fox R.S. and Barnes R.D. 2004. ''Invertebrate Zoology'' 7th ed, Brooks/Cole.</ref> The [[pycnogonids]] may or may not be Chelicerates. This is in dispute, as [[genetic]] evidence suggests they may be an ancient sister group to all other living arthropods.<ref>{{Cite journal |author= Regier J.C. |display-authors = etal |url=http://www.nature.com/nature/journal/v463/n7284/full/nature08742.html |journal=[[Nature (journal)|Nature]] |date=25 Febru