The Chelicerata are a sub-phylum of the Arthropods. The sub-phylum consists of only two classes: the horseshoe crabs and the arachnids. The horseshoe crabs (Xiphosura) are a small, ancient group which has survived relatively unchanged for millions of years. Groups like this are often called living fossils. The Arachnida is the huge and successful terrestrial (air-breathing) group of spiders and their relatives.
The chelicerates originated as marine animals. The first confirmed chelicerate fossils, the eurypterids, date from 445 million years ago in the Late Ordovician period. Although only four marine species survive, all of them horseshoe crabs, there are over 77,000 species of air-breathing chelicerates, and there may be about 500,000 unidentified species.
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