The subphylum (or phylum) Chelicerata constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and inclu*es horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician period. Although only four marine species survive, all of them horseshoe crabs (according to the view that excludes the Pycnogonida, all of which are aquatic), there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species. Like all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate bauplan consists of two tagmata, the cephalothorax and abdomen, except that mites have no visible division between these sections. The chelicerae that give the group its name are the only appendages that appear before the mouth, and in most sub-groups are modest pincers used in feeding; however spiders' chelicerae form fangs that most species use to inject venom into their prey.