The chelicerate exoskeleton and its remarkable anatomical
derivations, the small body size of many species, and the
extraordinary diversity of form and habitat contribute to the
success of chelicerates as a phylum. Scorpions—present already
in the Silurian as aquatic forms and in the Carboniferous as
definitely terrestrial ones—are traditionally considered the most
primitive arachnids. An alternative phylogeny places them as
derived arachnid group, probably relatives of harvestmen. The
affiliations between chelicerates, crustaceans and insects (A-21)
are unclear. All arthropods may have derived from annelid
ancestry, but this theory is currently contested.