Member of an extinct order (Eurypterida) of horseshoe crab , that lived c.
505–245 million years ago. Frequently referred to as giant scorpions, most eurypterids were small, although Pterygotus buffaloenis, a species from the {{link=Silurian Period">Silurian period , was the largest arthropod ever known, reaching a length of about 10 ft (3 m). The eurypterids lived in brackish waters. Some were predators; others were probably bottom-dwelling scavengers.