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a group of extinct aquatic arthropods of the class Merostomata that lived from the Ordovician through the Permian. The more or less spindle-shaped body measured 10–20 cm in length, although some, such as the genus Pterygotus, reached lengths of 1–1.8 m. The body was divided into a prosoma and an opisthosoma, which ended with a telson in the form of a spine or two lobes. On the prosoma there were simple and compound eyes and six pairs of extremities, of which the first, claw-shaped chelicerae, were piercing or prehensile and sometimes highly developed.
The Eurypteroidea embraced approximately 30 genera. They probably inhabited fresh or somewhat brackish waters and were predators. The study of the Eurypteroidea is important for an understanding of the evolution of arthropods, inasmuch as scorpions evidently descended from them during the Silurian, becoming the first arthropods to adapt successfully to life on dry land.