GIANT WATER SCORPION

also called Sea Scorpion, any member of the extinct order Eurypterida of the arthropod class Merostomata, a group of large, scorpion-like, aquatic invertebrates that flourished during the Silurian Period (438 to 408 million years ago). Well over 200 species have been identified and divided into 18 families. They include the largest arthropod species known, Pterygotus rhenanius, which measures nearly 3 m (10 feet) in length. Several other eurypterid forms were almost as large. The fossils of giant water scorpions are usually found in brackish and freshwater deposits, but the animals probably first lived in shallow coastal areas and estuaries and moved into freshwater environments later. Only a few species appear to have been good swimmers. The reason for their extinction is unknown. Their close relative the horseshoe crab (q.v.), of the order Xiphosura, has survived to the present day.