member of an extinct order (Eurypterida) of unusual arthropods rarely preserved as fossils. The eurypterids appeared at the beginning of the Ordovician Period (about 488 million years ago) and became extinct at the end of the Permian Period (about 251 million years ago).

Frequently referred to as giant scorpions, most eurypterids were small animals, although Jaekelopterus rhenaniae (also called Pterygotus rhenanius or P. buffaloenis), a species from the Silurian Period (about 444 to 416 million years ago) in North America, was the largest arthropod ever known; it reached a length of about 2.5 metres (8 feet). Similar in body plan to the horseshoe crab, eurypterids consisted of many segments; the animal's anterior appendages served as oarlike swimming organs. Some eurypterids were active predators and could probably swim rapidly after prey, whereas others were probably bottom-dwelling scavengers. The eurypterids inhabited brackish-water environments and generally occurred among a specialized, often impoverished, faunal assemblage.