Eurypterids (sea scorpions) are an extinct group of arthropods related to arachnids, which include the largest known arthropods that ever lived. They are members of the extinct class Eurypterida (Chelicerata). The word Eurypterid comes from the Greek word eury meaning "broad" or "wide" and the Greek word pteron meaning "wing" . They predate the earliest fishes. The largest, such as Jaekelopterus, reached 2 meters (6 ft 7 in) or more in length, but most species were less than 20 centimeters (8 in). They were formidable predators that thrived in warm shallow water in the Ordovician to Permian from 460 to 248 million years ago. Although informally called 'sea scorpions', only the earliest ones were marine (later ones lived in brackish or freshwater), and they were not true scorpions. Accordin