Answer: eurypterids

AnswerCrossword Clue
eurypteridsAn extinct marine arthropod of a group occurring in the Paleozoic era. They are related to horseshoe crabs and resemble large scorpions with a terminal pair of paddle-shaped swimming appendages
eurypteridsEURYPTERID, any of an order of large extinct aquatic Paleozoic arthropods related to the horsehoe crab