Eurypterids (sea scorpions) are an extinct group of arthropods that include the largest known arthropods that ever lived. Eurypterids predate the earliest fishes. The largest reached 8 ft in length, but most species were less than 8 in. They were formidable predators that thrived in warm shallow water, in both seas and lakes 460 to 248 million years ago. They went extinct during the Permian–Triassic extinction event 251 million years ago, and their fossils have a near global distribution.
EXTINCT † - Helicoprion ("Spiral Saw") was a long-lived genus of shark-like cartilaginous fish that first arose in the oceans of the late Carboniferous 310 million years ago, survived the Permian-Triassic Extinction Event, and eventually went extinct during the early Triassic, 225 million years ago.