Eurypterids (
sea scorpions) are an extinct group of
arthropods that are related to
arachnids and include the largest known arthropods to have ever lived. They are members of the extinct order
Eurypterida (
Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species. The name Eurypterida comes from the
Greek words (meaning "broad" or "wide") and (meaning "wing"). This name was chosen due to the pair of wide swimming appendages on the first fossil eurypterids discovered. The largest, such as
Jaekelopterus, reached in length, but most species were less than . They were formidable
predators that thrived in warm shallow water, in both seas and lakes, from the mid
Ordovician to late
Permian .
