The largest, such as "Jaekelopterus", reached 2½ metres in length, but most species were less than 20 cm (8 inches). They were the largest arthropods of all time. They were predators which thrived in the warm, shallow seas and lakes of the Ordovician to the Permian periods, around 460 to 248 million years ago. Recent research suggsts their eyesight was not very good.