Sea scorpion (Jaekelopterus rhenaniae), artwork. This sea scorpion lived approximately 390 million years ago and is one of the largest arthropods ever discovered. Sea scorpions (order Eurypterida) were marine creatures related to arachnids that died out 252 million years ago. The fossilised remains of Jaekelopterus suggests that it measured up to 8.2 metres in length. It had a pair of long, articulated, claw-like chelicera that was used to grab prey and was thought to be the top predator at the time.