Pterygotus

Pterygotus was one of the largest arthropods ever. It was certainly the largest eurypterid (or sea scorpion) ever.

Pterygotus was the top predator of its time, feeding on anything from jawless fish to other eurypterids.

Pterygotus was also the biggest creature of its time, but it couldn't move onto land like smaller eurypterids (Megalograptus) to mate, so it lived permanently in water. But it could go into freshwater lakes and rivers of the timr, possibly to perform the mating process or in pursuit of prey.

Pterygotus was found all over the world, so it must have been a rare sight in the Silurian seas. But it went extinct when the Devonian period started, as the eurypterids were being preyed upon buy larger fish. The sea scorpions went extinct all together at the end of the Permian, in the greatest extinctions of all time.