Sea Scorpions

Roaming the sea bed (and ponds and lakes and rivers...) millions or years ago were some creatures known as sea scorpions, related to the same families as scorpions, horseshoe crabs and spiders. Unlike these creatures that hang around with us now, sea scorpions could grow pretty big -- most could grow between 20cm up to a couple of feet big.

Then there was Jaekelopterus rhenaniae, who was, in a word, massive. 2.5 metre massive, with 46cm feeding claws, and was the largest arthropod the world has ever seen.