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And at the time that these rocks were laid down, Africa, where we are now, and where these are found, was down by the South Pole. Spectacular though these are, they were by no means the largest arthropods in the ocean at the time. The trilobites had remote cousins, also arthropods, that had grown into monsters. Their remains are much rarer, and often fragmentary, but some of the most complete have been found in Scotland. ALARM SOUNDS One of the best is held in the vaults of Edinburgh's National Museum. Gosh! Well, this is a magnificent example of just how big an animal can grow if it has an external skeleton. This is a creature called the Eurypterid, or a sea scorpion. And it was a hunter. It had a pair of powerful pincers at the top, just behind its head. It was obviously a monster, a terror of the seas. And this is by no means the biggest of the eurypterids. Sea scorpions were the top predators of their day. As far as we know, they were the biggest arthropod that has ever existed. The discovery of a large fossilised claw suggests that they could grow up to two and a half metres, eight feet in length. So arthropods of one kind or another were certainly dominant million years ago. The seas were full of life. From huge complex Online Radio like this sea scorpion creeping along the bottom, to simple creatures, like jellyfish, floating on the surface waters. But the land was barren and without Online Radio of any kind. But there was food up there, simple plants, and that tempted some Online Radio to venture out of the water. Surviving on land, however, was a problem for them. Coming from the sea, they had to evolve ways of preventing their bodies from drying out. And even more difficult, they had to develop a method of breathing air. The very first Online Radio had simply absorbed dissolved oxygen from the water through the skins of their soft bodies. As they began to move and grow bigger, they needed more energy, more quickly. And that meant they had to improve their method of collecting dissolved oxygen.