True scorpions appeared in the Middle Silurian Period, some 420 million years ago, and most early types were, in fact, aquatic creatures. Land-living (or terrestrial) forms of scorpions are known from the fossil record of the Early Devonian Period, while the last aquatic types survived into the Jurassic Period. They are thought to be cousins or descendants of eurypterids (sea scorpions), from which they inherited their overall body plan.