Horseshoe crabs are relatives of the Eurypterids (sea scorpions), whose first fossils appeared at the same time. They are quite ancient. They were definitely present in the early Ordovician, 450 million years ago, in a Burgess Shale-type environment. Their body shape has not changed much since then, though fossils in this group are scarce. The Atlantic horseshoe crab itself has no fossil record at all, and the genus "Limulus" goes back only some 20 million years, not 200 million.