The horseshoe crabs belong to a group of animals called the Merostomata, which first appeared in the fossil record during the lower Cambrian Period (570 million years ago) in the United States. We first see true horseshoe crabs in the fossil record of the Mesozoic Era (230 to 65 million years ago). They are especially abundant in the Solenhofen lithographic stone of the Upper Jurassic Period (140 million years ago).
One interesting thing about horseshoe crabs is that their blood is blue. Scientists have discovered that the blood of horseshoe crabs can be used to test certain drugs for the presence of compounds that might cause a fever in patients. When these compounds, called "pyrogens", are present, they cause the horseshoe crab's blood to clot.
The Merostomata are nearly extinct, and only five species in three genera survive today. The modern genus Tachypleus is not seen in the fossil record until the Miocene Epoch (22.5 million years ago) in central Europe. The other two modern genera (Carcinoscorpinus and the king crab) do not appear at all in the fossil record.
Remember, an animal's first appearance in the fossil record is only the first time that we have an actual record of its existence. The animal could exist for millions of years without leaving any traces in the fossil record. The Merostomata have a thin, uncalcified cuticle, which does not fossilize well. The fossils that we do have are mostly of their shells, but not their legs or other softer body parts.
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