Mazon Creek Fossil Fauna
Horseshoe crabs are really not crabs at all. Though they resemble crustaceans, they belong to a separate class, Chelicerata, and are closer to the eurypterids. Their nearest living relatives are scorpions, ticks, and land spiders. They are considered to be “living fossils” because of the great similarity between modern and fossil forms. Ranging from the Late Ordovician (roughly 450 million years ago) to the present, they are little changed in the last 250 million years.