Horseshoe crab fossil Fossilised horseshoe crab Limulus sp embedded in rock Horseshoe crabs also called king crabs first appeared in the Ordovician period million years ago By the beginning of the Triassic Period million years ago horseshoe crabs had evolved virtually the same body shape that they still possess today Horseshoe crabs are arachnids that are more closely related to spiders than true crabs They live on the sandy or muddy floors of shallow seas eating small invertebrates and growing to lengths of centimetres Once widely distributed only five geographically restricted species remain