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Horseshoe crabs are stem group chelicerates,[17] thus distant relatives of spiders. They were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. They may have evolved in the shallow seas of the Paleozoic Era (570-248 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods. Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years.[11]