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When did animals resembling modern horseshoe crabs first appear?
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horseshoe crab
Horseshoe crab (Limulus polyphemus).
Fossil relatives of horseshoe crabs are recognized as far back as the Ordovician Period (approximately 485.4 million to 443.8 million years ago). Forms that closely resemble modern-day horseshoe crabs first appeared during the Jurassic Period (about 201.3 million to 145 million years ago).1