Ectocochleate cephalopod Temporal range:
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The ectocochleate (externally shelled)[1] cephalopods are the oldest known representatives of their class, dating back to the Cambrian period.[1] Their aragonitic shells are not prone to fossilisation.[2] They contain the modern Nautilus and many fossil forms including the ancient Ellesmerocerida and the ammonoids.[1]