Ammonite Fossil DIshFossilized ammonite with a smooth polish on the outside and a concave dish portion for storing small objects. A relic from millenia past. Ammonite Fossil Ammonites began appearing over 410 million years ago, during the Early Devonian period, vanishing soon after the Cretaceous mass extinction event 66 million years ago. They are most closely related to living octopuses, squid, and cuttlefish. The name "ammonite" was inspired by the spiral shape of