Ammonites are a distinctive class of extinct invertebrates within the Phylum Mollusca. These spectacular looking marine animals thrived in the Paleozoic and Mesozoic Eras, which equates to approximately 408 to 65 million years ago. The Ammonoidea are ancient cephalopods that are closely related to the modern day chambered Nautilus, cuttlefish (e.g. the Cretaceous Actinosepia canadaensis), octopus, and squid. Their unique, chambered shell shapes have fascinated collectors as well as scientists who use ammonites as index fossils to date the relative age of rocks in which the fossils appear. The proliferation of ammonite species from the Devonian to the Cretaceous has contributed to an abundance of samples around the world and has become useful for indicating the locations of ancient seas within the geologic record.
