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Ammonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
Ammonites’ closest living relative is probably not the modern Nautilus (which they outwardly resemble), but rather the subclass Coleoidea (octopus, squid, and cuttlefish). Their name came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled rams horns. Pliny the Elder (d. 79 A.D. near Pompeii) called fossils of these animals ammonis cornua (“horns of Ammon”) because the Egyptian god Ammon (Amun) was typically depicted wearing ram’s horns.
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