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Ammonites (Ammonoidea) were Cephalopoda class molluscs. They appeared during the Devonian period and died out at the end Cretaceous period alongside with dinosaurs. Ammonites were marine animals, so they lived only in the oceans.

The name ammonite was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder (d. 79 AD. 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.

Ammonites can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa joint the outer shell wall, and in general by their siphuncles.