Ammonites are an extict group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more closely related to living coleoids (i.e.octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilusspecies.

Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as heteromorphs).

The name ammonite, from which the scientific term is derived, 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. Often the name of an ammonite genus ends in -ceras, which is Greek for "horn".