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. These three are beautiful examples.
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 was typically depicted wearing ram's horns.