Ammonites are an extinct group of marine mollusks that lived during the Mesozoic era. They are often found as fossils and are characterized by their distinctive spiral shape. The term 'AMMONITE' comes from the Greek word for 'HORNED, ' referring to the horns of the god Ammon, which were thought to resemble the shape of the fossilized shells. Ammonites are important to geologists because they can be used to date rocks and other geological formations.