ammonite, one of a type of extinct marine cephalopod mollusk, related to the nautilus and resembling it in having an elaborately coiled and chambered shell. Unlike the interiors of nautilus shells, the chambers of ammonite shells display intricately shaped septa and sutures. The type included numerous species, which were widely distributed during the Mesozoic era, about 200 million years ago. Ammonites are classified in the phylum Mollusca, class Cephalopoda, subclass Ammonoidea.