ammonite

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.

ammonite Any of an extinct group of shelled cephalopod molluscs. Most ammonites had a spiral shell, and they are believed to be related to the nautiloids, whose only surviving form is the pearly nautilus. They are common as fossils in marine rocks.

ammonite An extinct aquatic mollusc of the class Cephalopoda. Ammonites were abundant in the Mesozoic era (225–65 million years ago) and are commonly found as fossils in rock strata of that time, being used as index fossils for the Jurassic period. They were characterized by a coiled shell divided into many chambers, which acted as a buoyancy aid. The external suture lines on these shells increased in complexity with the advance of the group.