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.

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.

am·mo·nite / ˈaməˌnīt/ • n. an ammonoid that belongs to the order Ammonitida, typically having elaborately frilled suture lines.

ammonite XVIII. — modL. ammōnītēs, f. L. name cornu Ammōnis ‘horn of Ammon’ given to these fossils from their resemblance to the involuted horn of Jupiter Ammon; see prec. and -ITE.