Nautiloidea

a superorder of invertebrates of the class Cephalopoda. The nautiloids appeared in the Cambrian and reached their zenith in the Ordovician and Devonian; they became rare in the Mesozoic and Paleocene. Modern representatives of the nautiloids constitute the single genus Argonauta.

The shell structure of nautiloids is similar to that of the ammonites. The nautiloids, however, have simpler septa to divide the shell into chambers and a siphon that, as a rule, is not situated along the shell wall. The structure of the initial parts of the shell in nautiloids differs from that in ammonites.

The extinct nautiloids made up approximately 700 genera, embracing more than 2,000 species. These predators lived in the sea; their modes of locomotion included creeping, soaring, and swimming.

The Nautiloidea are important in the study of the development of the Cephalopoda and in the stratigraphy of lower Paleozoic deposits.