The modern nautilus is the only surviving shelled cephalopod. From the modern nautilus we can learn much about the function of the shell of the extinct ammonites. Although there are many similarities between nautiloids and ammonites, it appears that ammonites had closer affinities to today’s squids and octopi. Ammonites may have evolved from an ancient nautiloid more than 400 million years ago during the Silurian Period. Many recent squids and especially octopi seem to have descended from various ammonites during the Cretaceous Period.
The modern nautilus is probably the direct descendant of the nautilus Eutrephoceras, which is found throughout North American Cretaceous marine sediments. Fossil nautilus are common but not abundant.
The fundamental position of the body parts and the purpose and use of the nautilus shell is essential in understanding these principles in fossil shelled cephalopods.