The fossil nautiloids and ammonites (represented today only by Nautilus) were primitive, less-specialized forms, probably leading a rather inactive sluggish life.
The first cephalopods were probably provided with a simple, caplike shell. With elongation of the shell and the formation of septa or partitions, the nautiloid shell could be formed (Late Cambrian, 510 million to 488.3 million years ago). The primitive elongate shell of Orthoceras became unmanageable and coiling resulted, as in the Gastropoda.
The Ammonoidea are usually considered to have evolved from Devonian (416 million to 359.2 million years ago) straight-shelled forms (Bactrites) with certain nautiloid traits. Coiled ammonites appeared in the Late Devonian Epoch (Goniatites). The subclass became extinct in the Cretaceous.