Ammonites are one of the few groups of fossil invertebrates that are known to the general public as animals with coiled shells, some of them reaching significant sizes. The name Ammonites means 'image of Ammon': Ammon was an Egyptian god whose sacred animal was the ram, the curled horns of which ammonites were supposed to resemble. Ammonites were Mesozoic representatives of a larger group of cephalopods, the Ammonoidea, which also included a number of Palaeozoic lineages.
Among extant cephalopods, only extant members of the Nautilidae, the chambered nautiluses, have permanent external shells. Nautilus shells bear a general resemblance to those of ammonoids, and as a result ammonoids have often been assumed to have resembled nautiluses in life. However, there are numerous reasons to think that this may not have been the case. Ammonoids are more closely related to the other living cephalopods, the shell-less coleoids (octopods and squid). Study of the fossil record indicates that the coiled shells of ammonoids and nautiluses are due to convergence: both groups derived separately from straight-shelled ancestors. Soft-body remains of Michelinoceras, a straight-shelled cephalopod that was related to the ammonoid + coleoid clade, suggest that ammonoids probably possessed ten relatively large tentacles like modern squid, rather than the very numerous small tentacles of a nautilus (Jacobs & Landman 1993). Jacobs & Landman (1993) also argued that ammonoids are likely to have had an expansive mantle like that of coleoids, and could probably extend the body partially out of the shell. Many ammonoids had lateral extensions of the shell at the aperture that would have required some forward extension of the mantle to grow, and some even show evidence of external shell deposition. Palaeozoic ammonoids often have a sinus on the lower edge of the aperture like that of a nautilus: in the nautilus, this marks the position of the siphon used to propel the animal. Mesozoic ammonites, however, lack such a sinus, and may have had a more dorsally placed siphon, closer to the shell's centre of buoyancy. This would have allowed more direct, steady propulsion than that of a nautilus, but would have restricted the nautilus' ability to bend the siphon and use it to propel itself forwards as well as backwards.
As generally presented, the story of ammonoid evolution is the story of sutures. The septa dividing the chambers within the shells of ammonoids had a tendency to become increasingly complex over time, and the form of the sutures between septa and shell are one of the main characteristics used in distinguishing ammonoids. In many species of goniatites, one of the more basal Palaeozoic ammonoid groups, the sutures had only a small number of simple lobes. In other ammonoids, the number of lobes increased, and the individual lobes tended to develop their own complications. By the appearance of the ammonites, the sutures had become massively complicated, with almost fractal-appearing folds and folds within folds. The reasons for this complexity are uncertain: one possibility is that, if the ammonoids were more mobile than the modern nautilus, the crenulated sutures may have helped the animal in withstanding the hydrodynamic pressures involved with faster movement, by breaking up the flow of water within the body chamber (Hewitt & Westermann 2003).
Hewitt, R. A., & G. E. G. Westermann. 2003. Recurrences of hypotheses about ammonites and Argonauta. Journal of Paleontology 77 (4): 792-795.
Jacobs, D. K., & N. H. Landman. 1993. Nautilus-a poor model for the function and behavior of ammonoids? Lethaia 26: 101-111.