The name Ammonite was first used in 1546 by Georg Bauer, who used it to describe peculiar coiled fossils bearing a likeness to the horns of Ammon, after the ancient Egyptian king of the gods, Amūn. Amūn is portrayed with a ram's head, and this may be why ammonites are traditionally illustrated upside down like a ram's horn. Ammonites lived during the Jurassic and Cretaceous periods, during the time of the dinosaurs, and are found worldwide in rocks that formed in marine environments of that time. They were highly complex animals that achieved top predator status in the Mesozoic oceans, and their fossils are of fundamental importance in unraveling the ages of rocks. Ammonites were a diverse group ranging from complex shell forms like Eubostrychoceras to smooth forms like Placenticeras, and ranging in size from a couple of centimeters to several meters. True ammonites grew in a planispiral pattern, a spiral and on one plane with consecutive coils touching or overlapping. Heteromorph ammonites started there life the same way but grew into loosely coiled  or angled whorled patterns. 

Ammonite shell shapes were directly linked to their environment. Narrow streamlined shells could withstand a more open water environment, whereas more robust and ornamented forms were structurally stronger but less agile. Jacques Cousteau showed that the modern Nautilus can move quite fast when it wants to, streamlined ammonites were likely capable of even faster movement. Recent evidence shows that the more ornamented forms seem to have favored shallower environments, about 30 meters.

 The interior of the ammonite shell was divided into sections by curved walls, called septa. A new born ammonite shell the protoconch had one buoyancy chamber and would continue to grow adding septa. Contact points where the septa join the outer shell show complex suture patterns. A tube called a siphuncle connects all of the septa, and fluid forced through the siphuncle filled the hollow chambers, raising or lowering the ammonite in the ocean water. The shell's function was most likely the same as that of the modern Nautilus, except the Nautilus siphuncle is located through the middle of each septum instead of on its margin as in the ammonites. Ammonites had two jaws and a radula like the modern Nautilus, however ammonite jaws were far more specialized in function. Only about four types of nautiloid jaw are known from the fossil record, whereas there are almost as many types of ammonite jaws known as there are different types of ammonites. Evidence shows that a medium sized Baculites fed on plankton, some fossils show ostracods and foraminifera as intestinal Contents, larger ammonites may have fed on the same things as today's cephalopods. They, in turn, were eaten by a multitude of marine predators including octopus and possibly larger ammonites

Heteromorphs include the orthoconic or straight Baculites and Eubostrychoceras to the bizarre Nipponites.  Every genus of heteromorph had a different distinct pattern but the reason for the deviating growth patterns is a mystery. Heteromorphs had the same chambered buoyancy control system as other ammonites. It's unlikely they had a strictly benthic lifestyles as shell takes a lot of energy to produce, especially so for complicated  forms so there must be a specific reason for control of buoyancy.  Fluid transfer is not quick, it is a slow process in the modern Nautilus and likely the same for ammonites. The shell of ammonites was thinner than the nautilus but was stronger in structure, the septal structure of  an ammonite shell is effectively like 3 dimensional corrugation. The thinner shell likely meant faster growth and less energy expended to produce it. Fossil environments show heteromorphs were living sub-surface to about 200 meters or 655 ft. The modern Nautilus that can be found at 300 meters or 984 feet, planispiral ammonites may have lived at even deeper depths.

Dimorphism in ammonites is common, the female is thought to be bigger for egg  production but the reason dimorphism is not as obvious in some ammonites is not known. Observations in Nautilus show that excluding size, male and female shells are based on formation of the animal and in the case of the female a slight almost undetectable constriction. One study of N. pompilius showed that it was not uncommon for the male that is normally bigger by about an inch or two centimeter to also have a deformed spadex organ, which is responsible for the male Nautilus bowl shaped aperture. Animal deformity or immaturity would make the male and female Nautilus shells by themselves virtually indistinguishable. The distinguishing factor that draws the microconch (small form) and macroconch (the large form) into the same species of ammonite is the early shell structures. Examination of both ammonite conchs side by side show the same shape, structure and ornamentation of  initial whorls but, the macroconchs continue to grow another whorl or two sometimes even changing ornamentation. 

Most cephalopods are represented in the fossil record with soft body impressions, even a nautilus hood has been noted in the fossil record; interestingly the same locations that yield exquisite octopus, squid and belemnite fossils only have ammonite shells. To date, other than retractor muscle scars, rare indications of internal organs and egg sacs, direct evidence is lacking about the ammonite soft-body structure. Retractor muscle scars are found on the inside of the living chamber of the shell, and they represent the muscles the modern Nautilus uses to retract its body into its shell.


Ammonites could move backwards using a directional water siphon. Water was sucked into the mantle, then forced under pressure out through the siphon, the same as octopus, squid and Nautilus. The siphon, or hyponome of the Nautilus differs from other cephalopods in that it is a one-piece flap that is folded over, not only is it directional but volume can also be controlled. In other cephalopods, such as octopus, squid, and cuttlefish, the hyponome is a muscular tube. The flap-like hyponome may be an adaptation to confinement in a shell that both nautiloids and ammonites shared.

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