The standard explanation, often give, is that nautiloids, represented by the living Nautilus has a central siphuncle while ammonioids (i.e. ammonites) have siphuncles that run along the outer rim of the shell. True for most ammonites by far and certainly true for Nautilus, Another explanation is that nautiloids have simple sutures whre the septa join the outer shell while ammonites have complex sutures. Certainly some ammonites have very complex, crenelated suture patterns, far more complex than any nautiloid. On the other hand no ammonite has the complexity to its siphuncle as found in some nautiloids., So what is the primary difference between the two that can be found in their shells.
The answer is actually rather simple. The answer is found in their septa, the partitians that separate the chambers in that portion of the shell, known as the phragrocone, that indicates that the physiology of the two was rather distinct. As viewed from the front, looking back, all nautiloids have concave, dish-shaped septa. Ammonoids have symmetrically contoured, convex septa. Nautiloids can simply secrete a new septum from the back of the mantle after moving forward. Ammonoids have to shape their mantles before secreting new septa
The picture to the left shows the vertical cross section of a gonititid ammonite (left) and of a recent Nautilus (right). Note the difference in the profiles of the respective septa -- arched forward (convex) in the ammonite, dish-shaped (concave) in Nautilus, representing the Nautiloidea.
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