A search on the web for terms such as "golden spiral" turns up sites which maintain that the Nautilus shell is a logarithmic spiral with a growth ratio equal to the Fibonacci ratio (1.618). This "golden spiral" is supposed to endow the nautilus with some mystical or spiritually significant property. The purpose here is to demonstrate that the main precept of these ideas is wrong. The nautilus shell does NOT conform to the Fibonacci sequence. Figure 1 illustrates a real fossil nautilus with a "golden spiral" curve overlay.
Ammonites come in diverse shell shapes, sizes, and ornamentation. Some, especially the ammonites in the Phylloceras family, can be confused with fossil nautilus shells. The nautilus may be readily separated from ammonites if the siphuncle tube position can be identified. The siphuncle tube in the ammonoids runs along the top or bottom side of the shell. The siphuncle tube in the nautiloids runs through the approximate center of the septum wall with the septal collar in the retrochoanitic (pointing to the previous cavity) position. The typical nautiluoid septal penetration by the siphuncle is clearly seen in figure 1 above. In this case we are fortunate to have a nautilus that has been split and prepared for examination. Even lacking that, it is frequently possible to discern the siphuncle position. See figure 2.
The shape of the septa is about the best means of differentiating between ammonoids and nautiloids. Nautiloids have a basic simple structure and pattern as seen in figure 3.
The ammonoids that might be confused with nautiloids all have the complex ammonite suture line. If a complex suture line is present, the fossil is certainly not a nautilus! The last illustration, figure 4 is of an ammonite specimen shipped to us from Madagascar as a nautilus. The complex "phylloceratid" suture pattern leaves no doubt that this is NOT a nautilus!