"The Golden Mean is represented by the Greek letter phi, (with the decimal representation of 1.6180...) is one of those mysterious natural numbers that seems to arise out of the basic structure of our cosmos. Phi appears regularly in the realm of things that grow and unfold in steps just as the Nautilus shell grows larger on each spiral by phi."
Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell as pictured above. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs.
Squid, Octopi & Cuttlefish all have shells. The shell may either be internal or very, very reduced, indicating an evolution from the dependence of a protective shell like the chambered nautiluses that has existed for millions of years.