Ammonites are a common find in parts of the world as diverse as Morocco and Canada and they often are used as index fossils to help date unknown fossils found in the same strata. The Egyptians were familiar with ammonites, and named them after their god of life and procreation, Amon, who was often depicted with coiled ram's horns. However, it wasn't until the late 17th century that they were recognized as fossils.
Relation to the modern Chambered Nautilus
Though not all of them had a tightly coiled spiral shell, ammonites most closely resemble the modern chambered nautilus. Paleontologists theorize that the creature which possessed that shell was quite similar to the nautilus, with a soft body filling the last chamber and tentacles extending outside the shell. The other chambers could be filled with either fluids or gas, depending on whether the creature needed buoyancy or enough weight to sink deeper in the water. A central tube hints at the ability to propel itself by means of a concentrated stream of water, as does the modern squid. Ammonites are thought to have been predators, and to have been relatively quite intelligent -- able to learn and to take care of their offspring.
An important difference between the chambered nautilus and ammonites is the appearance of the septa, the divisions between the shell's chambers. Nautiloids show a simple curve in cross-section, and the sutures -- the connections between septa and shell -- are also simple. The septa of the ammonoids tend to be more complex, and ammonite specimens whose shells have worn away will show an intricately folded suture pattern. The complex sutures gave more strength to the thin shell and incidentally has provided paleontologists with a good way to identify a particular species.
Many ammonite specimens have an iridescent surface. This is original shell material, not a mineral replacement, although it is in part mineral. The shells consist of alternating layers of conchilin, a protein, and aragonite, a carbonate closely related to calcite. Calcite itself is often found in ammonites as a replacement mineral as are pyrite (also known as fool's gold), agate, and jasper. The processes of permineralization and petrifaction can produce spectacularly beautiful specimens where the inner chambers are filled with a pastel calcite or quartz mineral, and the septa have been replaced with shining pyrite.