Ammonites are the ancient equivalent to the modern-day chambered Nautilus. These sea faring squids built a chambered shell in which they would pump air into the empty chambers and float at different levels of the ocean in search of food. This unique ability to float at different depths of the ocean is what inspired Jules Vern (the famous science fiction writer) to call the first submarine the Nautilus. Ammonites lived approximately 400 million years ago.

Source:  http://www.brenardine.com/fossil-jewelry.htm