Ammonite was once thought to be the petrified remains of snakes! Modern science, however, tells us that these fascinating fossils are actually the remains of an ancient aquatic mollusk. A mollusk is an invertebrate with a soft, unsegmented body. The soft body of an ammonite was protected by a hard outer shell. The shells of ammonites ranged from an inch to nine feet! Each shell is divided into many different chambers. The walls of each chamber are called septa. The septa were penetrated by the ammonite’s siphuncle, a tube-like structure that allowed the ammonite to control the air pressure inside its shell. Ammonites were aquatic creatures, and being able to control the air pressure inside their shells meant being able to control their buoyancy.