Chambered Ammoite Fossil
Ammonites were free-swimming mollusks of the ancient oceans, living around the same time that the dinosaurs walked the Earth and disappearing during the same extinction event. They came in a range of sizes, from tiny species only a couple of centimeters across, to large ones reaching over two meters in diameter. The animal would have lived in the last and largest of a chain of spiraled chambers. Filling these chambers with fluid or gas allowed the ammonite to sink like a stone to avoid predators, though ammonite shells with tooth marks on them have been found, evidence that it didn't always work. Fossilized shells are usually, but not always, beautiful spirals.