Ammonites were huge, extinct relatives of the chambered nautilus. Some grew to 10 feet in diameter, and they balanced the heavy weight of their shells by filling them with gas, so they were buoyant. The ammonites, had exterior ribs on their shells, and complicated inner shell dividers. The ammonite on display has the coiled outer shell worn away so that the inner shell dividers can be seen. It is thought that ammonites became extinct at about the same time as the dinosaurs did.
Also displayed in the case beside this large ammonite are fossilized squid, and ancestors of other head-footed mollusks.