Late Late Cretaceous. Ammonites are the ancient relatives of the modern nautilus. Like the nautilus, ammonites had hollow chambers inside their shell that could be filled with air or water to allow the animal to float or sink respectively. So the shells were not homes, but buoyancy tanks, like what modern submarines have. The wavy lines radiating from the center of the fossil are sutures that separated different sections inside the shell, before the open spaces inside were filled in with minerals to make the piece one solid fossil.