Ammonites appeared during the Silurian Period (435 million to 410 million years ago) and were abundant in the seas of the Jurassic and Cretaceous periods (175 million to 65 million years ago). They are extinct members of the Cephalopod class. They are related to the modern day nautilus, octopus, and squid. Ammonite shells had hollow chambers separated by walls called septa. A tube called the siphuncle, connected the body with the chambers allowing the animal to fill them with water or air, changing its buoyancy in order to rise or drop in the ocean. Only the living animal occupied the last chamber. Ammonites are important index fossils meaning they often link the rock layer in which they are found to specific geological time periods.