Ammonites appeared during the Silurian (435-410 million years ago) and were abundant in the seas of the Jurassic and Cretaceous (175 million years ago). They are extinct members of the Cephalopod group. Ammonite shells had hollow chambers separated by walls called septa. A tube called a siphunculus connected the body to the chambers, allowing the animal to fill them with water or air, changing its buoyancy to rise or fall in the ocean. Only the living animal occupied the last chamber. Ammonites are very important fossils for geological dating. In some of the specimens in these lots, the natural sutures of the shell can be seen.
