Ammonites are well suited as standard fossils, and geologists can use them to date the strata that contain them. The closest relation of Ammonites to the living cephalopods is probably that of Octopatha rather than that of the Nautilus. The amceratidae first appeared on Earth from about the late Silurian to the early Devonian, and finally died out along with the dinosaurs in the late Cretaceous. Shell structure The ammonite shell is a conical tube with calcium carbonate as its main component. The beginning end of the shell tube is small, usually spherical or barrel-shaped, and is called the fetal shell. The shells of most ammonites swirl in a plane with the shell as the center, and a few shells are straight shells, spiral shells or other irregular shapes. Each shell ring of the planar spiral shell is called a spiral ring, the outer part is the abdomen, the side surrounding the inner spiral ring is the back, and the shell surface between the abdomen and the back is the side. Ammonite animals periodically move forward in the process of growth and secrete a septum behind it from the mantle to support the animal body, increase buoyancy and strengthen the shell. Therefore, the shell can be divided into two parts: the part of the animal body that lives without next door, called the abode; The part with a series of septum is the gas shell, the space separated by two adjacent septum is called the gas chamber, the septum is a curved surface, usually with a complex shape, especially the fold is very strong near the shell wall. The line of contact between the septum and the shell wall is called the suture line and is a very important sign in the classification of ammonites. Each septum has a circular septum hole for the location of the body tube. The body tube may play the role of connecting the software with the shell and adjusting the buoyancy to make the shell sink or float. It is usually located on the abdominal margin, but a few classes of body canals are in the back or near the center.