Cephalopods are the most advanced of all molluscs, having external or internal chambered shells with the gas filled chambers linked by a siphuncle and which gave buoyancy. In many cephalopods the shell is weighted by deposits which allow the animal to stay in its correct position in the water. The junction between the chamber walls and the outer shell wall is called a suture, and comprises various forms and degrees of complexity in different groups.
Cephalopods have a properly defined head with elaborate sense organs, and move by jet propulsion of water from the mantle cavity. They are carnivores. The modern Nautilus, ammonites, belemnite, goniatites and nautiloids (extinct relatives of Nautilus) are shown here.
The shells are straight (see right) or slightly curved in the older orders but become loosely to tightly coiled (see below) in later representatives. The siphuncle runs centrally through chambers. They often have well-developed cameral deposits, which are used to weight the shell so that it lies in the correct position while the animal is alive.
The pearly nautilus (Nautilus pompilius, see below) and some other species are the sole remaining representatives of this group.
Range: Cambrian – Recent
The ammonoids are tightly coiled with a marginal siphuncle and characteristic complex sutures. The goniatites and ammonites are included in this group. Goniatites have a more globose shell form and less complex sutures. Ammonites usually have a highly ornamented surface and a complex suture.
Range: Devonian – Cretaceous
This group includes belemnites, squids, cuttlefish and octopoids. The fossil record is relatively poor as the mineralized part is reduced and internal. Belemnites are the exception. They possess an internal chambered shell (the phragmocone) which is surrounded by a robust guard.
Range: Devonian – Recent