Plain-language Summary:
Ammonites are well known fossil molluscs, belonging to the group known as cephalopods, and close relatives of the octopus, cuttlefish, squid and the pearly nautilus. [For further general information, see The Cephalopod Page at:http://is.dal.ca/~ceph/wood.html.]
They dominated the Mesozoic seas, filling many of the niches fish do today. Although the pearly nautilus, Nautilus, has a similar external shell, it has never been clear to what extent this shell can be used as a functional analogue for the ammonites.
Both ammonite and pearly nautilus shells are chambered. Most of the chambers are empty except for the final chamber, known as the living chamber. In the pearly nautilus the living chamber is completely filled by the soft body parts of the animal. A key difference is that where pearly nautilus has a short and broad living chamber, ammonites often have very long and narrow living chambers.
We suggest that the ammonite's soft body parts might have not filled the entire living chamber, but instead could have been much smaller. With a small body, during feeding, the animal would be positioned near the opening. The tentacles could be extended to feed or pull the animal about like an octopus. If it was threatened, the animal could withdraw the body back into the living chamber which would have provided an effective defence.
If this was the case, a small, mobile body within a long living chamber profoundly affects the orientation of the ammonite in the water column. In the case of heteromorphs, many were considered to be planktonic because their shells were oriented with their heads pointing upwards. Using our model, when the head and tentacles were extended the shell would tilt down towards the sea bed, allowing them to have been bottom dwellers. When the body is pulled back in, the shell would tip upwards and away from danger.
Animations are included in this paper to help illustrate and explain these ideas.
Ammonite: Cephalopod mollusc with an external chambered shell. All ammonites lived in the sea, and used the chambered shell to provide neutral buoyancy (see: Buoyancy).
Buoyancy: A property of any object placed in water or other fluid. Negatively buoyant objects sink to the bottom, while positively buoyant ones float to the surface. Neutrally buoyant objects neither sink nor float but remain at the same point in the water column.
Centre of buoyancy: The point through which the lifting or floating forces act.
Centre of mass: The point through which the weight of an object acts.
Heteromorphs: Any ammonites with an uncoiled shell, but usually refers to a group of mainly Cretaceous ammonites, the Ancyloceratina.
Orientation: The alignment of the shell relative to the sea floor. Normally, the shell will be oriented with the centre of mass vertically below the centre of buoyancy.
Neale Monks and Jeremy R. Young, Department of Palaeontology, Natural History Museum, Cromwell Road, South Kensington, London, SW7 5BD, UK.