Most people are familiar with the spirally coiled ammonite shell. Externally the shell looks like that of a snail, but internally it is very different. The accompanying picture shows an ammonite that has been cut in half and polished. You can see the inside is divided into many small chambers. This is very different to a snail shell which is a simple open spiral inside. The closest comparison to an ammonite we have is a living sea creature called the nautilus. The nautilus is related to creatures such as the octopus and squid but it has a spirally coiled shell just like the ammonite. As nautilus is a living creature we can look at its soft parts and see how it uses the shell. In a living nautilus the chambered sections of the shell contain nitrogen gas and water. A spiral tube called the siphuncle runs through all the chambers to the centre of the shell. The fleshy body of the nautilus is only in the final chamber of the shell. This chamber makes up about half the final whorl. The living nautilus uses the siphuncle to adjust the proportion of water and gas in the chambers of its shell. This allows it to move up and down in the water, just like a submarine. The video below shows how a nautilus swims.
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This picture shows a modern nautilus shell that has been section in the same way as the fossil ammonite, the similarity between the two is readily seen. Nautilus catches its food with a mass of tentacles, just like a squid. In the middle of the tentacles is the animals mouth. Like an octopus it has a sharp beak to tear and cut its food. Below the tentacles is a large open tube called the siphon. This is used to draw water to the gills, where the oxygen is extracted. When the water is squirted back out it pushes the animal backward. In this way nautilus is jetted propelled through the water.
The study of modern day creatures is an invaluable way of gaining an insight into what ancient creatures may have looked like and how they could have behaved and interacted together.
A good example of this is the strange fossil shown in the photo. It is clearly a fossilised bone, but what might it be? It looks a bit like a jawbone, along the right edge are curved and pointed hooks that could easily be teeth. Despite these looks it is in fact the bone from a fossilised Jurassic shark.
This is unusual as sharks skeletons are mostly made of cartilage, a gristle-like material that has the advantage of being lighter than bone. This is useful for the shark, because unlike bony fishes they do not have an air-filled bladder to keep them bouyant in the water.
Unfortunately for palaeontologists (people who study fossils) cartilage normally decays before it can be fossiled. This is one of the reasons why most fossil shark remains only consist of teeth. Jurassic sharks had two of these bones. They are fin spines that were attached to the sharks back infront of each of its two dorsal fins.
A small species of modern shark has similar spines, it is called the Port Jackson Shark. It uses the spines as protection from predators, anything thing biting down on the shark would get the barbed spine stuck in the roof of its mouth.
In the Jurassic sea the large ichthyosaurs would have been the sharks main predators. Fossilised ichthyosaur jawbones have been found with the tips of shark fin spines wedged between the teeth. This would certainly be very painful for the ichthyosaur.
This image shows a drawing of a Port Jackson Shark. The two spines on its back are clearly visible. If you wish you can watch a short clip of the Port Jackson Shark swimming.
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