Nature’s greatest innovators
Molluscs are animals like mussels, clams, snails, slugs, cuttlefish and octopus. They include extinct creatures such as ammonites and belemnites. Molluscs are a hugely diverse group of animals. Although they look very different, they have a common basic body structure. They have soft bodies, and a muscular ‘foot’ to help them move. Many molluscs also have a hard shell.
Almost one in four of all animals that live in the sea belong to this group including most of the common shells we find on the shore today. Although most molluscs live in the sea, some are land dwellers such as the snail and slug.
Mollusc fossils are usually well preserved because of their hard shell. But those without a shell, such as the slug and octopus, are rarely found as fossils.
Like squids with spiral shells
Ammonites are one of the most commonly-found fossils. They are extinct marine creatures – predators that moved through the water by jet-propulsion. We know this because the nautilus, the closest living relative to the ammonite, still survives today in the Pacific and Indian Oceans. Ammonites belong to a group known as cephalopods, from the Greek ‘cephalo’, meaning head and ‘pod’ meaning foot. They have a big head, large eyes and tentacles that developed from a primitive ‘foot’. Modern cephalopods include the nautilus, squid, cuttlefish and octopus.
Sink or swim
Ammonites have a spiral shell divided into chambers. The soft body of the ammonite only took up the last half whorl of the shell. Only the shells of ammonites have ever been found as fossils. When the animal was alive, a small tube called the siphuncle filled the chambers with gas and water to control buoyancy. They moved by sucking water through the mouth, pumping it over the gills, then squirting it out of a tube below the tentacles. This propelled the animal through the water – backwards!
Fossil folklore – Snakestones
Before the mid-18th century, the origin of fossils was shrouded in superstition and myth. In England, particularly around Whitby, people thought that ammonites were coiled snakes that had been turned to rock. Hence they were known as snakestones. The name ‘ammonite’ comes from the ancient Greeks who believed the shells resembled the horns of the ram. So they were named after the Ram-god Ammon.
Ammonite or nautilus?
Although similar, there are differences between the ammonite and the nautilus. The nautilus has simple chamber walls, whereas the ammonite has complex walls that fold to form suture lines (you can see them when the shell is removed). The other difference is that in the nautilus, the tube or syphuncle that links the chambers runs through the middle of the chambers, but in an ammonite it runs around the outer edge.
Ammonites as tools of time
Different species of ammonites lived during different time periods, so scientists can use them to determine the relative age of the rocks in which they’re found. Such fossils are called zone fossils. Many ammonite species evolved rapidly but only survived for about 250,000 years before evolving into something different. So the rock layers can be dated based on the appearance and disappearance of different ammonites. These time periods are known as ammonite zones and their names relate to the ammonite species.
Harpoceras elegans, Jet Rock Series, Whitby. The rocks in both localities must be the same age as they contain the same type of ammonite that only existed for a short period of time. Ammonites are also really important in helping us understand the evolution of other animals. For example, the huge pliosaur skull in Dorset County Museum comes from an ammonite zone known as the Baylei zone. But the similar Westbury pliosaur comes from the younger Eudoxus zone. So the ammonites tell us that the two creatures lived about 750,000 years apart, and that the Westbury specimen is younger. Without the relative aging of the rocks we would not be able to determine the evolution of life.
Other zone fossils
Many rocks formed in environments that were not suitable for ammonites. So other creatures can be used as zone fossils, such as the tiny ostracod. But the Triassic rocks of East Devon formed in deserts where fossils are very rare, so zone fossils can’t be used to date the rocks.
Radioactive dating is a method of absolute dating of the rocks by measuring the decay of radioactive minerals. It is used in igneous rocks like lava and granite. The dating of the earth’s rocks is a complex process involving thousands of measurements from around the world. You can see the latest results on www.stratigraphy.org
Bullets on the beach
Belemnites are probably the most common fossils found on the beaches, especially around Charmouth. When the animal was alive, the pencil or bullet-shaped shell was surrounded by a soft body, and the creature looked very like a squid. Like ammonites, belemnites belong to the group known as cephalopods.
Belemnites had ten arms, equipped with hooks for grabbing prey. But belemnites were themselves prey for larger marine creatures – plesiosaurs and pliosaurs have been found with belemnite guards and hooks in their stomachs.
Jet-propulsion and ink clouds
Just like squid, belemnites swam by sucking water through their mouth and over their gills before squirting it out of a small tube below the head. So they jet-propelled themselves backwards! If threatened, they could squirt a cloud of ink into the water to aid their escape. Very occasionally, the soft parts of belemnites have been preserved, including the tentacles and even an ink sack.
The phragmacone or chambered shell which sat inside the solid shell (guard) The belemnite’s shell, known as a guard, is formed from calcite crystals. If you look closely, you can often see growth rings, like those in a tree. The pointed end of the shell is known as the guard, while other end housed a chambered shell called the phragmocone. This was filled with gas and water so the animal could control its buoyancy. Before fossilisation, the phragmocone was often crushed or filled with mud.
Look out for the beautiful, iridescent belemnites in our collections. This is where the shell around the phragmacone is preserved as fossilised mother of pearl. Many mollusc shells are lined with mother of pearl (also called nacre) – the iridescence is caused by thousands of thin layers breaking up the light into rainbow colours.
Fossil Folklore – Thunderbolts
Because of their pointed shapes, people once believed that belemnites had been cast down from the heavens during thunderstorms. This led to them being called thunderbolts and even ‘devil’s bullets’.