Ammonites Ammonites were sea creatures that lived before and at the same time as the dinosaurs. They were a bit like a modern squid or octopus with a shell. Ammonite fossils have been found all over the world. The fossils resemble a coiled snake, and for a time, people thought they were the remains of snakes, earning them the nickname snake stones!
Ammonites created additional chambers in their shell as they grew, giving the shell a distinctive spiral shape.
How did the Ammonites swim?
The hollow inner chambers of an ammonite's shell helped it float and control its buoyancy. Ammonites had a siphon system that allowed them to diffuse gas in and out of the empty shell to change its buoyancy. Only the outermost chamber housed the soft body.
They squirted water to propel themselves. Modern cephalopods (octopuses, squids, cuttlefish and nautilus) use this same method of jet propulsion to move.
When did the Ammonites live?
Ammonites are thought to have first appeared about 400 million years ago and became extinct about 66 million years ago during the same extinction event that killed off non-avian dinosaurs and many other animal and plant species on Earth.
What did the Ammonites eat?
Ammonites are thought to have eaten plankton and small crustaceans and were consumed by dinosaurs such as mosasaurs, ichthyosaurs and plesiosaurs.
Ammonite fossils
Ammonites were extremely abundant in the ocean for a long time, so fossils They are abundant. Scientists use the number and type of ammonite fossils found in a particular layer of rock to date other fossils in the same layer. These are known as index fossils.
Ammonite fossils are found in the sea. sedimentary rockIf you're in the UK, Lyme Regis in Dorset is a great place to go ammonite hunting.
Research on ammonites
How did ammonites control their buoyancy?
An ammonite's soft body only lived in the outermost section of its shell. The other chambers were sealed off as the ammonite grew and were used to control its buoyancy.
The Ammonites had a siphuncle system This allowed them to pump gas in and out of the empty shell to change its buoyancy. The siphunculus also pumped water out of the empty chambers as the body grew.
We can demonstrate how changing the density of the ammonite shell would have changed its buoyancy with a simple demonstration.
You will need
A jar or a large bottle
Small test tube with stopper or airtight lid
Buttons, clay or other small objects
Water
Instructions
Fill the jar or bottle with water almost to the brim. The jar should be deep enough so that the test tube is completely covered with water when submerged.
Place the empty test tube with the cap on inside the jar. It should float on the surface of the water. The test tube represents an ammonite.
Add buttons or clay to the test tube until it begins to sink below the surface of the water.
Add some water to the test tube containing the buttons and observe what happens. You should notice that the position of the test tube in the water changes as you add more water.
Continue adding more water to the test tube until it sinks to the bottom.
Adding water changes the density of the test tube. When the density of the test tube is greater than the density of the water in which it is placed, the test tube will sink.
When water is added to the test tube, its entire density changes, meaning its position in the water changes. This is how ammonites control their buoyancy. The siphunculus pumps water out of the newly emptied shell chambers and gases in to allow the ammonites to float at the level they desire.
More about ammonites
Find out how X-rays reveal what the ammonites ate.
Read about the The inspiring Mary Anning and make an ammonite out of clay.
Last updated on June 29, 2023 by Emma Vanstone
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