Ammonites are amongst the best known fossils. They belong to the cephalopods, a class of mollusc. They were well adapted as carnivores because they had a head with tentacles, a nervous system, an advanced brain and good eyesight. They were jet-propelled, squirting out water under high pressure from a tube called a hyponome.
Their shells are divided into many chambers. The outer chamber housed the soft parts of the animal. The other chambers were empty, but were connected by a tube called a siphuncle. The animal filled the empty chambers with gas to control their buoyancy, rather like a modern submarine.
Ammonites evolved rapidly into hundreds of species, and are used to date Jurassic rocks. Different species of ammonites occur in the Lower Jurassic, the Middle Jurassic, and the Upper Jurassic. Their closest living relatives today is the Nautilus, found in the Pacific Ocean.
