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Ammonites are a group of extinct marine animals
They had coiled external shells, similar to that of a modern day nautilus
Their shells were seperated into chambers
The ammonite itself only lived in the first chamber
The other chambers were filled with gas or fluid which the ammonite could regulate to control buoyancy
Their shells make for excellent index fossils
Ammonites first appear in the early Jurassic, around 200 million years ago
They went extinct at the end of the Cretaceous, just over 65 million years ago