They were cephalopod molluscs, related to squid and octopus.
They are found as fossils, formed when the shells of the animals became buried in sediment which later solidified into rock.
The oldest ammonite fossils are found in Jurassic rocks, from around 200 million years ago.
They became extinct at the end of the Cretaceous around 65 million years ago.
Ammonites had shells made of chambers. The air in the chambers helped them to swim.
There are hundreds of types of ammonites, with different shapes.
Here are just a few examples.