Ammonites are a type of cephalopod, distantly related to the modern day octopus, cuttlefish and nautilus.  They had coiled shells which contained gas chambers - a feature of many cephalopods.

Only the final chamber next to the opening was occupied by the animal.  As it grew, the animal periodically moved forwards and formed a new phragmacone chamber at the rear of the body.  Older chambers were connected by tube called the septum which was used to moved liquid  between chambers, the density of this liquid could be altered to allow the animal to move up and down in the water column.

Ammonites were named after the ancient Egyptian god "Ammon" who was depicted as having ram's horn on his head.

Ammonites had well developed eyes, jaws and tentacles.They swam using a type of jet propulsion by ingesting water through their mouths and shooting any excess water through a hose - like structure under the body propelling the animal backwards through the water.  The largest ammonite ever discovered was the size of a tractor tyre.