Ammonites are extinct shelled cephalopods. All of them had a chambered shell that they used for buoyancy.'
The group Cephalopoda is divided into three subgroups: coleoids (including squids, octopuses and cuttlefishes), nautiloids (the nautiluses) and ammonites.
Ammonites' shells make the animals look most like nautiluses, but they are actually thought to be more closely related to coleoids.
It’s more likely that ammonites would have had eight arms rather than lots of tentacles like a nautilus, though the shell is more similar to that of a nautilus.
Ammonites were born with tiny shells and, as they grew, they built new chambers onto it. They would move their entire body into a new chamber and seal off their old and now too-small living quarters with walls known as septa.
The subclass Ammonoidea, a group that is often referred to as ammonites, first appeared about 450 million years ago.
Ammonoidea includes a more exclusive group called Ammonitida, also known as the true ammonites. These animals are known from the Jurassic Period, from about 200 million years ago.
Most ammonites died out at the same time as the non-avian dinosaurs, at the end of the Cretaceous Period, 66 million years ago.