Ammonite skull

An unusual carved ammonite fossil.

Ammonites are extinct shelled cephalopods. All of them had a chambered shell they used for buoyancy.  The group Cephalopoda is divided into three subgroups: coleoids (including squid, octopus and cuttlefish), 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.  

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.  

Ammonites' growing shells typically formed into a flat spiral, known as a planispiral, although a variety of shapes did evolve over time. Shells could be a loose spiral or tightly curled with whorls touching. They could be flat or helical. Some species would begin growing their shell in a tight spiral but straighten it out through later growth phases.  Most ammonites died out at the same time as the non-avian dinosaurs, at the end of the Cretaceous Period, 66 million years ago.

The spiral draws in negative energy, filtering it through the chambers and releasing fresh, positive energy.  Ammonite fossils are believed to help with ailments like blood pressure and degenerative disorders, such as those affecting the ears and lungs.