Ammonites are part of the cephalopod family, along with Nautiloids and Coleoids (which include squid and octopuses). Although Ammonites look similar to Nautiloids, both species inhabit a coiled shell, they are actually believed to be more closely related to Coleoids. Ammonites were born with one tiny shell and built new chambers as they grew. They would have moved into the new chamber, sealing off the older, smaller chambers with walls known as septa.
Ammonites first appeared around 450 million years ago, during the Paleozoic Era, and becoming extinct around 66 million years ago, at the end of the Cretaceous period. Ammonites became extinct at the same time as the dinosaurs, in a mass extinction believed to have been caused by an asteroid collision.