 Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian ("circa" 409 million years ago) and became virtually extinct at the close of the Cretaceous (66 Mya) along with the dinosaurs.The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers.The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived