Ammonoidea - Classification of The Ammonoidea

Classification of The Ammonoidea

Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous (65.5 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; by these and other characteristics we can divide subclass Ammonoidea into three orders and eight known suborders. While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles (or peaks) and lobes (or valleys).