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 (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; 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).
The Ammonoidea can be divided into eight orders, listed here starting with the most primitive and going to the more derived:
- Anarcestida, Devonian
- Clymeniida, Upper Devonian
- Goniatitida, Middle Devonian – Upper Permian
- Prolecanitida, Upper Devonian – Upper Triassic
- Ceratitida, Permian – Triassic
- Phylloceratida, Triassic – Cretaceous
- Lytoceratida, Jurassic – Cretaceous
- Ammonitida, Lower Jurassic – Upper Cretaceous
In some classifications, these are left as suborders, included in only three orders: Goniatitida, Ceratitida, and Ammonitida.
Taxonomy of the Treatise on Invertebrate Paleontology
The Treatise on Invertebrate Paleontology (Part L, 1957) divides the Ammonoidea, regarded simply as an order, into eight suborders, the Anarcestina, Clymeniina, Goniatitina, and Prolecanitina from the Paleozoic; the Ceratitina from the Triassic; and the Ammonitina, Lytoceratina, and Phylloceratina from the Jurassic and Cretaceous. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea.
The extinction of the ammonites, along with other marine animals and non-avian dinosaurs, has been attributed to the K-Pg extinction event, marking the end of the Cretaceous Period.
Eight or so species from only two families made it almost to the end of the Cretaceous, the order having gone through a more or less steady decline since the middle of the period. Six other families made it well into the upper Maastrichtian (uppermost stage of the Cretaceous), but were extinct well before the end. All told, 11 families entered the Maastrichtian, a decline from the 19 families known from the Cenomanian in the middle of the Cretaceous.
One reason given for their demise is the Cretaceous ammonites, being closely related to coleoids, had a similar reproductive strategy in which huge numbers of eggs were laid in a single batch at the end of the lifespan. These, along with juvenile ammonites, are thought to have been part of the plankton at the surface of the ocean, where they were killed off by the effects of an impact. Nautiloids, exemplified by modern nautiluses, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a bolide strike, and thus survived.
Many ammonite species were filter-feeders, so they might have been particularly susceptible to marine faunal turnovers and climatic change.