The subclass Ammonoidea is phylogenetically divided into nine different orders that persisted from the Devonian period all the way up to the start of the Tertiary period. The first four and oldest of the Ammenoids, were the Anarcestida, Clymeniida, Goniatitida, and the Prolecantitida. These species all radiated around the middle to late Devonian period where they went extinct, while some others like the Goniatitida and the Prolecanitida persisted into the Carboniferous and the Permian periods. The most important survivor of the four, Prolecanitida, eventually gave rise to the most diverse groups of the Ammonoidea: the Ceratidia, Ancyloceratida, Lytoceratida, Ammonitida, and the Phylloceratida. The Ceratidia went extinct during the time of the Permian-Triassic extinction event along with the Prolecantidia, and the Goniatidia. The remaining species live well into the Mesozoic Era until the most famous extinction event of the Cretaceous-Tertiary occurred which wiped out all of the Ammonoids as well as the Dinosaurs.
THE AGE OF AMMENOIDS
Ammonite taxa radiation and extinction throughout the fossil record in comparison to other cephalopod groups. Placenticeras meeki, and the Baculites originate from the late crutacious era.
Three dimensional renderings of morphological structures in ammonites. A: Shell of Baculites sp. D/E: Radula of Baculites sp. F: Sketch of the first tooth of the Baculites sp. radula.
The Ammenoidea are a subclass under the Mollusks, and as such they have very similar body features such as the mantle, foot, and visceral mass, and the radula. The radula can be considered the most important feature to look at on the Ammonite because it is precisely what lead to their extinction. Ammonoid feeding habits were quite limited, by the radula specifically, to only feeding on small organisms soft tissues unlike the Nautiloids which could consume their prey whole.
FEEDING BEHAVIOR
The genus Baculitidae, under the suborder Ancyloceratina, resemble many other Ammonite suborders such as the aptychophorans. Their buchal mass specifically their jaw and radula are much smaller compared to their Nautiloid cousins. The buchal apparatus in nautloids living today have beaklike jaws with large radular teeth which allowed them to eat much larger prey and physically chew on them. The Baculitidae on the other hand show that the Ammenoidea had much smaller jaws and radular that could not physically accommodate large organisms for consumption, and so their diet was limited to mainly plankton, as well as tiny crustaceans and juvenile ammonites.
BACULITES
EXTINCTION
As stated before, their primary source of food came from consuming plankton which inhabited regions close to the surfaces of water bodies. During the Jurassic-Cretaceous boundary period there was a decrease in water body levels which lead to the extinction of a lot of shallow water body Ammonites, where as the deep sea adapted Ammonites lived to produce further generations. The following period after the Crutaceous was the Tertiary period, and it is here that we see the end of the Ammonites. The first stage of their extinction started with the acidifcation of sea surface levels which was thought to have killed off much of the plankton that lived near the surface which lead to a devastating food loss. The Ammenoids on the other hand usually laid their eggs at the end of their lifespan, and so the second stage came with the Chixculub meteor impact that was thought to have killed off the possible egg stages of the Ammonites. This decimated their offspring and their lineages as a result. The combination of decreased sea levels, water acidification, and meteor impact lead to the conclusion of this subclass Ammenoida. Other close relatives of the Ammenoids like the Nautiloids continued to live on after the K-T extinction event because primarily their egg laying stages occured throughout their lives and at deeper portions of the sea. The Ammenoids on the other hand usually laid their eggs at the end of their lifespan, and they weren't so reliant on plankton as their food source. This combination of factors like their habitat, feeding, and offspring production proves that the Ammenoid extinction was no accident, but rather a result of evolution and natural selection.
Three dimensional rendering of the morphology of the radula and teeth of Baculites sp.
Baculites were seen as the walking stick rocks of the cephalopods that lived predominantly in the upper certaceous period. They are specificially in the suborder of the Ancyloceratida which went extinct with the rest of the ammonites during the K-T extinction event.
