Ammonoids are extinct spiral shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living coleoids than they are to shelled nautiloids. The earliest ammonoids appeared during the Devonian, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event. They are often called ammonites, which is most frequently used for members of the order Ammonitida, the only remaining group of ammonoids from the Jurassic up until their extinction.
Mariella is an ammonoid genus, named by Nowak (1916) from the upper Albian and Cenomanian stages of the mid Cretaceous, included in the Turrilitidae. Its type is Turrilites bergeri
The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and became one of the most distinctive components of Cretaceous marine faunas. They have been recorded from every continent and many are used as zonal or index fossils. The most distinctive feature of the majority of the Ancyloceratina is the tendency for most of them to have shells that are not regular spirals like most other ammonites. These irregularly-coiled ammonites are called heteromorph ammonites, in contrast to regularly coiled ammonites, which are called homomorph ammonites.
Euhoplites is an extinct ammonoid cephalopod from the Lower Cretaceous, characterized by strongly ribbed, more or less evolute, compressed to inflated shells with flat or concave ribs, typically with a deep narrow groove running down the middle. In some, ribs seem to zigzag between umbilical tubercles and parallel ventrolateral clavi. In others the ribs are flexuous and curve forward from the umbilical shoulder and lap onto either side of the venter.
Abrytasites (Abrytusites) is an extinct genus of cephalopods belonging to the Ammonoidea subclass.
Acanthohoplites is an extinct genus of ammonites in the family Parahoplitidae that lived in the Aptian and Early Albian stages of the Early Cretaceous.
Aspidoceras is an extinct ammonoid cephalopod genus belonging to the family Aspidoceratidae.
Audaxlytoceras is an extinct genus of lytoceratid ammonites.
Balatonites is a genus of extinct cephalopods belonging to the ceratitid family Balatonitidae. There are at least four known species: B. balatonicus, B. oyama, B. shoshonensis, and B. zitteli.
Beudanticeras is an extinct cephalopod genus from the Late Cretaceous period; Albian and Cenomanian, belonging to the ammonoid subclass and included in the family Desmoceratidae.
Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post-Triassic ammonites.
Turrilitoidea is a diverse superfamily of Cretaceous ammonites generally considered as heteromorphic and commonly included in the suborder Ancyloceratina. Shells of this diverse group do not coil planospirally, as typical for most ammonites, but rather take on a variety of unique forms. Separation of Turrilitoidea along with Ptychoceratoidea and Scaphatoidea into a different suborder, Turrilitina, was proposed by Beznosov and Mikhailova in 1983 however this view is not accepted by most authors.
Prolecanitida is an order of extinct ammonoid cephalopods, the major Late Paleozoic group of ammonoids alongside the order Goniatitida. Prolecanitids had narrow shells, discoidal (disc-shaped) to thinly lenticular (lens-shaped). They retained a retrochoanitic siphuncle, a simple form with septal necks extending backwards. As is typical for ammonoids, the siphuncle sits along the ventral margin of the shell.
Oxynoticeratidae is a family of true ammonites included in the superfamily Psiloceratoidea.
Turrilites is a genus of helically coiled ammonoid cephalopods from the lower part of the Upper Cretaceous ; generally included in the Ancyloceratina. Previously it was included in the ammonoid suborder, Lytoceratina.
Melchiorites is a desmoceratid ammonite genus included in the subfamily Puzosiinae. Member species are characterized by an essentially evolute shell in which the early whorls are smooth, with sinuous radial or oblique constrictions but in which later whorls have feeble intermediate ribs on the outer part of the sides and venter.
Spitidiscus is a genus of ammonites placed in the family Holcodiscidae.
Holcodiscus is an extinct ammonite genus placed in the family Holcodiscidae. Species in this genus were fast-moving nektonic carnivores. The type species of the genus is Ammonites caillaudianus.
Dufrenoyia is an extinct genus of Cretaceous ammonites included in the family Parahoplitidae. These fast-moving nektonic carnivores lived in the Cretaceous period. The type species of the genus is Ammonites dufrenoyi.
Halorites is an extinct genus of Triassic ammonoids belonging to the family Haloritidae.
