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 living group of ammonoids from the Jurassic up until their extinction.
Hamites is a genus of heteromorph ammonite that evolved late in the Aptian stage of the Early Cretaceous and lasted into the Cenomanian stage of the Late Cretaceous. The genus is almost certainly paraphyletic but remains in wide use as a "catch all" for heteromorph ammonites of the superfamily Turrilitoidea that do not neatly fit into the more derived groupings. In an attempt to identify clades within the genus, it has been divided up into a series of new genera or subgenera by different palaeontologists, including Eohamites, Hamitella, Helicohamites, Lytohamites, Planohamites, Psilohamites, and Sziveshamites.
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.
Scaphites is a genus of heteromorph ammonites belonging to the Scaphitidae family. They were a widespread genus that thrived during the Late Cretaceous period.
Ancyloceras is an extinct genus of heteromorph ammonites found throughout the world during the Lower Cretaceous, from the Lower Barremian epoch until the genus extinction during the Lower Aptian.
Anaklinoceras is a genus of extinct heteromorph ammonite cephalopod that lived in marine environments in what is now Western North America during the Campanian division of the Cretaceous period. Their shells were very similar to the related ammonites of the genus Ainoceras, where the young ammonite's shell coiled helically, and upon reaching adulthood, the youngest coil arched over the older coils.
Astreptoceras is an extinct upper Cretaceous ammonoid cephalopod named by Henderson in 1970. Fossils belonging to this genera have been found in Antarctica and New Zealand.
Annuloceras is an extinct genus of ammonite cephalopod. Its fossils are found in Lower Barremian sediments from California. The genus is currently placed in the family Aegocrioceratidae.
Anisoceratidae is an extinct family of heteromorph ammonites which belong to the Ancyloceratina superfamily Turrilitoidea. Members of the family range is from the lower Albian to the upper Turonian. The family is possibly derived from a member of the Hamitidae.
Nostoceratidae is a diverse family of heteromorph ammonites found throughout the oceans of the world during the Late Cretaceous. The nostoceratids are famous for the bizarre coiling of their shells. Many genera, such as Yezoceras, Ainoceras, Anaklinoceras, and some species of Bostrychoceras and Eubostrychoceras, display, as young shells at least, a helical coiling very similar to the shells of the related family, Turrilitidae. As adults, though, the coils then curve away from the axis of coiling, either as an oxbow-like curve around the juvenile coils as in Ainoceras and Anaklinoceras, or in a simple curved loop beneath the juvenile coils, as in Yezoceras. Other genera form loose coils, sometimes in a spiral, such as those of Madagascarites, Muramotoceras, Hyphantoceras, and the infamously convolute Nipponites.
Nipponites is an extinct genus of heteromorph ammonites. The shells of Nipponites form "ox-bow" bends, resulting in some of the most bizarre shapes seen among ammonites.
Bostrychoceras is a genus of heteromorph ammonite from the family Nostoceratidae. Fossils have been found in Late Cretaceous sediments in Europe and North America.
Exiteloceras is an ammonite genus from the Late Cretaceous.
Heteroceras is a genus of Lower Cretaceous heteromorph ammonites belonging to the ancyloceratoidean family, Heteroceratidae.
Neoptychites is an extinct ammonoid cephalopod genus from the Turonian stage of the Upper Cretaceous, with a worldwide distribution.
Eubostrychoceras is a genus of helically wound, corkscrew form, heteromorph ammonite which lived during the Upper Cretaceous. The genus is included in the ancycleratid family Nostoceratidae.
Macroscaphites is an extinct cephalopod genus included in the Ammonoidea that lived during the Barremian and Aptian stages of the Early Cretaceous. Its fossils have been found throughout most of Europe and North Africa.
Lytoceras is an ammonite genus that was extant during most of the Jurassic and Cretaceous periods, and is the type genus for the family Lytoceratidae. These cephalopods were fast-moving nektonic carnivores.
Phyllopachyceras is an extinct genus of ammonoid cephalopods belonging to the family Phylloceratidae. These nektonic carnivores lived in the Cretaceous, from Hauterivian to Maastrichtian to age.
Diplomoceras is a genus of ammonites included in the family Diplomoceratidae. Fossils of species within this genus have been found in the Late Cretaceous sediments. D. maximum had coiled shell length about 1.5 m (4.9 ft), uncoiled shell being 3 m (9.8 ft) to over 4 m (13 ft), with body chamber around 2 m (6.6 ft). Some partial specimens may belong to shell with length around 1.7 m (5.6 ft). Studies of Diplomoceras suggest that members of this genus could reach lifespans of over 200 years.
