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Sphenodiscus is an extinct genus of acanthoceratacean ammonite.The genus has been found from many continents and is thought to have had a large global distribution during the Maastrichtian stage of the Late Cretaceous.
This list of ammonites is a comprehensive listing of genera that are included in the subclass †Ammonoidea, excluding purely vernacular terms. The list includes genera that are commonly accepted as valid, as well as those that may be invalid or doubtful ( nomina dubia ), or were not formally published ( nomina nuda ), as well as junior ...
Fagesia is a small, subglobular ammonite (suborder Ammonitina) belonging to the vascoceratid family of the Acanthocerataceae that lived during the Turonian stage of the Late Cretaceous, 92–88 Ma ago.
Polyptychoceras is a heteromorph ammonite, meaning that its shell does not curl up into the tight spiral shape which shells of ammonites from the subclass Ammonoidea typically do. Polyptychoceras shells have an abrupt weight increase after formation of the initial shaft, which represents the shell's automatic balance condition. [ 4 ]
All ammonites were wiped out during or shortly after the K-Pg extinction event, caused by the Chicxulub impact. It has been suggested that ocean acidification generated by the impact played a key role in their extinction, as the larvae of ammonites were likely small and planktonic, and would have been heavily affected. [42]
Ammonites were cephalopods, that thrived in tropical seas until becoming extinct along with the dinosaurs at the end of the Mesozoic era. The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains—this area is known today as the Cretaceous or Western Interior Seaway .
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 was a small ammonite with shells of diameters of at most a few centimetres in diameter, depending on the age, species and possibly gender of the individual.