Pachydiscus

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Pachydiscus
Temporal range: Upper Cretaceous
Pachydiscus neubergicus.jpg
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Desmocerataceae
Family: Pachydiscidae
Genus: Pachydiscus
Zittel, 1884
Synonyms

Parapachydiscus

Pachydiscus is an extinct ammonite genus from the Upper Cretaceous with a world-wide distribution, and type for the desmoceratacean family Pachydiscidae. Its type is P. neubergicus. Altogether some 28 species have been described.

The shell of Pachydiscus is compressed and high-whorled, with an oval or flat sided section. Ribs tend to be differentiated into short umbilical and separate ventro-lateral sets, with a smooth area between.

Pachydiscus includes two subgenera, P.(Pachysiscus) from the Campanian in which the ribs persist, and P (Neodesmoceras) from the Maastrichtian in which ribs disappear early, leaving an almost smooth shell.

Biostratigraphic significance[edit]

The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of Pachydiscus neubergicusas as the defining biological marker for the start of the Maastrichtian Stage of the Cretaceous, 72.1 ± 0.2 million years ago.

Sources[edit]

See also

  • A Pictorial Guide to Fossils by Gerard Ramon Case
  • Cephalopods Present and Past: New Insights and Fresh Perspectives by Neil H. Landman, Richard Arnold Davis, and Royal H. Mapes