Pachydiscus

Pachydiscus is an extinct genus of ammonite from the Late Cretaceous with a worldwide distribution, and type for the desmoceratacean family Pachydiscidae. The genus' type species is P. neubergicus. Altogether some 28 species have been described.

Pachydiscus
Temporal range: Upper Cretaceous
Scientific classification
Kingdom:
Animalia
Phylum:
Mollusca
Class:
Cephalopoda
Subclass:
Ammonoidea
Order:
Ammonitida
Superfamily:
Desmoceratoidea
Family:
Pachydiscidae
Genus:
Pachydiscus

Zittel, 1884
Synonyms

Parapachydiscus

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

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

Distribution

Fossils of Pachydiscus have been found in Antarctica, Australia, Austria, Belgium, Brazil, Canada (British Columbia), Chile, Denmark, France, Germany, Haiti, India, Iran, Japan, Mexico, the Netherlands, New Zealand, Nigeria, Oman, Romania, the Russian Federation, Saudi Arabia, South Africa, Spain, Turkey, Ukraine, the United Arab Emirates, and the United States (Alaska, Arkansas, California, Delaware, Mississippi, New Jersey, Oregon, Texas, Washington).[1]

References
  1. Pachydiscus at Fossilworks.org

Further reading
  • Treatise on Invertebrate Paleontology, Part L, Ammonoidea. R.C. Moore, ed. Geological Soc. of America and Univ. Kansas press. p L377-L380.
  • 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
  • Kennedy, W. J. (1986). Campanian and Maastrichtian Ammonites from Northern Aquitaine, France (No. 36). Palaeontological association. page 30
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