Pachydiscus

Pachydiscus Temporal range: Late Cretaceous-Early Paleocene,
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	†Ammonoidea
Order:	†Ammonitida
Family:	†Pachydiscidae
Genus:	†Pachydiscus Zittel, 1884
Synonyms
Parapachydiscus

Pachydiscus is an extinct genus of ammonite from the Late Cretaceous and Early Paleocene 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.

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. Some Hungarian specimens of this genus reached 40 cm (16 in) in diameter.

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, Poland, 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).

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