''Pachydiscus'' is an extinct genus of

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

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 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 The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratig ...

(ICS) has assigned the

First Appearance Datum First appearance datum (FAD) is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest fossil discovered, to date, of a partic ...

of ''Pachydiscus neubergicus'' as the defining biological marker for the start of the Maastrichtian Stage of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, 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).''Pachydiscus''
at

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.org

References

, 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 Desmoceratoidea Ammonitida genera Campanian life Maastrichtian life Campanian genus first appearances Maastrichtian genus extinctions Cretaceous ammonites Index fossils Ammonites of Africa Cretaceous Africa Ammonites of Asia Cretaceous Asia Ammonites of Australia Cretaceous animals of Australia Late Cretaceous ammonites of Europe Cretaceous Europe Late Cretaceous ammonites of North America Cretaceous Canada Cretaceous Mexico Cretaceous United States Ammonites of South America Cretaceous Brazil Cretaceous Chile Cretaceous Caribbean {{Ammonitina-stub

Biostratigraphic significance

Distribution

References

Further reading