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 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.

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.

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).