Eupachydisus is a large, coarse-ribbed Pachydiscid ammonite genus from the Upper Cretaceous, found in Coniacian

Coniacian

The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...

 to Campanian

Campanian

The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

 age strata in Europe, Madagascar, Japan, and British Columbia.

The whorl section in Eupachydiscus is inflated and depressed, almost round in the later growth stages. Ribs are narrow but prominent in the early whorls, becoming thick and strong in the later.

Eupachydiscus is distinct from the mostly later, Campanian - Maastrichtian

Maastrichtian

The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

, compressed and high-whorled Pachydiscus

Pachydiscus

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....

, but somewhat similar to the coeval, inflated and coarse ribbed, Coniacian-Santonian

Santonian

The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...

, Pachydiscoides