Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch (or, in chronostratigraphy: the fifth of six stages in the Upper Cretaceous series). The Campanian spans the time from 83.6 ± 0.7 Ma to 72.1 ± 0.6 Ma (million years ago). It is preceded by the Santonian and it is followed by the Maastrichtian.
Achelousaurus
"Achelousaurus" is known from the Two Medicine Formation, which preserves coastal sediments dating from the Campanian stage of the Late Cretaceous Period, between 83 and 74 million years ago. "Achelousaurus" specimens are found in the highest levels of the formation, probably closer to the end of that timeframe, 74 million years ago.