Coryphodon had one of the smallest brain/body ratios of any mammal, living or extinct, possessing a brain weighing just 90 grams (3.2 oz) and a body weight of around 500 kilograms (1,100 lb).

The body size then decreased until C. eocaenus appears at the Clarkforkian-Wasatchian transition (55.4 Ma, near the PETM), from where Coryphodon evolved into the large species C. radians.

C. radians in its turn evolved into two contemporaneous species that appear in the Early Eocene, the small C. armatus and the very large C. lobatus.

[7] Coryphodon had a semi-aquatic lifestyle, likely living in swamps and marshes like a hippopotamus, although it was not closely related to modern hippos or any other animal known today.

Though the climate of the Eocene was much warmer than today, plants and animals living north of the Arctic Circle still experienced months of complete darkness and 24-hour summer days.

However during the extended periods of darkness when plant photosynthesis was impossible, Coryphodon would switch to a diet of leaf litter, twigs, evergreen needles and most revealingly fungi, an organism and food source that does not require light to grow.