Trogosus castoridens (Leidy 1871, Eocene 54-33 mya; 35 cm skull length) is known from a large skull (up to 35 cm long). Formerly considered the largest tillodont, Trogosus nests here as a tree shrew.This bear-like herbivore left no descendants.

OC Marsh (1875) wrote, "These animals are the among the most remarkable yet discovered in American strata, and seem to combine characters of several distinct groups, viz: Carnivores, Ungulates, and Rodents. In Tillotherium (=Trogosus), the type [specimen] of the order, the skull has the same general form as in the Bears, but its structure resembles that of Ungulates. The molar teeth are of the ungulate type, the canines are small, and in each jaw there is a pair of large scalpiform incisors faced with enamel, and growing from persistent pulps, as in Rodents."

Tillodon fodiens (Gazin 1953) is known from a complete skull about 42cm long. It has a large overbit and probably a sliding jaw joint, as in rodents by convergencew, but closer to tree shrews.