Arctocyon primaevus – from Late Paleocene-Early Eocene of North America had medium and large sizes and most probably lead a wild lifestyle. In the Early Paleogene niche of large carnivores mostly was empty, and at different times specimens from different groups of archaic mammals have made more or less successful attempts to fill them. Arctocyonidaes are one of such attempts. Cheek teeth of these ancient ungulates had flattened crowns, which suggests for them rather than meat the omnivorous diet. However, in many ways it can be assumed that over time some Arctocyonidaes increasingly began to include in their diet vegetarian food, in parallel increasing in sizes. The second half of Paleocene Arctocyonidaes, for example, have already reached the size of wolves or even small bears.