Andrewsarchus mongoliensis (Andrews + Greek ἀρχός, "ruler"), was a mammalian Mesonychid predator.
This large heavily-built, wolf-like, mammal lived during the Eocene, about 45 to 36 million years ago. It walked on four short legs and had a long body, a long tail, and feet with hoofed toes. It had a long snout with large, sharp teeth and flat cheek teeth that may have been used to crush bones.
Because Andrewsarchus is only known from a skull and a few other bones, whether it was an active predator or merely a large scavenger is open to debate, as is its exact time range.