Amphicyon, or the Bear Dog, was a new predator to North America during the Miocene. They presumably crossed Beringia from Asia. By the time Amphicyon came, Dinohyus and Hyaenodon both became extinct. Paleontologists believe the Bear Dog outcompeted these 2 apex predators to extinction. The Bear Dog had advantages that Dinohyus and Hyaenodon could not deal with. For Dinohyus, Bear Dogs were more intelligent, faster, and had sharper teeth. For Hyaenodon, the Bear Dog had the same niche as Hyaenodon. That's like having 2 fast food restaurants right next to each other. The Bear Dog also had more intelligence than Hyaenodon. With these aptations, Amphicyon drove Dinohyus and Hyaenodon into extinction.