What happened to the ancient apex carnivore that suddenly disappeared from the fossil record?

Described as the "king of the ancient Egyptian forest," the hypercarnivorous hyaenodonts were enormous mammals—imagine a hyena twice the size of a Kodiak bear—so successful they spread across the globe. 25 millions years ago, though, they vanish from the fossil record. The discovery of the most complete skull yet (from a more modestly-sized species of t…