Hyaenodon was a genus of credonts (and thus not related to hyenas, despite the name, who are carnivorans) that existed from 42-16 million years ago, lasting for a whopping 26 million years. As typical of credonts, they had enormous heads with extremely powerful jaws and a bite force that far exceeds any modern carnivorous mammal's to go with it. The genus was quite large, with 42 known species ranging in size from a small truck to a weasel, but most species were likely the apex predators in their respective environments for most of their history, with entelodonts (such as Archaeotherium) likely being the only real competitors they had. They lived all across the globe, save for South America, Australia, and Antarctica. Casts of their brain cavity show that they had a powerful sense of smell, and their nasal cavity extended much further into the back of the throat than in most animals, which gave them the ability to cram a lot more food into their mouth and likely allowed them to clean a carcass more quickly. Recently, comparisons have been drawn with modern animals of similar builds and it is believe they could have ran as fast as 35 miles an hour-- fast enough to catch one of their most beloved prey items, Mesohippus. Due to their low lying bodies, it is believed that Hyaenodonts were ambush predators, using the brush as their ally. As their environments got drier, and vegetation more scarce, the fossil record has actually been able to show us that they moved to the more verdant watering holes to hunt, waiting for animals to drink. The mass quantities of bones founds at such locations with Hyaenodont wounds show that this was a very prolific hunting strategy.
Despite all these apparent advantages, they had important disadvantages that proved to be their downfall with the arrival of carnivorans as competitors (notably Amphicyon). For one, they had significantly smaller brains incapable of forethought and cooperation, so carnivorans tendency to pack hunt put all creodonts at a deep disadvantage. Furthermore, their wrists didn't have the appropriate structure required for rotation-- the paws could only move forward and back. Carnivorans, however, are able to move their wrists and use them grab and manipulate prey, where as creodonts like Hyaenodon could only use their jaws (one reason for their massive size and bite force, a feature that would be unnecessary in carnivorans). This gave Hyaenodon significantly fewer hunting strategies, allowing Carnivora to dominate the niche.
