Size and Weight:
The unusually long persistence of Hyaenodon in the fossil record--specimens of this prehistoric carnivore have been found in sediments dating from 40 million to 20 million years ago, from the Eocene to the early Miocene epochs--can be explained by the fact that this genus comprised a number of species, which ranged widely in size and had a nearly worldwide distribution. The largest species of Hyaenodon was about the size of a wolf, and probably led a predatory wolf-like lifestyle (supplemented with hyena-like scavenging of dead carcasses), while the smallest was about the size of a house cat.
You might assume that Hyaenodon was directly ancestral to modern wolves and hyenas, but you'd be wrong: this was an example of a creodont, a breed of carnivorous mammals that arose about 10 million years after the dinosaurs went extinct and went extinct themselves about 20 million years ago, leaving no direct descendants (one of the biggest creodonts was the amusingly named Sarkastodon). The fact that Hyaenodon so closely resembled modern meat-eaters can be chalked up to convergent evolution, the tendency for creatures in similar ecosystems to develop similar appearances and lifestyles.