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Hyaenodon (Greek for "hyena tooth"); pronounced hi-YAY-no-don
Plains of North America, Eurasia, and Africa
Late Eocene-Early Miocene (40-20 million years ago)
Size and Weight:
Varies by species; about one to five feet long and five to 100 pounds
Slender legs; large head; long, narrow, tooth-studded snout
The unusually long persistence of Hyaenodon in the fossil record--various specimens of this prehistoric carnivore have been found in sediments dating from 40 million to 20 million years ago, all the way from the Eocene to the early Miocene epochs--can be explained by the fact that this genus comprised a large number of species, which ranged widely in size and enjoyed a nearly worldwide distribution. The largest species of Hyaenodon, H. gigas, 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 species, the appropriately named H. microdon, was only 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: the "hyena tooth" was a prime example of a creodont, a family 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, with its four slender legs and narrow snout, 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. (However, bear in mind that this creodont didn't much resemble modern hyenas, except for the shape of some of its teeth!)
Part of what made Hyaenodon such a formidable predator was its almost comically oversized jaws, which had to be supported by extra layers of musculature near the top of this creodont's neck. Like roughly contemporary "bone-crushing" dogs (to which it was only distantly related), Hyaenodon would likely snap the neck of its prey with a single bite, and then use the slicing teeth in the back of its jaws to grind down the carcass into smaller (and easier to handle) mouthfuls of flesh. (Hyaenodon was also equipped with an extra-long palate, which allowed this mammal to continue breathing comfortably as it dug into its meal.)
What Happened to Hyaenodon?
What could have edged Hyaenodon out of the spotlight, after millions of years of dominance? The"bone-crushing" dogs referenced above are possible culprits: these megafauna mammals (typified by Amphicyon, the "bear dog") were every bit as lethal, bite-wise, as Hyaenodon, but they were also better adapted for hunting scurrying herbivores across the wide plains of the later Cenozoic Era. One can imagine a pack of hungry Amphicyons denying a Hyaeonodon its recently killed prey, thus leading, over thousands and millions of years, to the eventual extinction of this otherwise well-adapted predator.