Creodonts were the apex-predators of the early Tertiary period, but only in the North Hemisphere - in the south, in South America and Australia, giant marsupials, flightless birds and reptiles ruled instead. Unlike the more advanced carnivorans, such as the bear-dogs, creodonts had much smaller brains and were less intelligent, they had less keen hearing and they were slower runners.
Creodonts were generalized carnivores, but were roughly divided into two groups. Oxyaenids were rather cat-like, while hyaenodontids (including Hyaenodon and its close relatives) were more dog-like. They hunted the more primitive herbivores of Palaeogene (New Dawn episode features a small creodont, much smaller than Hyaenodon) and when by early Oligocene herbivores began to evolve more specialized forms, the creodonts began to die out. Oxyaenids vanished first, while hyaenodontids lasted until the Miocene Epoch (as shown in the WWB episode Land of Giants), after which the whole group died out and was replaced by more modern carnivores.