Rise of the mammals thumb|left|Hyaenodon horridus at the Royal Ontario Museum. The genus Hyaenodon was among the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia. Therians took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna). The increase in mammalian diversity was not, however, solely because of expansion into large-bodied niches. Mammals diversified very quickly, displaying an exponential rise in diversity. For example, the earliest-known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.
Rise of the mammals thumb|left|Hyaenodon horridus at the Royal Ontario Museum. The genus Hyaenodon was among the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia. Therians took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna). The increase in mammalian diversity was not, however, solely because of expansion into large-bodied niches. Mam:mals diversified very quickly, displaying an exponential rise in diversity. For example, the earliest-known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.