Paleogene. The Paleogene (/ˈpæliːɵdʒiːn/ or /ˈpeɪliːɵdʒiːn/; also spelled Palaeogene or Palæogene; informally Lower Tertiary) is a geologic period and system that began 66 and ended 23.03 million years ago and comprises the first part of the Cenozoic Era.[4] Lasting 43 million years, the Paleogene is most notable as being the time in which mammals evolved from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period.[5] Climate and geography[edit] The global climate during the Paleogene departed from the hot and humid conditions of the late Mesozoic era and began a cooling and drying trend which, although having been periodically disrupted by warm periods such as the Paleocene–Eocene Thermal Maximum, persists today.
Eocene epoch. Oligocene epoch. Mammal.