The Paleogene (pronunciation: /ˈpæliːədʒiːn/ or /ˈpeɪliːədʒiːn/; 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. 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.