The Paleogene (alternatively Palaeogene) period is a unit of geologic time that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic era.Formerly the time period covered by the Paleogene was called the first part of the Tertiary, which usage is no longer official. http://www.stratigraphy.org/geowhen/TQ.html "Whatever happened to the Tertiary and Quaternary?" Lasting 42 million years, the Paleogene is most notable as being the time in which mammals evolved from small, simple forms into diverse animals in the wake of the mass extinction that ended the preceding Cretaceous Period. Some of these mammals would evolve into large forms that would dominate the land, while others would become capable of living in marine, specialized terrestrial and even airborne environments.