Palaeogene or Paleogene Period (both spellings are accepted): A unit of geologic time consisting of the Paleocene, Eocene, and Oligocene Epochs. The Paleogene Period follows the Cretaceous Period and is followed by the Miocene Epoch of the Neogene Period. The terms 'Paleogene System' and 'Lower Tertiary System' have also been used to describe what is currently called the 'Paleogene Period'. The somewhat confusing terminology seems to be due to attempts to deal with the comparatively fine granularity of time units as time approaches the present and more information is preserved. By dividing the Tertiary Era into two 'Periods' instead of 7 'epochs', the periods are more closely comparable to the duration of 'periods' in the Mesozoic and Paleozoic Eras.
The Paleogene Covers roughly 34 Million years. During the Paleogene mammals evolved from small, simple forms to diverse animals -- some of great size -- capable of living in marine, terrestrial and even airborne environments. Birds also evolved considerably. Most other forms were relatively unchanged. Some continental motion took place. Climates cooled somewhat over the duration of the Paleogene. Inland seas retreated from North America early in the Period.