This subperiod is part of theTertiary period.
Palaeogene (alternatively Paleogene) period is a unit of geologic time that began 65 and ended 23 million years ago. Lasting 42 million years, this period 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. Birds also evolved considerably during this period changing into roughly-modern forms. Most other branches of life on earth remained relatively unchanged in comparison to birds and mammals during this time period. Some continental motion took place. Climates cooled somewhat over the duration of the Paleogene and inland seas retreated from North America early in the Period.
This period consists of the Paleocene, Eocene, and Oligocene Epochs. The Paleogene 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.