The previous eras (Paleozoic and Mesozoic) are divided in a simple way because the information that is had of them is smaller, in comparison with the one that is had about the Cenozoic era. The Cenozoic Era has more recent fossil records, well preserved, the rocks are younger and it is easier to divide into short and very defined periods of time, unlike the previous eras, where there are many gaps in the history of the earth to define it . That great information and greater precision, is the reason for a different classification, and the names given to the periods, within the Cenozoic era.