Fossils and the history of life - How can we estimate the age of a fossil?
This figure shows the main time divisions of the geological history of the Earth during the past 65 million years. Earlier divisions are recognized too, and it is the past 600 million years which are paleontologically the most important because fossils are much less common before this time.
The time divisions were recognized by 19th century geologists on the basis of characteristic fossil faunas. The times of transition between two eras are times of transition between different characteristic fossil faunas: the fossils of the Permian, for instance, characteristically differ from those of the Triassic and at the Permo-Triassic boundary there is a relatively sudden transition between the fossil faunas.
Figure: the geological time scale for the Cainozoic era.