 To better study Earth and the many changes it has experienced, scientists use the geologic time scale.

 Divides Earth’s long history into separate periods of time  Divisions are based on important events  Examples – climatic changes, presence of major life forms

 Largest geologic time divisions  Two earliest eons cover nearly a billion years

EonMillions of Years Ago PhanerozoicCurrent eon Proterozoic2,500 Archean3,800 Hadeon4,600

 Eons are divided into eras  Eras are divided into shorter periods  The most recent periods, the Tertiary and Quaternary, are divided into epochs

 Scientists study fossils to learn about Earth’s History

 Fossil evidence indicates that dinosaurs and other reptiles dominated the Mesozoic era

 This era is often called the Age of Reptiles