1. 12.2 The Geologic Time Scale KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.

2. 12.2 The Geologic Time Scale Index fossils are another tool to determine the age of rock layers. Index fossils can provide the relative age of a rock layer. –existed only during specific spans of time –occurred in large geographic areas Index fossils include fusulinids and trilobites. Fig. Fusulinids are fossils ranging in size from less than a centimeter to 5 centimeters long. They make good index fossils, as they are very abundant in marine sediment, widely distributed, and existed during a specific period of time.

3. 12.2 The Geologic Time Scale The geologic time scale organizes Earth’s history. The history of Earth is represented in the geologic time scale. 100 250 550 1000 2000 PRECAMBRIAN TIME Cyanobacteria This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.

4. 12.2 The Geologic Time Scale Eras last tens to hundreds of millions of years. –consist of two or more periods –three eras: Cenozoic, Mesozoic, Paleozoic

5. 12.2 The Geologic Time Scale Periods last tens of millions of years. –most commonly used units of time on time scale –associated with rock systems. Epochs last several million years.