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