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

2. Index fossils are another tool to determine the relative age of rock layers.
Definition: Fossils of organisms that existed only during specific spans of time over large geographic areas
Example: Fusulinids and trilobites.
Characteristics: Common, easy to identify, found widely around the world, only existed for a relatively brief time
Nonexample (I don’t get it either): Anything that is rare to find or that are only found in a certain region

3. Epochs: The smallest units of time, lasting several millions of years
Geologic Time Scale: Representation of the history of the Earth that is organized by major changes or events
Eras: Last tens to hundreds of millions of years & consist of two or more periods
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.
Periods: Most commonly used units of time, lasting tens of millions of years
Epochs: The smallest units of time, lasting several millions of years

4. Eras:
Era: Cenozoic (Most recent, currently still underway)
Description: Evolution of primates, diversification of mammals, flowering plants & continues today

5. Eras:
2. Era: Mesozoic (“Age of the Dinosaurs”)
Description: Evolution & extinction of dinosaurs, evolution of ferns, cycads & mammals

6. Eras:
3. Era: Paleozoic (Oldest era, “Ancient life”)
Description: All existing animal phyla developed, earliest land plants arose

7. Here’s the whole thing…