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

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

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

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

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

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

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