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

The geologic time scale divides Earth’s history based on major past events. Index fossils are vital for determining rock layers' relative age, representing organisms existing during specific time spans. They are common, easy to identify, and found worldwide for a brief time period. Precambrian time comprises the oldest rocks and fossils, including cyanobacteria and eukaryotes. Learn about the geologic time scale's divisions - Eras, Periods, and Epochs, with focus on the Cenozoic, Mesozoic, and Paleozoic Eras.

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

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  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. 100 250 550 1000 2000 PRECAMBRIAN TIME 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. Cyanobacteria 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 • 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…

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