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The Geologic Time Scale

* Paleontologists have been able to divide Earth’s history into time units based on the life-forms that lived only during certain periods.

* This division of Earth’s history makes up the geologic time scale.

• Four major subdivisions of geologic time are used— eons, eras, periods, and epochs.

• The longest subdivisions— eons—are based upon the abundance of certain fossils.

•Next to eons, the longest subdivisions are the eras, which are marked by major, striking, and worldwide changes in the types of fossils present.

• Eras are subdivided into periods.

• Periods are units of geologic time characterized by the types of life existing worldwide at the time.

• Periods can be divided into smaller units of time called epochs.

• Epochs are also characterized by differences in life-forms, but some of these differences can vary from continent to continent.

Trilobites

Precambrian Time

Precambrian time is the longest part of Earth’s history. It includes the Hadean, Archean, and Proterozoic Eons.

Precambrian time lasted from about 4.5 billion years ago to about 544 million years ago.

Precambrian Time

Even though the Precambrian was the largest interval of geologic time, we do not know much about the organisms that lived during this time.

Many Precambrian rocks have been changed by heat and pressure over the years.

The Paleozoic Era •The Paleozoic Era, or era of

ancient life, began about 544 million years ago and ended about 248 million years ago.

• An abundance of organisms

with hard parts (like shells) marks the

beginning of the Paleozoic

Era.

Paleozoic Life

•Many of the life-forms scientists know about were marine, meaning they lived in the ocean.•Trilobites were common, especially early in the Paleozoic.

•Vertebrates, or animals with backbones, also evolved during this era.

Paleozoic Life

• Armored fish with jaws lived during the Devonian Period

• Forests appeared during the Devonian period, and vertebrates began to adapt to land during this time.

Mountain Building

• The Appalachian mountains were formed during this time.

• Mountains were created when Africa/Europe collided with N. America

Mass Extinction

90% of marine life and 70% of land species died.

• Climate change

• Low sea levels• Volcanic

activity?

Your Project

1. What is your scale?

2. When did all of the events happen? (Find year,

eon/era/period

3. Make sure to answer 2/4 questions about each event

(can be turned in on a separate sheet of paper but

must be neat!)