Index Fossils

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SC: I can answer questions about Earth’s history using fossil evidence

Why learn about sedimentary rocks and fossils?

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They store evidence of past environments. They tell us the stories of what life was like in the past.

What are fossils?

• Fossils are the remains of ancient plants or animals (or other past stuff)
• They are one of the ways used to date layers of rock

Types of Fossils

1. Petrification – minerals take the place of the once living plant or animal

• Petrified wood is one example

Types of Fossils

• 2. Trace – Imprints, tracks, impressions, and waste left behind by plants and animals
• Examples:
• dinosaur footprint
• leaf imprint
• coprolites

Types of Fossils

• 3. Preserved – actual remains of a plant or animal that have not decayed
• Examples:
• Mummification
• Insects in amber

Types of Fossils

• Examples:
• Remains in tar

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• Freezing

Questions

• Why are fossils only found in sedimentary rock?
• Why are there few fossils of bacteria?
• What do you call a scientist who studies fossils?
• What is the name we give to fossils that are especially important in dating the age of rocks?

Index Fossils

• Index fossils are those used to date the age of rocks.
• Because radiometric dating doesn’t work on sedimentary rock.
• Four Criteria to be an index fossil:

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Scattered Over a Wide Area

• Found on 2 or more continents.
• Most ocean dwelling creatures are widespread due to the few natural barriers in the ocean.
• The more places they are found the more they can be used to correlate with other layers.

Distinguishing Features

• A body structure found in fossils which can help scientists tell this species apart from similar species.

Lived for a short period of time

• The shorter amount of time the species existed the better index fossil the species makes.
• Because the date of the rock they are found in will be narrower.
• Not individuals, the whole species.

Great Good Enough

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100,000Years 3-10 Million Years

Abundant = Large Number

• The more there are of them the easier they are to find and use.

Trilobites

• Trilobites are an important index fossil that lived 400 mya (million years ago)

Clues to the Past

• Fossils give clues to the past!
• For instance, fossils of marine (ocean) animals have been found all around Colorado.
• What does this tell us about Colorado?

Index Fossils

Principle of Correlation

A way to relatively date rock layers by comparing the rock layers and fossils in different locations. 

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Correlating Rock Layers Rock layers that contain the same fossils are the same age.

Concept of Correlation 

Concept of Correlation