1. Chapter F3 Fossilized Organisms
Section 4 Looking at Fossils
Fossilized Organisms
The trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock is called a fossil.
Fossils are most often preserved in sedimentary rock, but other materials can also preserve evidence of past life.

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Fossils in Rocks When an organism dies, it either begins to decay or is consumed by other organisms. Sometimes dead organisms are quickly buried by sediment, which slows down decay.
Shells and bones are more resistant to decay than soft tissues, so when sediments become rock, the harder structures are more commonly preserved.

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Section 4 Looking at Fossils
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Organisms occasionally become trapped in soft, sticky tree sap, which hardens and becomes amber.
Insect fossils have often been preserved in this way, but frogs and lizards have also been found in amber.

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Petrifaction is a process in which minerals replace and organism’s tissues.
One form of petrifaction is called permineralization, a process in which the pore space in an organism’s hard tissue is filled up with mineral.
Replacement is a process in which an organism’s tissues are completely replaced by minerals.

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Fossils in Asphalt There are places where asphalt wells up at the Earth’s surface. These thick, sticky pools can trap and preserve organisms.
Frozen Fossils Since cold temperatures slow down decay, many types of fossils have been found preserved in ice.

6. Chapter F3 Other Types of Fossils
Section 4 Looking at Fossils
Other Types of Fossils
Trace Fossils are naturally preserved evidence of animal activity. Preserved animal tracks are an example of a trace fossil.
Other types of trace fossils include preserved burrows or shelters that were made by animals, and coprolite, which is preserved animal dung.

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Section 4 Looking at Fossils
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Molds and Casts are two more examples of fossils.
A mold is a mark or cavity made in a sedimentary surface by a shell or other body.
A cast is a type of fossil that forms when sediments fill the cavity left by a decomposed organism.

8. Using Fossils to Interpret the Past
Chapter F3
Section 4 Looking at Fossils
Using Fossils to Interpret the Past
The Information in the Fossil Record The fossil record offers only a rough sketch of the history of life on Earth. The fossil record is incomplete because most organisms never became fossils.
Scientists know more information about organisms that had hard body parts and that lived in environments that favored fossilization.

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History of Environmental Changes The fossil record reveals changes in an area’s climate over time. By using the fossils of plants and land animals, scientists can reconstruct past climates.
History of Changing Organisms By studying the relationships between fossils, scientists can interpret how life has changed over time.

10. Using Fossils to Date Rocks
Chapter F3
Section 4 Looking at Fossils
Using Fossils to Date Rocks
Scientists have learned that particular types of fossils appear only in certain layers of rock.
By dating the rock layers above and below these fossils, scientists can determine the time span in which the organisms that formed the fossils lived.

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If a type of organism existed for only a short period of time, its fossils would show up in a limited range of rock layers. These fossils are called index fossils.
Index fossils are fossils that are found in the rock layers of only one geologic age, and can be used to establish the age of the rock layers.

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Ammonites An example of an index fossil is the fossil of a genus of ammonites called Tropites.
Tropites, a marine mollusk similar to a modern squid, lived between 230 million and 208 million years ago.

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Trilobites Fossils of a genus of trilobites called Phacops are another example of an index fossil.
Trilobites are extinct and lived approximately 400 million years ago. When scientists find Phacops in a rock, they assume that the rock is approximately 400 million years old.