chapter
9
Every person’s history may be revealed by photos, letters, and other things they own. Earth has a history, too. What things might be used to reveal Earth’s history?
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■ how fossils form how fossils are used to tell rock ages
■ how fossils explain changes in Earth’s surface and life forms
It’s likely that you’ve read about dinosaurs and other animals that lived on Earth in the past. But how do you know that they were real? How do you know what they were like? The answer is fossils. Paleontologists, scientists who study fossils, can learn about extinct animals from their fossil remains.
Fossils are the remains, imprints, or traces of animals or plants that died long ago. Scientists have used fossils to determine when life first appeared, when plants and animals first lived on land, and when organisms became extinct.
Fossils can tell a lot about the past.
Most animals and plants decay soon after they die. Some animals may eat and scatter the remains of dead organisms.
Fungi and bacteria may cause the remains to rot. In time, no trace is left. But some dead animals and plants do become fossils. Sometimes conditions are just right for fossils to form.
Study Coach
Two-Column Notes As you read, organize your notes in two columns. In the left column, write the main idea of each paragraph. Next to it, in the right column, write details about it.
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Compare and Contrast
Make a Foldable as shown below to compare and contrast the ways that fossils are preserved.
Mineral
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1.
Explain What is one reason that hard parts of organisms have a better chance of becoming fossils than soft parts do?
2.
Identify What is the source of the minerals that form permineralized remains?
Dead organisms that are protected from scavengers and other harmful forces may leave fossils. One way a dead organism can be protected is for sediment to bury the body quickly. For example, a dead fish might sink to the bottom of a lake. If it is quickly buried by sediment dropped from a stream, it would be protected from scavengers. Over time, the fish may become a fossil in a layer of rock. But quick burial alone isn’t always enough to make a fossil.
Organisms with hard parts, such as bones, shells, or teeth, have a better chance of becoming fossils. These hard parts are less likely to be eaten by scavengers than soft parts. Hard parts also decay more slowly than soft parts do. Most fossils are the hard parts of organisms.
You may have seen the bones of a dinosaur in a museum.
You may have seen drawings of dinosaurs in books. Artists who draw dinosaurs study their fossil bones. What preserves fossil bones?
The hard parts of living things, such as bones, teeth, and shells, have tiny spaces in them. When organisms are alive, the spaces can be filled with cells, blood vessels, nerves, or air. When organisms die, the soft parts decay and leave empty spaces. If the hard part is buried, groundwater can seep into these spaces and deposit minerals. The result is a type of fossil.
Permineralized remains are fossils in which the spaces are filled with minerals from groundwater. Sometimes minerals replace all of the original hard parts of an organism.
Scientists learn about past forms of life from remains that are permineralized. Other types of fossils can be found as well.
The tissues of living organisms contain carbon. Some fossils are made only of carbon. Fossils usually form when sediments bury a dead organism. As sediment builds up, heat and pressure force all the gases and liquids out of the organism. Then, just a thin layer, or film, of carbon is left. It looks like a shadow of the organism’s body. A carbon film is a thin film of carbon left from an organism and preserved as a fossil.
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Large amounts of dead plants may build up in swamps.
Over millions of years, heat and pressure change the plant material into coal which contains large amounts of carbon.
Coal is another kind of fossil, but it doesn’t reveal much about the past. As the coal forms, the structure of the original plant is usually lost.
Sometimes the hard parts of a dead organism fall into a soft sediment, such as mud. Then more sediment buries the object. In time, pressure and cementation turn the sediment into rock. Cementation is when minerals from water are deposited in the spaces between sediment particles. Then water and air flow through open spaces in the rock and dissolve the organism’s hard parts. This leaves a hole, or cavity, in the rock called a mold. A mold is a body fossil that forms when an organism decays, or dissolves, and leaves a cavity in rock.
Later, water and minerals may enter the mold and form new rock. This produces a copy, or cast, of the original object. A cast is a type of body fossil that forms when minerals fill a mold and harden into rock. The figure below shows a cast resulting when a mold fills with minerals.
3.
Describe What does a mold in a rock look like?
Shell dissolves
Mold forms
Minerals fill mold
Cast is done
4.
Explain What is the key difference between the shell on the left and the cast fossil on the right?
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5.
Identify List three different trace fossils.
6.
Define What are index fossils?
From time to time, conditions are just right for the soft parts of an organism to be preserved. Insects can be trapped in amber, a tree sap that turns into a solid. The amber surrounds and protects the insect’s original parts. Some organisms, such as mammoths, have been found preserved in frozen ground in Siberia. Some original remains have been found in natural tar deposits.
Animals that walked on Earth long ago left tracks in soft mud. Some of those tracks have been preserved. They are trace fossils. Trace fossils may be footprints, trails, burrows, or any marks that tell something about how an animal moved and lived. In some cases, tracks can tell more about how an organism lived than any other type of fossil.
Some trace fossils are burrows and trails left by worms and other animals. These fossils give scientists clues about an animal’s way of life. For example, examining fossil burrows can reveal how firm the sediment was that the animal lived in. From this information, scientists can learn more about the animal who dug the burrow.
By studying fossils, scientists can learn that species of organisms have changed over time. Some species lived on
Earth for a long time without changing. Other species changed a lot in a short time. Scientists use these organisms as index fossils.
Index fossils are the remains of species that lived for a short time, were numerous, and were found in many places.
Organisms that became index fossils lived only during a specific time. Because of this, scientists can estimate the ages of rock layers based on the index fossils they contain.
But not all rocks have index fossils. There is another way to estimate when these rocks formed. It is done by comparing time spans in which more than one fossil appears. Look at the figure at the top of the next page which shows the time spans of three different fossils in sedimentary rock. The estimated age of the rock layer is the time period where the different fossil ranges overlap.
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Fossils in a Sequence of Sedimentary Rock
Fossil Range Chart
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7.
Interpret According to the chart, during what span of time did all three fossils appear together on Earth?
Euomphalus Illaenus Rhipidomella
Scientists can use fossils to learn what an area was like long ago. Using fossils, they can determine whether an area was land or covered by ocean. If the area was covered by ocean, it might even be possible to learn how deep the water was.
Fossils also can give clues about the past climate of an area. For example, rocks in parts of the eastern United
States contain fossils of tropical plants. But today the environment of this area isn’t tropical. Because of these fossils, scientists know that the climate was tropical when these plants were living.
Crinoids are animals with many arms that usually live in warm, shallow waters. But fossils of crinoids have been found in deserts in parts of western and central North
America. What do scientists learn from these fossils? When the fossil crinoids were living, a shallow sea must have covered this area of North America.
Fossils give clues about past life on Earth. Fossils give information about plants and animals that are now extinct.
They provide information about the rock layers that contain them and about the ages of the rock layers. By studying fossils, scientists learn about the climate and environment that existed when the rocks formed.
8.
Infer If fossil sea shells are found on the tops of mountains today, what does that tell you about the location of the mountains long ago?
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carbon film: thin film of carbon left from an organism and preserved as a fossil cast: body fossil that forms when a mold fills with sediment or minerals and then hardens into rock fossil: the remains, imprints, or traces of animals or plants that died long ago index fossil: the remains of species that lived for a short time, were numerous, and were found in many places mold: body fossil that forms when an organism decays or dissolves and leaves a cavity in the rock permineralized remains: fossils in which the spaces are filled with minerals from groundwater
1.
Review the terms and their definitions in the Mini Glossary. Then write a sentence that explains why a mold and a cast fossil may be found together.
2.
Complete the concept map below.
Original remains
Types of fossils
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Carbon films
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