Fossils

1. Fossils Chapter 10 Sections 1+2

2. - Fossils How a Fossil Forms • Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

3. - Fossils Changes Over Time • The fossils record provides evidence about the history of life and past environments on Earth. The fossil record also shows that different groups of organisms have changed over time. Fossils of many different kinds of organisms were formed in this ancient lakeshore environment that existed in Wyoming about 50 million years ago.

4. - Fossils Changes Over Time • From fossils, scientists have reconstructed the paleomastodon. This animal had a short trunk and short tusks on both the upper and lower jaws. The paleomastodon is an ancestor of the modern elephant.

5. - Fossils Using Prior Knowledge • Before you read, look at the section headings and visuals to see what this section is about. Then write what you know about fossils in a graphic organizer like the one below. As you read, write what you learn. What You Know Fossils come from ancient organisms. Fossils are found in hardened rock. Fossils show us how some present-day organisms looked different in the past. What You Learned Molds and casts are types of fossils. Organisms are also preserved in amber, tar, and ice. Fossils tell us about past climates, changes in Earth’s surface, and how organisms have changed over time.

6. - The Relative Age of Rocks The Position of Rock Layers • According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

7. - The Relative Age of Rocks Determining Relative Age • To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.

8. - The Relative Age of Rocks Determining Relative Age • An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

9. - The Relative Age of Rocks Using Fossils to Date Rocks • Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

10. - The Relative Age of Rocks Index Fossil Activity • Click the Active Art button to open a browser window and access Active Art about index fossils.

11. Question Answer - The Relative Age of Rocks Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. What does the position of rock layer reveal? The oldest layers—and the oldest fossils—are at the bottom. How do geologists determine the relative age of a rock? They examine the position of rock layer, extrusions and intrusions of igneous rock, faults, and gaps in the geologic record. How are fossils used to date rocks? The age of an index fossil tells the age of the rock layer in which it occurs.

12. - The Relative Age of Rocks Rock Layers • Click the Video button to watch a movie about rock layers.

14. - The Relative Age of Rocks Index Fossils • Click the Video button to watch a movie about index fossils.