Unit 10 Lesson 2 What Was Ancient Earth Like? Copyright © Houghton Mifflin Harcourt Publishing Company

How Rocks and Fossils Tell a Story Unit 10 Lesson 2 What Was Ancient Earth Like? How Rocks and Fossils Tell a Story Sedimentary rock forms in layers. Many sedimentary rocks contain fossils. The oldest rock layers are at the bottom, and the youngest rock layers are at the top. The relative age of a layer of rock is the age of that layer when compared to other layers. Copyright © Houghton Mifflin Harcourt Publishing Company

How Rocks and Fossils Tell a Story Unit 10 Lesson 2 What Was Ancient Earth Like? How Rocks and Fossils Tell a Story Fossils form when sediment buries dead organisms. The fossil record is made up of all the fossils in Earth’s rock layers. The fossil record contains information about Earth’s history and the history of life on Earth. Copyright © Houghton Mifflin Harcourt Publishing Company

How Rocks and Fossils Tell a Story Unit 10 Lesson 2 What Was Ancient Earth Like? How Rocks and Fossils Tell a Story Over time, the movement of Earth’s crust can cause layers of rock to become tilted. It can also lift rocks formed in the ocean to above sea level. The rock layers of the Grand Canyon were tilted and lifted millions of years ago. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Divisions in Time Scientists developed the geologic time scale to divide Earth’s history into manageable units. The fossils each unit contains define it. Flowering plants, such as grasses, and saber-toothed cats existed during the Ice Age. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Divisions in Time Trees that produce seeds in cones appeared at the beginning of the Mesozoic Era. The middle and end of this era is known as the Age of Dinosaurs. Large animals, such as Stegosaurus, lived during this time. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Divisions in Time Large tree ferns were common during the late Paleozoic Era. The large coal deposits in the eastern United States formed from plants like these. Trilobites were so common that the earliest part of the Paleozoic Era is called the Age of Trilobites. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Divisions in Time Index fossils help identify a very short period of Earth’s history and must meet four requirements. The organisms from which they formed lived during a short period of Earth’s history. The organisms must have had large populations so that many fossils formed. Finally, the fossils must be widespread and must be easily recognized. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Big Changes on Earth Fossils tell how life on Earth has changed over time. Fossils also tell how Earth’s continents and landforms have changed. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Big Changes on Earth Scientists have found fossils of a Mesosaurus—a small, lizard-like reptile that lived in fresh water—on both sides of the Atlantic Ocean. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Big Changes on Earth Mesosaurus was too small to have been able to swim across the Atlantic. At the end of the Paleozoic Era, all of Earth’s masses were joined. These masses began to break apart during the Mesozoic Era. The single landmass explains how Mesosaurus could move across land. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? Big Changes on Earth The landmasses breaking apart supports the idea of continental drift, and helps scientists understand the movement of Earth’s plates. Copyright © Houghton Mifflin Harcourt Publishing Company

Changing Environments Unit 10 Lesson 2 What Was Ancient Earth Like? Changing Environments Fossils provide clues about changing environments. Finding a fossil sea snail at the top of a mountain would lead to the conclusion that the environment has changed. Copyright © Houghton Mifflin Harcourt Publishing Company

Changing Environments Unit 10 Lesson 2 What Was Ancient Earth Like? Changing Environments Fossils of plants and animals at La Brea Tar Pits, in Los Angeles, California, show that the climate has not changed much in the last 40,000 years. Copyright © Houghton Mifflin Harcourt Publishing Company

Changing Environments Unit 10 Lesson 2 What Was Ancient Earth Like? Changing Environments Falls of the Ohio is a state park in Indiana where summers are hot and winters are cold and snowy. Fossils of coral, clams, and other organisms that lived in shallow, warm, tropical seas show how the climate has changed in the last 380 million years. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? The Great Die Offs Several times in history, many species became extinct at once. These large events, caused by climate change, are called mass extinctions. Worldwide volcanic eruptions blow large amounts of ash and dust into the air, blocking sunlight so plants can’t grow. Other plants die as ash settles on them and smothers them. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 10 Lesson 2 What Was Ancient Earth Like? The Great Die Offs At the end of the Mesozoic Era, an asteroid sent huge amounts of dust into the air, blocking sunlight and changing the climate. These changes may have led to the extinction of many organisms, including dinosaurs. Copyright © Houghton Mifflin Harcourt Publishing Company