52 Habitats Then and Now-High

Habitats Then and Now FOCUS Question How does the fossil record help us understand how Earth has changed over time?

The Changing Earth Throughout Earth's history, habitats have changed a great deal. Some changes happen quickly. When beavers build dams, they change their habitat. We can see the difference in just a few weeks. Other changes happen very slowly, over thousands or millions of years. In some places, an ocean became a desert and a rainforest became frozen tundra. How do we know these changes have occurred? Fossils— the remains of living things preserved in rock— show us the plants and animals that lived long ago. Fossils also show us how the environment has changed over time. This evidence is part of the fossil record.

Time Recorded in Rocks Fossils are often found buried deep in rocks. Rock is laid down over time in layers. In places where the layers are undisturbed, younger rock layers rest upon older rock layers. This is called the law of superposition. One way scientists can learn the age of fossils is by studying the rock layer in which they are found. Fossils found in deeper layers come from plants and animals that lived further in the past. The types of fossils in each rock layer provide clues about how a fossil organism's habitat has changed over time. Word Wise Super means "over," and position means "to place." Superposition occurs when new rock layers are naturally placed over old ones.

A Grand Process The Grand Canyon provides an example of the law of superposition. The canyon is over 1,600 meters (1 mi.) deep. It has many different layers of rock. Fossils found in these layers are evidence that the environment in and around the canyon has changed over time. More than a billion years ago, the area that is now the Grand Canyon was at the bottom of a sea! Over millions of years, the sea advanced and retreated in this area many times. As the sea advanced, the water left behind layers of limestone. As the water retreated, layers of shale, slate, and mudstone formed.

Forces inside Earth eventually pushed up an area of this land. A layer of rock from the bottom of a sea was now at the top of tall mountains! This layer is the Kaibab (KY-bab) limestone. The Colorado River began to cut through the uplifted land. As the Grand Canyon formed, older layers of rock became visible. When the Kaibab limestone was forming under the sea, many creatures became fossils within the rock. Fossils of sea lilies, called crinoids, are found in this layer. Brachiopods— animals similar to clams— are also found there. These fossils reveal that millions of years ago, this area was very different. It was under water! Do You Know? The newest rock layer in the Grand Canyon. Kaibab Limestone, is still very old. It was formed even before dinosaurs existed!

Super Continent The Himalayas (him-ah-LAY-uz) are the world's highest mountain range. Yet people have discovered ammonite fossils there. Ammonites are sea creatures. This means that the rocks that make up Mount Everest were once on the seafloor. About 225 million years ago, all the continents were one big landmass called Pangaea (pan-GEE-uh). As Pangaea broke apart, a new sea was created. The ammonites lived in this sea. One piece of Pangaea became India. It slowly collided with Asia. The impact caused the seafloor to lift up, creating the Himalayas.