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The Rock and Fossil Record Objectives • Explain how geologic time is recorded in rock layers. • Identify important dates on the geologic time scale. • Explain how environmental changes resulted in the extinction of some species. Geologic Time • The Rock Record and Geologic Time One of the best places in North America to see the Earth’s history recorded in rock layers is in Grand Canyon National Park. These layers represent almost half, or nearly 2 billion years, of Earth’s history. • The Fossil Record and Geologic Time Fossils of plants and animals are common in some rocks. These fossils contribute to our understanding of geologic time. I. The Geologic Time Scale A. Divisions of Time Geologists have divided Earth’s history into sections of time. The largest divisions of geologic time are eons. Eons are divided into eras, eras into periods, and periods into epochs. • The Appearance and Disappearance of Species At certain times during Earth’s history, the number of species has increased or decreased dramatically. An increase or decrease in the number of species often comes as a result of either a relatively sudden increase or decrease in competition among species. The Geologic Time Scale B. The Paleozoic Era—Old Life The Paleozoic era lasted from about 543 million to 248 million years ago. It is the first era well represented by fossils. Ended with the largest mass extinction in Earth’s history. C. The Mesozoic Era—The Age of Reptiles The Mesozoic era began about 248 million years ago. The Mesozoic is known as the Age of Reptiles because reptiles, such as dinosaurs, inhabited the land. D. The Cenozoic Era—The Age of Mammals The Cenozoic era began about 65 million years ago and continues to the present. Science Journal Entry #2 • Draw the geologic time scale. • Draw and give examples of animals alive during the Paleozoic, Mesozoic, and Cenozoic Eras.