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Name CHAPTER 9 Class Date A View of Earth’s Past 2 Precambrian Time and the Paleozoic Era SECTION KEY IDEAS As you read this section, keep these questions in mind: • How is evolution related to geologic change? • What are two characteristics of Precambrian rock? • What were the major events of the Paleozoic Era? What Is Evolution? Earth’s history is recorded in rock layers. The fossils from each layer show which species of organisms existed when the layer formed. Scientists study and compare fossils from different times in Earth’s history. By studying fossils, scientists have discovered evidence that species have changed over time. Scientists use the term evolution to describe this change. Evolution is the process in which new species develop from existing species. Scientists find evidence for evolution when they compare fossils to living organisms. This huge species existed from 110 million to 90 million years ago. READING TOOLBOX Outline As you read, make a chart showing major events that happened during Precambrian time and the Paleozoic Era. LOOKING CLOSER 1. Analyze How do these two skulls support the theory of evolution? The group that includes modern crocodiles has existed for 65 million years. Modern crocodiles look very similar to their crocodilian ancestors. EVOLUTION AND GEOLOGIC CHANGE Geologic and climatic changes affect the ability of many organisms to survive. Scientists use evidence from rocks and fossils to find out how environmental changes affected organisms in the past. The fossil record shows that some species survived environmental changes but that some species disappeared. Scientists study fossils to find out why some species changed over time and why some stayed almost the same. They also try to find out why species became extinct. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 127 A View of Earth’s Past Name SECTION 2 Class Date Precambrian Time and the Paleozoic Era continued Why Do Scientists Know Relatively Little About Precambrian Time? Critical Thinking 2. Analyze Methods Why do scientists not divide Precambrian time into smaller units based on the fossil record? Most scientists agree that Earth formed about 4.6 billion years ago. Earth formed as part of a cloud of dust, or nebula. Over time, particles from the cloud formed Earth and other planets. The formation of Earth was the beginning of Precambrian time. Precambrian time lasted about 4 billion years and makes up about 88% of Earth’s history. Scientists do not know much about what happened during Precambrian time. Geologic changes have deformed the rock layers from the time. The layers are not in their original order. Thus, the rock record is hard to interpret, or understand. PRECAMBRIAN ROCKS READING CHECK 3. Explain Give one reason Precambrian rocks do not have many fossils. Earth has many areas of exposed, or uncovered, Precambrian rocks. These areas are called shields. Precambrian shields result from millions of years of geologic changes. Precambrian rocks have very few fossils. Precambrian organisms did not have hard parts such as bones or shells that fossilize easily. Precambrian rocks are also very old. Volcanic eruptions, erosion, and movement of tectonic plates probably destroyed most Precambrian fossils. PRECAMBRIAN LIFE Scientists have discovered a few Precambrian fossils. The most common fossils are cyanobacteria. These fossils are found in stromatolites, or reeflike deposits that form in water. Scientists have also discovered Precambrian fossils of marine worms and jellyfish. Precambrian stromatolites show that shallow seas covered much of Earth during Precambrian time. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 128 A View of Earth’s Past Name Class SECTION 2 Date Precambrian Time and the Paleozoic Era continued What Changes Happened in the Paleozoic Era? The Paleozoic Era began 542 million years ago and ended 251 million years ago. When the Paleozoic Era began, many small continents were scattered over Earth. By the end of the Paleozoic Era, the smaller continents had moved and collided, or crashed into each other. This movement formed the supercontinent Pangaea. The collisions lifted large areas of land and formed mountain ranges. Paleozoic rocks have many fossils. These fossils show that the number of plant and animal species increased during the Paleozoic Era. Scientists have used the detailed fossil record to divide the Paleozoic Era into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. READING CHECK 4. Explain How did Pangaea form? TIMELINE OF THE PALEOZOIC ERA Carboniferous Period 299 Ma Permian mass extinction results from major environmental changes. Formation of Pangaea is complete. First reptiles appear. Crinoid 318 Ma 359 Ma Devonian Period Mississippian Pennsylvanian Period Period Permian Period 251 Ma Silurian Period 416 Ma a Ordovician Period 444 Ma Cambrian Period 488 Ma First amphibians appear. LOOKING CLOSER First arthropods appear on land. 5. Identify What was the earliest period in the Paleozoic Era? First land plants appear. Modern, oxygen-rich atmosphere has formed. Trilobite First possible vertebrates appear. 6. Identify During which period did the first land plants appear? 7. Interpret Diagrams Which group of organisms evolved first—amphibians or reptiles? 542 Ma Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 129 A View of Earth’s Past Name SECTION 2 Class Date Precambrian Time and the Paleozoic Era continued THE CAMBRIAN PERIOD READING CHECK 8. List Identify three kinds of organisms that existed during the Cambrian Period. Many marine, or sea, organisms appeared during the Cambrian Period. The warm, shallow waters on Earth were perfect for marine invertebrates, or animals without backbones. The most common Cambrian invertebrates were trilobites. Scientists use many kinds of trilobite fossils as index fossils. Cambrian invertebrates also included brachiopods, a kind of animal with a shell. Many kinds of brachiopods lived during this period, but very few exist today. Worms, jellyfish, snails, and sponges also existed during the Cambrian Period. THE ORDOVICIAN PERIOD During the Ordovician Period, brachiopods and other marine organisms, such as graptolites, became more common. At the same time, the number of trilobite species decreased. By this period, vertebrates had appeared. Vertebrates are animals with backbones. The earliest vertebrates were fish. Fish in the Ordovician Period did not have jaws or teeth. Their bodies had thick, bony plates. THE SILURIAN PERIOD Critical Thinking 9. Infer Land animals evolved only after land plants evolved. Why do you think events happened in this order? Vertebrate and invertebrate marine life thrived during the Silurian Period. Corals and relatives of modern sea stars became more common. Eurypterids, shown below, also lived during this period. Near the end of the Silurian Period, early land plants and animals evolved. Scientists have found fossils of giant eurypterids that are about 2 m long. THE DEVONIAN PERIOD The Devonian Period is called the Age of Fishes. Rocks from this period hold fossils of many bony fish. One group of fishes, lungfish, could breathe air. Another group of fishes, rhipidistians, may have used their strong fins to crawl on land. Early amphibians, such as the ancestors of modern frogs, probably evolved from rhipidistians. Land plants, such as ferns, also appeared during this period. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 130 A View of Earth’s Past Name SECTION 2 Class Date Precambrian Time and the Paleozoic Era continued THE CARBONIFEROUS PERIOD During the Carboniferous Period, Earth’s climate was generally warm. Humidity was often very high. Forests and swamps covered much of the land. Over many millions of years, these forests and swamps became coal deposits. These coal deposits exist today in places such as Pennsylvania and Ohio. In North America, scientists divide the Carboniferous Period into the Mississippian Period and the Pennsylvanian Period. Mississippian Pennsylvanian Period Period Carboniferous Period LOOKING CLOSER 299 Ma 318 Ma 10. Analyze Graphics About when did reptiles first appear on Earth? First reptiles appear. Amphibians and fish thrived during the Carboniferous Period. Crinoids were common ocean animals. Insects such as giant cockroaches and dragonflies were common land animals. Near the end of the period, early reptiles, which looked like large lizards, appeared. THE PERMIAN PERIOD The Permian Period is the last period in the Paleozoic Era. Many environmental changes happened during this period. By the start of the Permian Period, the continents had joined to form Pangaea. New mountain ranges formed, and areas of dry land developed. The warm, shallow seas became smaller. The environmental changes caused a mass extinction of Paleozoic organisms. In a mass extinction, a very large number of species becomes extinct at about the same time. As the seas became smaller, there were fewer habitats for marine species. During the mass extinction at the end of the Permian Period, most marine invertebrates, such as trilobites, died out. However, fossils show that many reptiles and amphibians survived the changes. Animals from these groups dominated Earth for millions of years after the Paleozoic Era. READING CHECK 11. Identify What group of organisms was most affected by changing environments at the end of the Permian Period? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 131 A View of Earth’s Past Name Class Date Section 2 Review SECTION VOCABULARY evolution the process of change by which new species develop from preexisting species over time Paleozoic Era the geologic era that followed Precambrian time and that lasted from 542 million years ago to 251 million years ago Precambrian time the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago 1. Summarize Complete the table below to describe the major developments in the six periods of the Paleozoic Era. Period When Period Began (Ma) Major Events and Biological Developments Cambrian Ordovician 488 Silurian Devonian Carboniferous Permian 2. Identify What are two characteristics of Precambrian rock? 3. Analyze Relationships How do geologic changes affect evolution? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 132 A View of Earth’s Past