Chapter 18 Fossils and Geologic Time

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426 CHAPTER 18: FOSSILS AND GEOLOGIC TIME cause later rocks have covered most of the rocks from this time, or the rocks were recycled by weathering and erosion, or altered by metamorphism. Most of the earliest forms of life inhabited the oceans, which provided them with nutrients and protected them from harmful solar radiation. Carbon dioxide and nitrogen probably dominated Earth’s early atmosphere, making it similar in composition to the present atmosphere of Venus and Mars. Photosynthesis eventually began in single-celled Precambrian organisms. They used carbon dioxide to store energy, releasing oxygen as a waste product. The addition of oxygen to the atmosphere had two very important results. Oxygen in the form of ozone absorbs harmful radiation, such as ultraviolet rays, from outside Earth. Oxygen also is necessary for air-breathing animals. Both of these factors made possible the later development of life outside the oceans. Paleozoic Era: The Origin of Complex Life-forms The name Paleozoic can be translated from its word origin as “the time of early life.” This era began a little more than half a billion years ago. The abundance of fossils that marks the beginning of the Paleozoic was a result of rapid evolution. Hard parts, such as skeletons and shells, allowed some organisms to move rapidly in search of food and others to protect themselves from becoming food. Many of the specialized features of today’s most complex life-forms, including our sense organs, can be traced back to structures that appeared early in the Paleozoic Era. The first fish as well as the first plants and animals to inhabit the land appeared in the Paleozoic along with the earliest insects and amphibians. Amphibians lay their eggs in water; but as adults, many move onto the land. Reptiles lay eggs with protective shells, and live and reproduce entirely on land. Trilobites appeared in the oceans early in the Paleozoic. They evolved many variations in shape before they became extinct at the end of the Paleozoic. The end of the Paleozoic Era is marked by the extinction of as much as 95 percent of living species. The cause of this ex-
Figure 18-6 Changes in the number of families of marine animals indicate mass extinctions that mark the ends of two geologic eras. The event at the end of the Paleozoic era wiped out about 50 percent of animal families, and a large number of species. Number or families tinction is not known, although some dramatic change in the world’s climates is suspected. This might have been related to the formation of the supercontinent Pangaea, the impact of a large meteorite, or great volcanic eruptions. Regardless of the cause, this catastrophic event led to the appearance of new life-forms in the next era. Figure 18-6 shows when major mass extinctions occurred in the geological past. Mesozoic Era: The Age of Dinosaurs HOW HAS GEOLOGIC TIME BEEN DIVIDED? 427 900 600 300 0 545 Paleozoic Cenozoic Mesozoic 400 200 Geologic age (million years ago) The word Mesozoic can be translated as “middle life.” This era began about 250 million years ago following the mass extinction at the end of the Paleozoic Era. Some forms of fish, insects, and reptiles survived the Paleozoic extinction. Mammals appeared in the Mesozoic, but they were small creatures. Dinosaurs dominated life on land. The evolution of dinosaurs produced a wide range of animals that could inhabit nearly every terrestrial environment. The first birds probably evolved from flying dinosaurs in the Mesozoic. When people or things are called dinosaurs, it may imply that they are not very intelligent or successful. However, dinosaurs existed for nearly 200 million years. Some of them probably achieved remarkable intelligence before they became extinct at the end of the Mesozoic. Recently discovered evidence has linked that extinction to the impact of meteorites and climatic change resulting from dust blown into the 0
Page 1 and 2: DINOSAURS Chapter 18 Fossils and Ge
Page 3 and 4: ally the soft parts of an organism
Page 5 and 6: Figure 18-2 Footprint Puzzle WHAT A
Page 7 and 8: Rock fragments from Mars have been
Page 9 and 10: ACTIVITY 18-3 VARIATIONS WITHIN A S
Page 11 and 12: HOW HAS GEOLOGIC TIME BEEN DIVIDED?
Page 13: Figure 18-5 The wide geographic ran
Page 17 and 18: First birds Extinction of dinosaurs
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Page 21 and 22: HOW DO GEOLOGISTS CORRELATE ROCK LA
Page 23 and 24: Figure 18-9 Correlation. HOW DO GEO
Page 25 and 26: Figure 18-11 An exercise in correla
Page 27 and 28: (2) The length of each age depends
Page 29 and 30: 8. What is the geologic age of most
Page 31: CHAPTER REVIEW QUESTIONS 443 17. Ex

Chapter 18 Fossils and Geologic Time

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