【正确答案】 D

【答案解析】[解析] 6-11 Although the existence of fossils had been known for centuries, it wasn't until the early nineteenth century that their significance as geologic tools became evident. That was when a British engineer and canal builder made an interesting discovery. He noticed that each rock layer in the canals he worked on contained fossils that were unlike those in the rocks either above or below. Each layer held fossils of different types of ancient plants or animals. Furthermore, the engineer noticed that the characteristic fossil content of each layer could be used to determine the age of the rock. Rock layers from some areas could be matched to the layers in other areas. The process of comparing sedimentary rocks at different locations to determine the relationship between them is known as correlation. Correlation involves comparing the rock types, the sequence of rock layers, and the fossils in different places. Correlation gives us a better understanding of geologic history. By correlating the rocks from one place to another, we've been able to develop a geologic time scale that applies to the whole planet. This leads us to one of the most important principles in historical geology: the principle of fossil succession. The principle of fossil succession states that fossil organisms succeed one another in a definite order. Fossils show a definite succession, a progressive change from simple to complex. Thus, fossils document the evolution of life through time. Therefore, we can identify the time period of rock layers by the types of fossils we find there. The principle of fossil succession tells us that different groups of fossils are found in different layers of rock. Fossils found at the base of a rock formation may extend part way up and then disappear. After a fossil has disappeared in a rock formation, it never reappears in a higher layer of that formation. The fossils at the bottom don't appear at the top, and those at the top don't appear at the bottom. This pattern means that fossil species existed for specific periods of the earth's history, and then they became extinct. Extinction is forever; thus, certain fossils disappeared from the record. Fossils are the most useful means of correlating rocks of similar age in different regions. We pay particular attention to certain fossils called index fossils, such as trilobites and fishes, which point to various ages in the evolution of life. For example, we can see an Age of Trilobites--small sea creatures---quite early in the record. Then, in succession, we find an Age of Fishes, an Age of Reptiles, and an Age of Mammals. These "ages" refer to life forms that were especially plentiful during particular time periods. This same succession of organisms, always in the same order, is found in rock formations on every continent. In addition to being important time indicators, fossils are important environmental indicators. We can learn a great deal of information about past environments by studying the fossil record. For example, when we find the fossils of certain shells, we can assume that the region was once covered by water. In the deserts of the American Southwest, fossils provide clues that the environment was much wetter in the past than it is today. We've found sandstone with ripple marks-delicate, wavy ridges that formed millions of years ago when mud and silt at the edge of a shallow sea were rippled by the action of the water. There are also numerous fossils of oyster shells and water-loving plants such as ferns. The main idea is that fossils provide clues to our geologic and environmental history. The professor says: ...their significance as geologic tools became evident; Correlation gives us a better understanding of geologic history; Fossils are the most useful means of correlating rocks of similar age in differen regions; In addition to being important time indicators, fossils are important environmental indicators.