Geologic Time. Life and Geologic Time

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1 chapter 10 3 Geologic Time section 1 Life and Geologic Time Before You Read Think about a giraffe you have seen. On the lines below, describe the giraffe and tell why you think it has a long neck. What You ll Learn how geologic time is divided how plate tectonics and other changes on Earth affect species Read to Learn Geologic Time A group of students is searching for fossils. By looking in rocks that are hundreds of millions of years old, they hope to find fossils of organisms called trilobites (TRI loh bites). Trilobites are small, hard-shelled animals that lived in ancient seas. Trilobites are considered to be index fossils. Index fossils lived over vast regions of the world during specific periods of geologic time. The students hope that by studying trilobite fossils, they can help piece together a puzzle. They want to know what caused the trilobites to disappear from Earth millions of years ago. What is the geologic time scale? The appearance or disappearance of types of organisms throughout Earth s history marks important events in geologic time. Paleontologists, scientists who study the prehistoric world, divide Earth s history into time units based on life-forms that existed only during certain periods. This division of Earth s history is known as the geologic time scale. Sometimes few fossils remain from a period. Then paleontologists use other methods to define a division of geologic time. Study Coach Read-and-Say Something Work with a partner. As you read each paragraph of the text, take turns saying something about the main idea of the paragraph. Help each other understand the information in the text. A Classify Make the following Foldable to help you organize geologic time periods and events into groups based on their characteristics. Reading Essentials 135

2 4, Identify What is the longest subdivision of geologic time? 2. Interpret What major event occurred at the end of the Paleozoic Era? Precambrian Time Hadean Eon 3,800 Archean Eon 2,500 Proterozoic Eon First life Origin of Earth 544 Cambrian Period 505 Ordovician Period 440 Silurian Period First trilobites Paleozoic Era 410 Devonian Period What are major subdivisions of geologic time? The fossil record is used to divide Earth s history into geologic time periods. The figure below shows the four major subdivisions of geologic time eons, eras, periods, and epochs. Eons are the longest subdivision and are based on the abundance of certain fossils. Eons are divided into smaller time periods called eras. An era is marked by major worldwide changes in the types of fossils present. For example, at the end of the Mesozoic Era, many kinds of invertebrates, birds, mammals and reptiles became extinct. Eras are subdivided into periods. A period is a unit of geologic time during which certain types of life-forms existed all over the world. Geologic periods are divided into epochs. An epoch is also characterized by differences in life-forms, but these may vary from continent to continent. s may be given names, like those in the Cenozoic Era or may be called simply early, middle, or late. What limits the divisions of geologic time? There is a limit to how finely geologic time can be subdivided. It depends on the kind of rock record that is being studied. Sometimes it is possible to distinguish different layers of rock that formed during a single year. In other cases, there is little information to help scientists subdivide geologic time. 360 Mississippian Period 325 Pennsylvanian Period 286 Permian Period Phanerozoic Eon 248 Mesozoic Era Triassic Period 213 Jurassic Period 145 Cretaceous Period 65 Paleocene 55.5 Eocene 33.7 Cenozoic Era Tertiary Period Oligocene Mass extinction First flowering plants 23.8 Miocene 5.3 Pliocene 1.8 Pleistocene Quaternary Period Holocene Millions of years ago 136 Geologic Time

3 Organic Evolution The fossil record shows that species (SPEE sheez), or types of life-forms, have changed over geologic time. Organic evolution is the change in species over geologic time. Most scientists believe that changes in the environment affect an organism s survival. Organisms that do not adapt to the changes are less likely to survive or reproduce. Over time, the disappearance of individuals that are not adapted to the new conditions can cause changes to species of organisms. What is a species? There are many ways to define the term species. Life scientists often define a species as a group of organisms that normally reproduce only with other members of their group. For example, horses generally reproduce only with other horses. Sometimes, members of different species can mate and produce offspring. Horses sometimes mate with donkeys and produce mules. However, the offspring of two different species are often sterile. What is natural selection? Charles Darwin was a naturalist someone who studies the natural world. Between 1831 and 1836, Darwin sailed around the world, carefully observing plants and animals. He collected samples of life-forms and studied them to learn how they were related. After returning home to England, Darwin explained his theory of natural selection. Darwin defined natural selection as the process by which organisms with characteristics suited to a certain environment have a better chance of surviving and reproducing than organisms that do not have these characteristics. Darwin understood that all organisms compete for resources, such as food and living space. He also knew that individuals within a species could be different, or show variation. An individual s differences might help or hurt its chances of surviving in a changing environment. Some organisms that were well suited to their environment lived longer and had a better chance of producing offspring. Organisms that were poorly adapted to their environment produced few or no offspring. Because many characteristics are inherited, the characteristics of organisms that are better adapted to the environment get passed on to offspring more often. According to Darwin, this can cause a species to change over time. 3. Identify What is the name for a group of organisms that normally reproduce only with other members of their group? 4. Identify Who proposed the theory of natural selection? Reading Essentials 137

4 5. Predict What might have happened to the animals in their new environment if none of them had had a longer neck? 6. Explain How could the animals with short necks survive without evolving? What is natural selection within a species? Suppose that an animal species exists in which a few of the individuals have long necks, but most have short necks. The main food for the animal is the leaves on trees in the area. Then suppose the climate changes and the area becomes dry. The lower branches of the trees might not have any leaves. Now which of the animals will be better suited to survive? In this case, the animals with the longer necks will be better able to eat the leaves. Clearly, the long-necked animals have a better chance of surviving and reproducing. Their offspring will have a greater chance of inheriting the important characteristic. Gradually, as the number of long-necked animals becomes greater, the number of short-necked animals decreases. Over time, the species might change so that nearly all of its members have long necks just like the giraffe. It is important to notice that individual, short-necked animals did not change into long-necked animals. A new characteristic becomes common in a species only under two conditions. First, some members must already have that characteristic. Second, the trait must increase the animal s chance of survival. If no animal in the species had a long neck in the first place, a long-necked species could not have evolved by means of natural selection. What is artificial selection? Humans have long used artificial selection to breed domestic animals. Animal breeders carefully choose individuals animals with desired characteristics to mate. Their offspring also have the desired characteristics. In this way, animal breeders have created many different breeds of cats, dogs, cattle, and chickens. How do new species evolve? Natural selection explains how characteristics change and how new species arise. Remember the animals with short necks? If they had moved, or migrated, to a different area, they may have survived. Then they may have reproduced in the new area, developing different characteristics from the long-necked animals. If the short-necked animals were different enough from the long-necked animals that they could no longer breed, then a new species would have evolved. 138 Geologic Time

5 Side lobe Middle lobe Picture This 7. Identify Look carefully at the picture of a trilobite. With a red pencil, outline the trilobite s lobes. With a blue pencil, circle its head, middle, and tail. Side lobe Cephalon (SE ful lahn) Thorax (THOR aks) Pygidium (pi JIH dee uhm) Trilobites Remember the trilobites you read about earlier? A trilobite is an ancient organism with a three-lobed exoskeleton. An exoskeleton is a hard outer skeleton. These ancient animals were given the name trilobite because they have a three-part shell. The figure above shows the three parts of the trilobite shell. The three parts, called lobes, run the length of its body. Trilobites also have a head (cephalon), a middle that is segmented or divided into sections (thorax), and a tail (pygidium). What do the changing characteristics of trilobites tell scientists? Trilobites lived in Earth s oceans for more than 200 million years. All through the Paleozoic Era, some species of trilobites became extinct and other species of trilobites evolved. Different periods of the Paleozoic Era had different species of trilobites. Each species of trilobites had its own particular characteristics that were different from all other species. Paleontologists use the differences in trilobite species to explain how trilobites evolved over geologic time. These changes tell how different trilobites from different periods lived. The changes also tell how trilobites responded to changes in their environment. 8. Identify What were the two things that happened to trilobite species during the Paleozoic Era? Reading Essentials 139

6 9. Identify What characteristic helps scientists figure out where different trilobite species lived? Picture This 10. Identify Highlight the body segments of the trilobites in the figure. What do trilobite eyes reveal? Trilobites may have been the first organisms on Earth with complex eyes as shown in the figure above. Trilobite eyes are the result of natural selection. The position of an organism s eyes tells how it lived. If its eyes are in the front of its head, it likely swam actively through the sea. If its eyes are located toward the back of its head, it likely lived on the bottom of the ocean. Most species of trilobites had eyes that were midway between the front and the back of the head. This clue tells us that trilobites were adapted to both active swimming and crawling on the ocean floor. What changes occurred in trilobite eyes? Over time, the eyes in some trilobites changed. Gradually, the eyes of many trilobite species became smaller and smaller. Eventually, their eyes disappeared completely. These blind trilobites, shown above, might have burrowed into sediments on the ocean floor. Or they lived in a part of the deep ocean where there was no light. Not all trilobite species lost their eyes. Some trilobite species developed highly complex eyes. One species of trilobite had compound eyes eyes with many individual lenses. These trilobites had excellent vision. Still other trilobite species developed complex eyes on stalks that extended from their head. They also could see their world very well. What changes occurred in trilobite bodies? The trilobite body also changed over geologic time, as shown in the figure below. Some early trilobite species had many segments in the middle part of the body. Later trilobites had fewer segments. Ordovician Period Devonian Period Cambrian Period Mississippian Period Silurian Period 140 Geologic Time

7 What evidence do fossils provide? The exoskeletons of trilobites changed as their environment changed. Each change in the trilobite body shows how different species of trilobite adapted to new conditions. Some species of trilobite could not adapt. These species disappeared, or became extinct. Plate Tectonics and Earth History Earth s crust is made up of several plates. These plates are in slow but constant motion. This motion, called plate tectonics, caused continents to split apart or to collide. At the time the trilobites dominated Earth s seas, Earth s plates were moving together. When all the continents collided, they formed a single, enormous continent known as Pangaea (pan JEE uh), shown in the figure below. Pangaea was one giant landmass, or supercontinent. When Pangaea was forming, sea levels were dropping. Because trilobites lived in the ocean, they could not survive in the changed environment. At the end of the Paleozoic Era, trilobites became extinct. Some scientists do not accept that the formation of Pangaea caused the extinctions at the end of the Paleozoic Era. Changes in the climate or other conditions may have led to the Paleozoic extinctions. As in all scientific debates, evidence must be considered carefully, and conclusions must be drawn based on the evidence. P A N G A E A 11. Explain What did each change in the trilobite body show? Picture This 12. Outline In the figure, outline the borders of the continents that crashed together to form the supercontinent, Pangaea. Reading Essentials 141

8 After You Read Mini Glossary eon: largest geologic time division that is based on the abundance of certain fossils epoch: geologic time division characterized by differences in life-forms, which may vary from continent to continent era: geologic time division marked by major worldwide changes in the types of fossils present geologic time scale: divisions of time in Earth s history natural selection: process by which organisms with characteristics suited to a certain environment have a better chance of surviving and reproducing than organisms that do not have these characteristics organic evolution: change in species over geologic time Pangaea: one giant landmass, or supercontinent, that formed at the end of the Paleozoic Era period: subdivision of geologic time during which certain types of life-forms existed all over the world species: group of organisms that normally reproduce only with other members of their group trilobite (TRI loh bite) : small, hard-shelled animal that lived in ancient seas 1. Review the terms and their definitions in the Mini Glossary. Then write two sentences about geologic time and natural selection. Use at least four vocabulary words in your sentences. 2. Fill in the correct term to show how the Geologic Time Scale is divided. Phanerozoic Eon Mesozoic Era Triassic Period Jurassic Period Cretaceous Period Paleocene Eocene Cenozoic Era Tertiary Period Oligocene Miocene Pliocene Pleistocene Quaternary Period Holocene a. b. c. d. End of Section 142 Geologic Time Visit blue.msscience.com to access your textbook, interactive games, and projects to help you learn more about geologic time and natural selection.