3 The Mesozoic and Cenozoic Eras

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1 CHAPTER 9 3 The Mesozoic and Cenozoic Eras SECTION A View of Earth s Past KEY IDEAS As you read this section, keep these questions in mind: What were the periods of the Mesozoic and Cenozoic Eras? What were the major geologic and biological developments during the Mesozoic Era? What were the major geologic and biological developments during the Cenozoic Era? What Changes Happened During the Mesozoic Era? During a mass extinction, large numbers of species become extinct around the same time. At the end of the Permian, more than 70% of land organisms and 90% of marine organisms became extinct. The Mesozoic Era followed this mass extinction. The mass extinction left space and resources for the species that survived. During the Mesozoic Era, many new species evolved. TIMELINE OF THE MESOZOIC ERA READING TOOLBOX Graphic Organizer As you read, make a concept map using the following terms: Mesozoic Era, Age of Reptiles, Jurassic, Triassic, Cretaceous, Cenozoic Era, Age of Mammals, Tertiary, and Quaternary Ma The last dinosaurs become extinct. Catastrophic asteroid impact occurs. Cretaceous First modern birds 146 Ma Jurassic First flowering plants (angiosperms) First primitive birds Tyrannosaurus rex LOOKING CLOSER 1. Identify What is one geologic change that happened in the Mesozoic Era? 200 Ma Triassic 251 Ma First mammals Pangaea begins to break apart. Archaeopteryx 2. Interpret Graphics When did the first mammals appear? Holt McDougal Earth Science 133 A View of Earth s Past

2 3. Describe Describe the general climate during the Mesozoic Era. GEOLOGIC AND BIOLOGICAL DEVELOPMENTS During the Mesozoic Era, Pangaea broke into smaller continents. Shallow seas and marshes covered much of the land. In general, the climate was warm and humid. The conditions were good for reptiles. Lizards, turtles, crocodiles, snakes, and many dinosaurs thrived during the Mesozoic Era. This era is often called the Age of Reptiles. The Mesozoic Era left a rich fossil record. Based on the fossil record, scientists divide the Mesozoic Era into three periods: Triassic, Jurassic, and Cretaceous. THE TRIASSIC PERIOD Many dinosaur species lived during the Triassic. Some dinosaurs were the size of squirrels. Others were nearly 30 m long. Most dinosaurs were about 2 m to 5 m long and moved quickly. Reptiles called ichthyosaurs lived in the oceans during the Triassic. New marine invertebrates evolved, such as the ammonite. On land, small mammals appeared in the forests. THE JURASSIC PERIOD Dinosaurs became the dominant organism during the Jurassic. Fossil records show that two major groups of dinosaurs evolved. Scientists distinguish the dinosaurs in these groups by their hip bones. The Jurassic also had flying reptiles, such as the pterosaur. The pterosaur flew on skin-covered wings, like modern bats do. Jurassic rocks also show evidence of early birds, such as Archaeopteryx. Talk About It Make Comparisons Just how big was a 30-m-long dinosaur? Imagine 30 meter sticks lying end to end. With a partner, discuss everyday objects or buildings that are about the same size as a large dinosaur. Share your ideas with the class. The forests of the Triassic were made up of conifers. Today, conifers include pine and fir trees. Holt McDougal Earth Science 134 A View of Earth s Past

3 The Two Major Groups of Dinosaurs Saurischians Ornithischians Meaning of the name lizard-hipped bird-hipped Herbivores (plant eaters) or carnivores (meat eaters)? herbivores and carnivores herbivores Example Apatosaurus Stegosaurus THE CRETACEOUS PERIOD Dinosaurs continued to dominate Earth during the Cretaceous. Tyrannosaurus rex was almost 6 m tall, which made it one of the largest carnivores from this time. Its sharp teeth were up to 15 cm long. The armored ankylosaurs and the horned ceratopsians were other dinosaurs of the Cretaceous. Flowering plants, or angiosperms, evolved during this period. Angiosperms include magnolia and willow trees. Later, trees such as maples, oaks, and walnuts became common. Angiosperms are the dominant type of land plant today. The Cretaceous ended in a mass extinction that included the dinosaurs. Some scientists think that changes such as volcanoes and continental shifts caused the extinction. However, many scientists use the impact hypothesis to explain the mass extinction. According to the impact hypothesis, an asteroid crashed into Earth 65 million years ago. The crash threw dust into the air. The dust blocked the sun and caused Earth s climate to cool. Many species could not survive the cooler climate. Over time, the dust settled over Earth and formed a layer of sediment. This sediment was full of the element iridium. Iridium is common in asteroids, but uncommon in rocks on Earth. What Changes Happened During the Cenozoic Era? The Cenozoic Era began 65 million years ago after the mass extinction at the end of the Mesozoic Era. Earth had many climate changes during the Cenozoic Era. At times, ice sheets covered almost one-third of Earth s land. New species that could survive in cool climates appeared. Mammals became the dominant organisms. Thus, the Cenozoic Era is called the Age of Mammals. Critical Thinking 4. Synthesize Graphics and Text To which group of dinosaurs did T. rex belong? Explain your answer. 5. Evaluate Hypotheses In what way does the presence of an iridium layer give evidence for an asteroid impact? Holt McDougal Earth Science 135 A View of Earth s Past

4 LOOKING CLOSER 6. Calculate How long did the Tertiary last? How long has the Quaternary lasted? 7. Interpret Graphics Which two epochs make up the Quaternary? 8. Identify Name the era, the period, and the epoch we live in today. 9. Identify During which epoch did the first primates evolve? TIMELINE OF THE CENOZOIC ERA Tertiary Quaternary present Holocene Ma Pleistocene 1.8 Ma Pliocene 5.3 Ma Miocene 23.0 Ma Oligocene 33.9 Ma Eocene 55.8 Ma Paleocene 65.5 Ma Modern humans Modern Ice Age begins. First hominids (early human ancestors) India collides with Asia, and Antarctica drifts over South Pole. First grasses Most modern mammal families, including whales, carnivores, hoofed animals, and primates, have appeared. Cave painting made by early humans between 15,000 and 13,000 years ago Small mammals This timeline is not drawn to scale. In other words, the length of an epoch on the timeline is not related to how long the epoch actually lasted. The Cenozoic Era is divided into two periods: the Tertiary and the Quaternary. These periods are divided into seven epochs: the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. During the Paleocene, many new mammal species evolved. The first primates also evolved during this time. The tarsier is a modern primate with ancestors from the Paleocene. During the Eocene, the first horses, whales, flying squirrels, and bats appeared. Small reptiles continued to thrive. At the end of the Eocene, Earth s climate cooled further. During the Oligocene, continents collided and formed the Himalayas in Asia. Earth s climate became cooler and drier. Many early mammals became extinct. However, large species of deer, pigs, horses, camels, cats, and dogs survived. Marine invertebrates, such as clams and snails, continued to thrive. Grasses and hardwood trees flourished in this climate. Holt McDougal Earth Science 136 A View of Earth s Past

5 THE MIOCENE EPOCH The Miocene had changes in currents and sea levels. The largest known land mammals existed during this epoch. Miocene rocks contain fossils of horses, camels, deer, rhinoceroses, pigs, raccoons, wolves, and foxes. They also contain fossils of saber-toothed cats, which are now extinct. THE PLIOCENE EPOCH During the Pliocene, predators such as bears, dogs, and cats evolved into their modern forms. Modern horses also appeared. Critical Thinking 10. Apply Concepts Scientists have a harder time dividing the Mesozoic Era into epochs than dividing the Cenozoic Era. Why do you think this is true? Giant ground sloths lived during the late Pliocene in parts of North America and South America. Near the end of the Pliocene, climate changes caused the ice sheets to grow. As more water froze, sea levels fell. A land bridge appeared between Eurasia and North America. Changes in Earth s crust formed a land bridge between North and South America. Organisms moved between the continents on these land bridges. THE PLEISTOCENE EPOCH During the Pleistocene, ice sheets changed in size many times. Some animals, such as the woolly mammoth, had thick fur that helped them survive the cold climate. Other species survived by moving to warmer areas. Some species became extinct. Pleistocene rocks have fossils of early modern humans. Cave paintings suggest that early humans may have been hunters. THE HOLOCENE EPOCH The Holocene began about 11,500 years ago as the last glacial period ended. The Holocene continues today. As the ice sheets melted, sea levels rose. Coastlines took on their present shapes. In the early Holocene, modern humans began to farm and use tools made of bronze and iron. 11. Explain How did climate changes affect Earth s surface in the Pliocene? Math Skills 12. Calculate About what percent of the Cenozoic Era does the Holocene represent? Holt McDougal Earth Science 137 A View of Earth s Past

6 Section 3 Review SECTION VOCABULARY Cenozoic Era the current geologic era, which began 65.5 million years ago; also called the Age of Mammals mass extinction an episode during which large numbers of species become extinct Mesozoic Era the geologic era that lasted from 251 million to 65.5 million years ago; also called the Age of Reptiles 1. Compare What do the start of the Mesozoic Era and the start of the Cenozoic Era have in common? 2. Summarize Complete the table below to identify the periods of the Mesozoic Era. For each period, describe at least one major life-form that evolved during the period or was dominant during that period. Major Life-Forms 3. Explain What do many scientists think caused the mass extinction of the dinosaurs? 4. Identify What is one major geologic development during the Mesozoic Era? What is one major geologic development during the Cenozoic Era? 5. Identify Relationships How could you identify the boundary between the Cretaceous and Tertiary s in a rock sample? Explain your answer. Holt McDougal Earth Science 138 A View of Earth s Past