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The Mesozoic and Cenozoic Eras, Visit the Earth Science Web Site at Earthgeu.Com 2424 TheThe MesozoicMesozoic andand CenozoicCenozoic What You’ll Learn • How plate tectonics shaped the landscape of EErasras western North America. • What the characteristics of a dinosaur are. • How dinosaurs and many other organisms became extinct at the end of the Mesozoic Era. Why It’s Important As the physical geology of Earth changed, so did the biosphere. Reptiles ruled the land during the Mesozoic, but their reign ended abruptly with the dawn of the Cenozoic. To find out more about the Mesozoic and Cenozoic Eras, visit the Earth Science Web Site at earthgeu.com Albertosaurus and Centrosaurus from the 624 Cretaceous of Alberta, Canada DDiscoveryiscovery LLabab Determining the Effect of Shape on Buoyancy How can the shape of a micro- 4. Drop both spheres into the beaker fossil help paleontologists determine at the same time and observe what whether it floated or whether it lived happens. on the seafloor? In this lab, you will Observe In your science learn how the shape of a fossil shell journal, record your observa- affected the animal’s buoyancy, and tions. What effect does the how this information helps paleontol- shape of the spheres have on ogists determine how the animal lived. their buoyancy? The photos 1. Fill a 1000-mL beaker with water. on this page show marine microfossils. Based on your 2. Using plasticene, make two solid observations in this lab, spheres that are each 1.5 cm diameter. which one do you think 3. Deform one of the spheres by pinch- floated in the water? Which ing the plasticene into 5 “spines” that one lived on the seafloor? extend around the sphere. Explain your reasoning. 24.124.1 Mesozoic Paleogeography OBJECTIVES The Mesozoic Era consisted of the Triassic, Jurassic, and Cretaceous • Explain the breakup periods, as shown in Figure 24-1 on page 626. Movies such as Jurassic of Pangaea. Park and The Lost World have popularized the Mesozoic as the age of the dinosaurs. What was the world like in which dinosaurs such as • Distinguish between the those pictured on page 624 lived? Why did this chapter in Earth’s his- different tectonic charac- tory end? These and other questions make the Mesozoic one of the teristics of the Mesozoic most fascinating times of study in Earth’s history. Orogenies. THE BREAKUP OF PANGAEA VOCABULARY An important event that occurred during the Mesozoic Era was the Cordillera breakup of Pangaea. Because heat causes solid objects to expand, the heat coming from within Earth beneath Pangaea caused the continent to expand. By the Late Triassic, the brittle lithosphere of Pangaea had cracked and broken apart. As some of the large cracks, or rifts, widened and as the landmasses spread apart, the ocean flooded the rift valleys. This resulted in the formation of new oceans that divided the newly separated continents. 24.1 Mesozoic Paleogeography 625 66 M.Y.B.P. The breakup of Pangaea resulted in the formation of the Atlantic Ocean. As North America rifted away from Europe and Africa, some of the spreading areas joined together to form a long, continuous rift system called the Mid-Atlantic Ridge. The Cretaceous Period Mid-Atlantic Ridge is still active today. 146 M.Y.B.P. ACTIVE TECTONISM IN WESTERN NORTH AMERICA Jurassic Period In contrast to the passive margin that existed along eastern North America during the Mesozoic, active subduction along Mesozoic Era 208 M.Y.B.P. the western coast continued through the Middle Triassic. Geologists refer to the mountain ranges that formed in west- Triassic Period ern North America during this time as the Cordillera, which means “mountain range” in Spanish. Deformation along the western margin of North America 245 M.Y.B.P. increased substantially when Pangaea broke apart. Three major Figure 24-1 Each Mesozoic episodes of orogenies—mountain building—occurred along period is distinct. Pangaea the western margin of North America during the Mesozoic. Different broke apart and new sea- types of deformation occurred during each of these orogenies. ways formed during the The oldest orogeny was characterized by a tremendous number Triassic. Dinosaurs domi- of igneous intrusions. Large bodies of granite called batholiths exist nated the land during the Mesozoic. Flowering plants throughout the cordillera. The spectacular exposure of Half-Dome evolved during the Creta- at Yosemite National Park was intruded during this orogeny. ceous. You’ll learn more The next orogeny was characterized by low-angle thrust faulting about these events as you and folding. This was caused by collisional tectonism along the west- study this chapter. ern margin of North America. This type of deformation began in the Late Jurassic and continued through the Late Cretaceous. The thrust faults run north-south and place older rocks on top of younger rocks in Utah, Idaho, Wyoming, western Canada, and Montana, shown in Figure 24-2. Figure 24-2 These Paleo- zoic limestones in the Sawtooth Mountains, MT were faulted and thrust eastward during the second Mesozoic orogeny. 626 CHAPTER 24 The third Mesozoic orogenic event was characterized by vertical uplifts. This orogeny mainly affected the area east of the folds and faults caused by the second orogeny. Deformation caused by the third orogeny began during the Late Cretaceous and continued into the Cenozoic. SEAWAYS AND SAND DUNES Throughout the Early and Middle Triassic, the superconti- nent Pangaea and a single global ocean defined Earth’s pale- ogeography. As Pangaea began to split apart, numerous rift basins formed in eastern North America, and large blocks of crust collapsed to form deep valleys. The Triassic ended with a rapid drop in sea level that caused sedimentation in the western United States to change dramatically during the Late Triassic and Early Jurassic. Western North America became much more arid, and it was covered by a thick blanket of sand. Strong winds shaped the sand into dunes. Evidence of this ancient desert is preserved in large-scale, cross-bedded sandstone deposits some of which are shown in Figure 24-3. Sea level rose again in the Jurassic, and a shallow sea cov- Figure 24-3 The effects of ered central North America. The Appalachian Mountains still rose the wind is preserved in high in the east, and the newly formed mountains of the Cordillera these sand dunes in the Vermillion Cliffs Wilderness, rose high in the west. As the mountains continued to uplift in the AZ. west, large river systems transported sediments from the mountains into the sea. The deposits of the Late Jurassic river systems are pre- served today as multicolored sandstones, siltstones, and mudstones. They are well known for large numbers of dinosaur fossils. The ocean continued to rise onto North America during the Cretaceous Period, and the Gulf of Mexico flooded the entire southeastern margin of North America. As a result, a sea covered the interior of North America from Texas to Alaska. 1. Explain why Pangaea rifted apart. SKILL REVIEW 2. Describe the difference between the 4. Recognizing Cause and Effect Explain three major episodes of mountain build- how heat that is generated in Earth’s ing that occurred during the Mesozoic. interior can cause continents to rift apart, 3. Thinking Critically What might happen if as in the case of Pangaea. For more help, a continent stops rifting apart? refer to the Skill Handbook. earthgeu.com/self_check_quiz 24.1 Mesozoic Paleogeography 627 24.224.2 Mesozoic Life OBJECTIVES The Mesozoic is commonly referred to as the Age of Reptiles. • Discuss why many However, the Mesozoic also was a time of some other very important paleontologists theorize biological firsts, such as the first mammals, the first birds, and the that birds are descended first flowering plants. The mass extinction at the end of the Paleozoic from dinosaurs. Era had left Earth’s biosphere quite barren, and therefore ripe for the appearance of new organisms. • Describe how paleontol- As Pangaea began to split apart during the Triassic, much of the ogists distinguish among habitat on the continental shelves that was lost during the formation reptile, dinosaur, and of Pangaea became available again. New marine organisms, which mammal fossils. are collectively called the modern fauna, evolved which filled this • Explain the evidence habitat. The modern fauna includes crabs, lobsters, shrimps, indicating that a mete- sponges, sea urchins, modern corals, snails, and clams. The major orite impact caused the marine vertebrate groups include bony fishes, sharks, aquatic rep- Cretaceous-Paleogene tiles, and aquatic mammals. mass extinction event. LIFE IN THE OCEANS VOCABULARY Much attention is given to the large animals of the Mesozoic. It is modern fauna important, however, to remember that then, as now, the base of the angiosperm food chain that supported all the large animals consisted of tiny, dinosaur ocean-dwelling organisms called phytoplankton, shown in Figure Ornithischia 24-4. Phytoplankton float near the surface of oceans and lakes and Saurischia make their own food through the process of photosynthesis. You ectotherm examined the relationship between shape and floating ability in the endotherm Discovery Lab on page 625. iridium Reef Builders Arise Again As you learned in Chapter 23, the Permo- Triassic Extinction Event wiped out the reef-building corals of the Paleozoic. By the end of the Environmental Triassic, the modern corals had Connection evolved to fill this same niche. In addition to corals, a new group of clams called rudists developed the ability to build Figure 24-4 These reefs during the Cretaceous. phytoplankton are Rudists are important because diatoms; tiny freshwater the reefs that they built were very or marine plants. Upon porous and today contain some of death, their silica shells contribute to the ocean the largest Cretaceous oil deposits in .floor sediment.
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