Geologic Time The Cenozoic Era Key Concepts What major geologic events occurred during the Cenozoic era? What does fossil evidence reveal about the Cenozoic era? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 7. Mammals evolved after dinosaurs became extinct. 8. Ice covered nearly one-third of Earth s land surface 10,000 years ago. Building Vocabulary As you read, underline any words you do not understand and look them up in a dictionary. Write the meanings in the margin close to the words. Reread the sentences containing the words to help you study the meanings. Make a shutter-fold book to record information about the changes that took place during the Cenozoic era. Geology Geology of the Cenozoic Era Have you ever experienced a severe storm, such as a hurricane, a blizzard, a thunderstorm, or a tornado? If so, what did your neighborhood look like after the storm? Piles of snow, rushing water, or broken trees might have made your neighborhood seem like a different place. In a similar way, the place where you live was much different in the Paleozoic and Mesozoic eras from how it is today. The landscapes and organisms have changed greatly since then. Although some unusual animals lived during the Cenozoic era, this era is more familiar to many people than any other era. Because the Cenozoic era is so recent, people know more about the Cenozoic than they know about any other era. Its fossils and its rock records are better preserved. Look at the time scale on the next page. The Cenozoic era is the most recent era of geologic time. The Cenozoic era spans the time from the end of the Cretaceous period, 65 mya, to the present. Biology 178 Geologic Time Reading Essentials
Cenozoic Era Eons Eras Eras Periods Cenozoic Quaternary Phanerozoic Cenozoic Tertiary Present 65.5 mya 1. Identify the two periods of the Cenozoic era. Precambrian Geologists divide the Cenozoic era into two periods the Tertiary (TUR shee ayr ee) period and the Quaternary (KWAH tur nayr ee) period. These two periods are further divided into seven epochs. The most recent epoch, the Holocene (HOH luh seen) epoch, began 10,000 years ago. You live in the Holocene epoch. Cenozoic Mountain Building Earth s continents continued to move apart during the Cenozoic era. The Atlantic Ocean continued to widen. As the continents moved, some landmasses collided. Early in the Tertiary period, India crashed into Asia. This collision began to push up the Himalayas the highest mountains on Earth today. At about the same time, Africa began to push into Europe. This collision formed the Alps another mountain range. Both the Himalayas and the Alps continue to get higher today. Recall that during the Mesozoic era, the western coast of North America pushed against the seafloor next to it and formed the Rocky Mountains. These plates are still colliding today, causing the Rockies to continue to rise. Plate collisions during the Cenozoic era also caused other mountain ranges the Cascades and the Sierra Nevadas to begin forming along the western coast of North America. On the eastern coast, there was little tectonic activity. The Appalachian Mountains, which formed during the Paleozoic era, continue to erode, or wear away. Math Skills The Cenozoic era began 65.5 mya. What percentage of the Cenozoic era is taken up by the Quaternary period, which began 2.6 mya? To calculate the percentage of a part to the whole, perform the following steps: a. Express the problem as a fraction. 2.6 mya 65.5 mya b. Convert the fraction to a decimal. 2.6 mya 65.5 mya = 0.040 c. Multiply by 100 and add %. 0.040 100 = 4.0% 2. Use Percentages What percent of the Cenozoic era is represented by the Tertiary period, which lasted from 65.5 mya to 2.6 mya? [Hint: Subtract to find the length of the Tertiary period.] Reading Check 3. Explain Why are the Appalachian Mountains relatively small today? Reading Essentials Geologic Time 179
4. Contrast How was the climate of the Pleistocene epoch different from the climate of the early Cenozoic era? 5. Summarize Why did sea level drop during the Pleistocene ice age? Pleistocene Ice Age Like the Mesozoic era, the early part of the Cenozoic era was warm. In the middle of the Tertiary period, the climate began to cool. By the Pliocene (PLY oh seen) epoch, ice covered Earth s poles as well as many mountaintops. Earth got even colder during the next epoch, the Pleistocene (PLY stoh seen). The Pleistocene epoch was the first epoch of the Quaternary period. During this time, glaciers advanced and retreated many times. They covered as much as 30 percent of Earth s land surface. An ice age is a time when a large proportion of Earth s surface is covered by glaciers. The map below shows how far the glaciers advanced in North America during the Pleistocene ice age. Sometimes, rocks carried by the glaciers created deep gouges or grooves in the land. Glacial grooves are grooves made by rocks carried in glaciers. Glacial grooves in Ohio are evidence that glaciers advanced far into North America during the Pleistocene ice age. The glaciers contained huge amounts of water. This water came from the oceans. Glaciers trapped so much water that sea level dropped. As sea level dropped, the inland seas that had covered the continents drained away, exposing dry land. When sea level was at its lowest, the Florida peninsula was about twice as wide as it is today. 6. Estimate Approximately what percentage of the United States was covered with ice? 180 Geologic Time Reading Essentials
Cenozoic Life The Age of Mammals The mass extinction event at the end of the Mesozoic era meant that there was more space for the surviving species. Flowering plants, including grasses, evolved and began to dominate the land. Flowering trees and grasses provided new food sources. The new food sources helped make it possible for many animals, including mammals, to evolve. Mammals were so successful during the Cenozoic era that this era is sometimes called the age of mammals. Mega-Mammals Recall that mammals were small during the Mesozoic era. Many new types of mammals appeared during the Cenozoic era. Some were very large, such as those shown below. The large mammals of the Cenozoic era are called mega-mammals. Some of the largest mega-mammals lived during the Oligocene and Miocene periods, from 34 mya to 5 mya. Others, such as woolly mammoths, giant sloths, and saber-toothed cats, lived during the cool climate of the Pliocene and Pleistocene periods. These periods lasted from 5 mya to 10,000 years ago. Many fossils of these animals have been discovered. A few preserved mammoth bodies also have been discovered in glacial ice. All of the Cenozoic mega-mammals shown below are extinct. Key Concept Check 7. Explain How do scientists know that megamammals lived during the Cenozoic era? 8. Compare the sizes of Cenozoic era animals in the figure to the human. Make a general statement about their sizes. Reading Essentials Geologic Time 181
ACADEMIC VOCABULARY hypothesize (verb) to make an assumption about something that is not positively known Reading Check 9. Contrast What major geologic events affected the evolution of marsupials in Australia? 10. Analyze Would the migration that took place 65.5 mya be possible today? Explain. Isolated Continents and Land Bridges The mega-mammal lived in the Americas, Europe, and Asia. Different mammal species evolved in Australia during this time. This is mostly because of the movement of Earth s tectonic plates. Recall that land bridges can connect continents that were once separated. Also recall that when continents are separated, members of a species that once lived together can become geographically isolated. Most mammals that live in Australia today are marsupials (mar SOO pee ulz). Marsupials are mammals, such as kangaroos, that carry their young in pouches. Some scientists suggest that marsupials did not evolve in Australia. Instead, scientists hypothesize that the ancestors of marsupials migrated to Australia from South America. This migration would have happened when South America and Australia were both connected to Antarctica by land bridges, as shown below. After the ancestors of marsupials arrived in Australia, Australia moved away from the other landmasses. Sea level rose and water covered the land bridges. Over time, the marsupials evolved into the types of marsupials that live in Australia today. Migration route North America South America North America Africa Eurasia 65.5 million years ago South America Antarctica ca Europe Africa Antarctica a Present day India Asia Australia Australia 182 Geologic Time Reading Essentials
Rise of Humans The oldest fossil remains of human ancestors have been discovered in Africa. The fossil evidence suggests that Africa is where humans first evolved. These fossils are nearly 6 million years old. Scientists discovered a skeleton of a 3.2-million-year-old human ancestor. They named the skeleton Lucy. Modern humans, called Homo sapiens, didn t evolve until the Pleistocene epoch. Early Homo sapiens migrated to Europe, Asia, and eventually North America. Early humans likely migrated to North America from Asia, using a land bridge that connected the continents during the Pleistocene ice age. Today, this land bridge is covered with water. Pleistocene Extinctions Earth s climate changed at the close of the Pleistocene epoch 10,000 years ago. The Holocene epoch was warmer and drier. Forests replaced grasses. The mega-mammals that lived during the Pleistocene became extinct. Some scientists suggest that mega-mammal species did not adapt to the changes in the environment. Future Changes Evidence suggests that present-day Earth is undergoing a global-warming climate change. Many scientists hypothesize that humans have contributed to this change because of their use of coal, oil, and other fossil fuels over the past few centuries. 11. Specify In what epoch did modern humans first appear on Earth? Key Concept Check 12. Describe How did climate change at the end of the Pleistocene epoch? Reading Essentials Geologic Time 183
Mini Glossary glacial groove: a groove made by rocks carried in glaciers Holocene (HOH luh seen) epoch: the most recent epoch, which began 10,000 years ago mega-mammal: a large mammal of the Cenozoic era Pleistocene (PLY stoh seen) epoch: the first epoch of the Quaternary period ice age: a time when a large proportion of Earth s surface is covered by glaciers 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains the order in which the Pleistocene and the Holocene epochs occurred. 2. Complete the diagram by listing different things that happened during the Cenozoic era. Cenozoic Era 3. The oldest well-preserved skeleton of a human ancestor was found in Africa. How old is the skeleton and what was it named? What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 184 Geologic Time Reading Essentials