1. The Geologic Column
geologic column an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom.
Evidence of changing conditions on Earth’s surface is recorded in the rock layers of Earth’s crust.
The geologic time scale outlines the development of Earth and of life on Earth.
No single area on Earth contained a record of all geologic time, so scientists combined observations to create a standard geologic column.

2. The Geologic Column, continued
Rock layers in a geologic column are distinguished by the types of rock the layers are made of and by the kinds of fossils the layers contain.
Fossils in the upper layers resemble modern plants and animals.
Many of the fossils discovered in old layers are from species that have been extinct for millions of years.

3. The Geologic Column, continued
Using a Geologic Column
Scientists use geologic columns to estimate the age of rock layers that cannot be dated radiometrically.
To determine the layer’s age, scientists compare a given rock layer with a similar layer in a geologic column that contains the same fossils or that has the same relative position.
If the two layers match, they likely formed at about the same time.

4. Divisions of Geologic Time
The geologic history of Earth is marked by major changes in Earth’s surface, climate, and types of organisms.
Geologists use these indicators to divide the geologic time scale into smaller units.
Rocks grouped within each unit contain similar fossils and each unit is generally characterized by fossils of a dominant life-form.

5. Divisions of Geologic Time, continued

6. Divisions of Geologic Time, continued
Eons and Eras
The largest unit of geologic unit of time is an eon.
Geologic time is divided into four eons: the Hadean eon, the Archean eon, the Proterozoic eon, and the Phanerozoic eon.
The first three eons are part of a time interval commonly known as Precambrian Time. This 4 billion year interval contains most of Earth’s history.

7. Divisions of Geologic Time, continued
Eons and Eras, continued
After Precambrian time the Phanerozoic eon began. This eon is divided into smaller units of geologic time called eras.
era a unit of geologic time that includes two or more periods.
The first era of the Phanerozoic eon was the Paleozoic Era, which lasted about 291 million years.

8. Divisions of Geologic Time, continued
Eons and Eras, continued
Paleozoic rocks contain fossils of a wide variety of marine and terrestrial life forms.
After the Paleozoic Era the Mesozoic Era began and lasted about 186 million years.
Mesozoic fossils include early forms of birds and reptiles.
The present era is the Cenozoic Era, which began 65 million years ago. Fossils of mammals are common in Cenozoic rocks.

9. Divisions of Geologic Time, continued
Periods and Epochs
period a unit of geologic time that is longer than an epoch but shorter than an era.
epoch a subdivision of geologic time that is longer than an age but shorter than a period.
Eras are divided into shorter time units called periods. Each period is characterized by specific fossils and is usually named for the location in which the fossils were first discovered.

10. Divisions of Geologic Time, continued
Periods and Epochs, continued
Where the rock record is most complete and least deformed, a detailed fossil record may allow scientists to divide period into shorter time units called epochs.
Epochs may be divided into smaller units of time called ages.
Ages are defined by the occurrence of distinct fossils in the fossil record.

11. Earth-History Clock
Click below to watch the Visual Concept.

12. Evolution
By examining rock layers and fossils, scientists have discovered evidence that species of livings things have changed over time. Scientists call this process evolution.
evolution the process of change by which new species develop from preexisting species over time
Scientists think that evolution occurs by means of natural selection. Evidence for evolution included the similarity in skeletal structures of animals.

13. Evolution, continued Evolution and Geologic Change
Major geologic and climatic changes can affect the ability of some organisms to survive.
By using geologic evidence, scientists try to determine how environmental changes affected organisms in the past.
Scientists use fossils to learn why some organisms survived long periods of time without changing, while other organisms changed or became extinct.

14. Evolution, continued

15. Evolution
Click below to watch the Visual Concept.

16. Precambrian Time
Precambrian time the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago.
The time interval that began with the formation of Earth and ended about 542 million years ago is known as Precambrian time, which makes up 88% of Earth’s history.
The Precambrian rock record is difficult to interpret. Most Precambrian rocks have been so severely deformed and altered by tectonic activity that the original order of rock layers is rarely identifiable.

17. Precambrian Time, continued

18. Precambrian Time, continued
Precambrian Rocks
Large areas of exposed Precambrian rocks, called shields, exist on every continent except Antarctica.
Nearly half of the valuable mineral deposits in the world occur in the rocks of Precambrian shields.
These valuable minerals include nickel, iron, gold, and copper.

19. Precambrian Time, continued
Precambrian Life
Fossils are rare in Precambrian rocks mostly because Precambrian life-forms lacked bones, or other hard parts that commonly form fossils.
Of the few Precambrian fossils that have been discovered, the most common are stromatolites, or layered, reef-like deposits.
The presence of stromatolite fossils in Precambrian rocks indicates that shallow seas covered much of Earth during that time.

20. The Paleozoic Era
Paleozoic Era the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago.
Paleozoic rocks hold an abundance of fossils. The number of plant and animal species on Earth increased dramatically at the beginning of the Paleozoic Era.
Because of this rich fossil record, the Paleozoic Era has been divided into seven periods.

21. The Paleozoic Era, continued

22. The Paleozoic Era, continued
The Cambrian Period
The Cambrian Period is the first period of the Paleozoic Era. Marine invertebrates thrived in the warm waters that existed during this time.
The most common of the Cambrian invertebrates were trilobites. Scientists use many trilobites as index fossils to date rocks to the Cambrian Period.
The second most common animals of the Cambrian Period were the brachiopods, a group of shelled animals. Fossils indicated that at least 15 different families of brachiopods existed during this period.
Other common Cambrian invertebrates include worms, jellyfish, snails, and sponges.

23. The Paleozoic Era, continued
The Ordovician Period
During this period, populations of trilobites began to shrink.
Brachiopods, bryozoans, and cephalopod mollusks became the dominant invertebrate life-form.
Colonies of graptolites also flourished in the oceans, and the first vertebrates appeared.
The most primitive vertebrates were fish, which did not have jaws or teeth and were covered with thick, bony plates.

24. The Paleozoic Era, continued
The Silurian Period
During the Silurian Period, echinoderms, relatives of modern sea stars, and corals became more common.
Scorpion-like sea creatures called eurypterids also existed during this period.
Near the end of this period, the earliest land plants as well as animals evolved on land.

25. The Paleozoic Era, continued
The Devonian Period
The Devonian Period is called the Age of Fishes because fossils of many bony fishes were discovered in rocks of this period.
One type of fish, called a lungfish, had the ability to breathe air.
Another type of fish, rhipidistians, were air-breathing fish that had strong fins that may have allowed them to crawl onto the land for short periods of time.
Ichthyostega, early amphibians that resembled huge salamanders, are thought to be ancestors of modern amphibians such as frogs and toads.
Land plants, such as giant horsetails, ferns, and cone-bearing plants also began to develop during this period.

26. The Paleozoic Era, continued
The Carboniferous Period
In North America, the Carboniferous Period is divided into the Mississippian and Pennsylvanian Periods.
During this time, the climate was warm, and forests and swamps covered most of the world.
Amphibians and fish continued to flourish, and the first vertebrates that were adapted to live on land appeared.

27. The Paleozoic Era, continued
The Permian Period
The Permian Period marks the end of the Paleozoic Era. A mass extinction of a several life-forms occurred at the end of this period.
During this time, the continents had joined to form the supercontinent Pangaea. Areas of desert and dry savanna climates developed, and the shallow inland seas disappeared.
As the seas retreated, several species of marine life became extinct.
Fossil records indicate that reptiles and amphibians survived the environmental changes.

28. The Mesozoic Era
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.
Earth’s surface changed dramatically during the Mesozoic Era. Pangaea broke into smaller continents, and the climate was warm and humid.
Lizards, turtles, crocodiles, snakes and a variety of dinosaurs flourished during this era.

29. The Mesozoic Era, continued

30. The Mesozoic Era, continued
The Mesozoic Era is known as the Age of Reptiles and is divided into three periods: the Triassic, the Jurassic, and the Cretaceous Periods.
The Triassic Period
Dinosaurs flourished during the Triassic period. Most dinosaurs were about 2 m to 5 m long and moved very quickly.
Reptiles called ichthyosaurs lived in the oceans. The ammonite, a marine invertebrate, was dominant, and serves as a Mesozoic index fossil.

31. The Mesozoic Era, continued
The Jurassic Period
Two major groups of dinosaurs evolved during the Jurassic Period
The saurischians, or “lizard-hipped” dinosaurs, included carnivores and herbivores, such as those of the genus Apatosaurus, once known as Brontosaurus.
The ornithischians, or “bird-hipped” dinosaurs, were herbivores. One of the best known ornisthischians were of the genus Stegosaurus.
Flying reptiles called pterosaurs were also common.

32. The Mesozoic Era, continued
The Cretaceous Period
Among the common Cretaceous dinosaurs was the carnivore Tyrannosaurus Rex. The ankylosaurs, the ceratopsians, and the hadrosaurs were also common.
The earliest flowering plants, or angiosperms, appeared during this period. The most common of these plants were magnolias and willows.
Later, trees such as maples, oaks, and walnuts became abundant.

33. The Mesozoic Era, continued
The Cretaceous-Tertiary Mass Extinction
The Cretaceous Period ended in another mass extinction. No dinosaur fossils have been found in rocks that formed after the Cretaceous Period.
Some scientists think this extinction was caused by environmental changes resulting from the movement of continents and increased volcanic activity.
Many scientists accept the impact hypothesis as the explanation for the extinction of the dinosaurs. In this hypothesis, a giant meteorite crashed into Earth about 65 million years ago. The impact of the collision raised enough dust to block the sun’s rays for many years. As the climate became cooler, plant life began to die, and many animal species to become extinct.

34. Mass Extinction
Click below to watch the Visual Concept.

35. The Cenozoic Era
Cenozoic Era the current geologic era, which began 65.5 million years ago; also called the Age of Mammals
During the Cenozoic Era, dramatic changes in climate have occurred. As temperatures decreased during the ice ages, new species that were adapted to life in cooler climates appeared.
Mammals became the dominant life-form and underwent many changes.
The Cenozoic Era is divided into two periods: the Tertiary Period and the Quaternary Period.

36. The Cenozoic Era, continued

37. The Cenozoic Era, continued
The Quaternary and Tertiary Periods
The Tertiary Period includes the time before the last ice age.
The Tertiary Period is divided into five epochs: the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs.
The Quaternary Period began with the last ice age and includes the present.
The Quaternary is divided into two epochs: The Pleistocene and Holocene Epochs.

38. The Cenozoic Era, continued
The Paleocene and Eocene Epochs
The fossil record indicates that during the Paleocene Epoch many new mammals, such as small rodents and primates, evolved.
Other mammals, including the earliest known ancestor of the horse, first whales, flying squirrels, and bats, evolved during this time.
Worldwide, temperatures dropped by about 4ºC at the end of the Eocene Epoch.

39. The Cenozoic Era, continued
The Oligocene and Miocene Epochs
During these epochs, the worldwide climate became significantly cooler and drier. The modern Antarctic icecap began to form. The Mediterranean Sea dried up and refilled several times.
This climate change caused many early mammals to become extinct. However large species of deer, pigs, horses, camels, cats, and dogs flourished. Also, the climate change favored grasses, cone-bearing, and hardwood trees. Earliest human ancestors appeared.

40. The Cenozoic Era, continued
The Pliocene Epoch
During the Pliocene Epoch, predators such as bears, dogs, and cats, evolved into modern forms. Herbivores, such as the giant ground sloth, flourished.
Dramatic climatic changes occurred, and the continental ice sheets began to spread. The Bering land bridge and the Central American land bridge formed, allowing various species to migrate between the continents.

41. The Cenozoic Era, continued
The Pleistocene Epoch
During the Pleistocene Epoch, ice sheets in Eurasia and North America advanced and retreated several times.
Some animals had certain features that allowed them to survive the cold climate, such as the thick fur that covered woolly mammoths.
Other species survived by moving to warmer regions, while some species eventually became extinct.
Fossils of the earliest modern humans were discovered in Pleistocene sediments. Evidence of modern human ancestors indicated that early humans may have been hunters.

42. The Cenozoic Era, continued
The Holocene Epoch
The Holocene Epoch began as the last glacial period ended. As the ice sheets melted, sea level rose about 140 m, and the coastlines took on their present shapes.
Modern humans developed agriculture and began to make and use tools made of bronze and iron.

43. Maps in Action
Fossil Evidence for Gondwanaland

44. Multiple Choice
Dinosaurs first became the dominant life-forms during which geologic period?
Quaternary Period
Jurassic Period
Triassic Period
Cretaceous Period

45. Multiple Choice
Dinosaurs first became the dominant life-forms during which geologic period?
Quaternary Period
Jurassic Period
Triassic Period
Cretaceous Period

46. Multiple Choice, continued
2. Pangaea broke into separate continents during
the Paleozoic Era.
the Mesozoic Era.
the Cenozoic Era.
Precambrian time.

47. Multiple Choice, continued
2. Pangaea broke into separate continents during
the Paleozoic Era.
the Mesozoic Era.
the Cenozoic Era.
Precambrian time.

48. Multiple Choice, continued
3. Why are fossils rarely found in Precambrian rock?
Most Precambrian organisms did not have hard body parts that commonly form fossils.
Precambrian rock is buried too deeply for geologists to study it.
Most Precambrian organisms were too small to leave fossil remains.
Precambrian rock is made of a material that prevented the formation of fossils.

49. Multiple Choice, continued
3. Why are fossils rarely found in Precambrian rock?
A. Most Precambrian organisms did not have hard body parts that commonly form fossils.
B. Precambrian rock is buried too deeply for geologists to study it.
C. Most Precambrian organisms were too small to leave fossil remains.
D. Precambrian rock is made of a material that prevented the formation of fossils.

50. Multiple Choice, continued
4. Which of the following statements describes a principle of natural selection?
The environment has more than enough resources to support all of the individuals that are born in a given ecosystem.
Only individuals well-suited to the environment are likely to survive and reproduce.
Individuals in a healthy population are identical and have the same traits.
Most species produce plentiful offspring that will all live until maturity and reproduce.

51. Multiple Choice, continued
4. Which of the following statements describes a principle of natural selection?
The environment has more than enough resources to support all of the individuals that are born in a given ecosystem.
Only individuals well-suited to the environment are likely to survive and reproduce.
Individuals in a healthy population are identical and have the same traits.
Most species produce plentiful offspring that will all live until maturity and reproduce.

52. Short Response What is the term for the largest unit of geologic time?
An era

53. Short Response, continued
What is the term for the gradual development of organism from other organisms by means of natural selection?
evolution

54. Short Response, continued
7. Why is the Cenozoic Era also known as the Age of the Mammals?
Mammals became the dominant life-forms and underwent many evolutionary changes during this era.