1. Preview Section 1 Geologic History Section 2 Looking at Fossils
The Fossil Record
Preview
Section 1 Geologic History
Section 2 Looking at Fossils
Section 3 Time Marches On
Concept Mapping

2. Warm Up! “The Present is the Key to the Past.”
Section 1 Geologic History
Warm Up!
“The Present is the Key to the Past.”
This phrase was the cornerstone of the uniformitarianist theory developed by geologist James Hutton in the late 1700s.
Write a few sentences about how studying the present could reveal the story of Earth’s history, processes that may have occurred millions of years ago that you can still see today.

3. Section 1 Geologic History
Learning Targets
Compare uniformitarianism, catastrophism, gradualism (know the difference).
Describe how the science of geology has changed over the past 200 years.
Contrast relative dating with absolute dating.

4. The Principle of Uniformitarianism, continued
Section 1 Geologic History
The Principle of Uniformitarianism, continued
In Theory of the Earth (1788), James Hutton introduced the idea of uniformitarianism.
Uniformitarianism assumes that geologic processes shaping the Earth today have been at work all throughout Earth’s history.

5. Uniformitarianism Versus Catastrophism
Section 1 Geologic History
Uniformitarianism Versus Catastrophism
But, during Hutton’s time, most scientists supported catastrophism, the principle that all geologic change occurs suddenly, as the result of some catastrophe, such as a global flood.
Thought that Earth’s mountains, canyons, and seas formed during rare, sudden events called catastrophes.

6. Hutton conjectured a very different reality: an older Earth.
Section 1 Geologic History
Late 1700s: most people, scientists included, believed the Earth was only a few thousand years old, perhaps 6K – 10K years.
Hutton conjectured a very different reality: an older Earth.
According to his ideas, Earth had to be much older, because gradual geologic processes he suggested would take much longer than a few thousand years.

7. Lyell helped the “new” idea of Uniformitarianism became popular.
Section 1 Geologic History
Uniformitarianism, requiring an old Earth, became geology’s guiding principle after Charles Lyell reintroduced the concept in his Principles of Geology ( ).
Lyell helped the “new” idea of Uniformitarianism became popular.

8. Section 1 Geologic History
Catastrophism remained the guiding principle of geology in the early 19th century.
Events such as meteor impacts, massive volcanic eruptions, tsunamis and/or earthquakes are considered catastrophies

9. Modern Geology—A Happy Medium
Section 1 Geologic History
Modern Geology—A Happy Medium
Late 20th century - Stephen J. Gould challenged the principle of uniformitarianism.
Believed that catastrophes sometimes play an important role in shaping Earth’s history.
Gould and such believe things haven’t always happened at the same rate.
Neither theory completely accounts for all geologic change.

10. Modern Geology—A Happy Medium, continued
Section 1 Geologic History
Modern Geology—A Happy Medium, continued
Gradualism – gradual change brought about by ice ages, warming periods, tectonic movements.
Most geologic change is gradual and uniform.
But catastrophes that cause geologic change have occurred during Earth’s long history.

11. Modern Geology—A Happy Medium, continued
Section 1 Geologic History
Modern Geology—A Happy Medium, continued
Some scientists think an asteroid strike 65 million years ago caused the dinosaurs to become extinct.

12. Modern Geology—A Happy Medium, continued
Section 1 Geologic History
Modern Geology—A Happy Medium, continued
Some scientists think the Earth passed through the freezing tail of a massive comet, flash-freezing wooly mammoths and possibly causing the dinosaurs to become extinct.
These are called “Extinction Events.”

13. Two methods : Relative Dating, Absolute Dating
Section 1 Geologic History
How do scientists determine the age of objects (like fossils) in sedimentary rocks?
Two methods : Relative Dating, Absolute Dating

14. Section 1 Geologic History
1. Relative dating
Fossils in different rock layers have different ages. Oldest at the bottom.
The age of the rock around the fossil tells how old the fossil is.
Using rocks to date the fossils.

15. And, Using Fossils to Date Rocks
Section 2 Looking at Fossils
And, Using Fossils to Date Rocks
Certain types of fossils appear only in certain layers of rock.
By dating rock layers above and below these fossils, scientists can determine the time span in which the organism lived.
If the organism lived for a relatively short period of time, its fossils would show up in limited layers.

16. Relative Dating, continued
Section 1 Geologic History
Relative Dating, continued
The bottom layers of rock: usually the oldest
The top layers: usually the youngest.
For example, fossils in the bottom layers are usually older than fossils in the top layers.
The rock tells the age of the fossil.

17. Relative Dating, continued
Section 1 Geologic History
Relative Dating, continued
Sometimes, though, you will read about the fossil being used to determine the age of the rock, and vice versa
This is called “circular reasoning,” and is invalid as an explanation.

18. Section 1 Geologic History
The Geologic Column : combination of data from all of the known rock sequences around the world.
An idealized sequence, a picture of rock layers that contains all of the known fossils and rock formations on Earth.
These layers are arranged from oldest to youngest.

19. Section 1 Geologic History

20. Some periods may be missing in places.
Section 1 Geologic History
Some periods may be missing in places.
But geological periods almost always appear in the proper sequence in the fossil record.
So, it is argued that the geological column is real and represents the order in which various life forms existed in the past.

21. However, this argument is circular.
Section 1 Geologic History
However, this argument is circular.
The geological periods are defined using index fossils, and the index fossils are chosen so that the resulting geological periods occur in order.

22. 2. Absolute Dating Use of Carbon-14 and Uranium-238 most common.
Section 1 Geologic History
2. Absolute Dating
Use of Carbon-14 and Uranium-238 most common.
Thought to be more precise than relative dating.
For fossils, rocks, and other things
Activity of atoms used to measure the age of fossils or rocks in years.
Atoms are the particles that make up all matter.

23. Absolute Dating, continued
Section 1 Geologic History
Absolute Dating, continued
Some atoms are unstable, and will decay over time.
When an atom decays, it becomes a different and more stable kind of atom.
Each kind of unstable atom decays at its own rate.

24. Absolute Dating, continued
Section 1 Geologic History
Absolute Dating, continued
Half-life: The time it takes for half of the unstable atoms in a sample to decay
Scientists compare amounts/ratio of unstable to stable atoms.
Since they know the half-life, they can determine the approximate age of the sample.

25. Section 1 Geologic History

26. Absolute Dating, continued
Section 1 Geologic History
Absolute Dating, continued
Uranium-238 used to date rocks or fossils thought to be millions of years old.
Longer half-life than carbon-14
Carbon-14 has a half-life of only 5,780 years.
Used to date fossils and other objects that are less than 50,000 years old, such as human artifacts.

27. Paleontology—The Study of Past Life
Section 1 Geologic History
Paleontology—The Study of Past Life
Paleontology is the science involved with the study of past life.
Paleontologists.:Scientists who study past life
Paleontologists study fossils, the remains of organisms preserved by geological processes.

28. Paleontology—The Study of Past Life,
Section 1 Geologic History
Paleontology—The Study of Past Life,
Vertebrate and invertebrate paleontologists study the remains of animals.
Paleobotanists study fossils of plants.
Other paleontologists reconstruct past ecosystems, study the traces that animals left behind, and piece together the conditions under which fossils formed.

29. Section 2 Looking at Fossils
Learning targets
DESCRIBE several ways in which different types of fossils form.
KNOW three types of fossils that aren’t formed from parts of organisms.
EXPLAIN how fossils can be used to determine the history of changes in environments and organisms.
UNDERSTAND how index fossils can be used to date rock layers. This is where the fossils are usedto date the rocks.

30. Section 2 Looking at Fossils
Fossilized Organisms
Fossil (definition): The remains or physical evidence of an organism preserved by geologic processes
Fossils in rocks : form when an organism dies and is quickly covered by sediment.
When the sediment becomes rock, hard parts of the organism are preserved.

31. Fossilized Organisms, continued
Section 2 Looking at Fossils
Fossilized Organisms, continued
If an insect is caught in sticky tree sap, the sap covers its entire body and hardens quickly.
Fossils in amber are entire organisms preserved inside hardened tree sap, called amber.
Some of the best insect fossils, as well as frogs and lizards, have been found in amber.

32. Fossilized Organisms, continued
Section 2 Looking at Fossils
Fossilized Organisms, continued
Organisms can also be preserved by petrifaction.
Petrifaction: when minerals replace the organism’s tissues.
Two Types of petrifaction for our consideration: permineralization and replacement.

33. Fossilized Organisms, continued
Section 2 Looking at Fossils
Fossilized Organisms, continued
Permineralization: pores in organism’s hard tissue (like bone or wood) is filled up with minerals.
Replacement, minerals completely replace the tissues of the organism.
Some samples of petrified wood are composed completely of minerals.

34. Fossilized Organisms, continued
Section 2 Looking at Fossils
Fossilized Organisms, continued
In some places, asphalt wells up and forms thick, sticky pools at Earth’s surface.
These asphalt pools can trap and preserve many organisms.
The La Brea asphalt deposits in Los Angeles, California have preserved organisms for at least 38,000 years.

35. Fossilized Organisms, continued
Section 2 Looking at Fossils
Fossilized Organisms, continued
Frozen Fossils In 1999, scientists removed a 20,000-year-old woolly mammoth that was “flash-frozen” in the Siberian tundra.
These mammoths became extinct about 10,000 years ago.
Because cold temperatures slow down decay, the mammoth was almost perfectly preserved.

36. Section 2 Looking at Fossils
Other Types of Fossils
Trace fossils are any naturally preserved evidence of animal activity, not parts of their bodies
Tracks: an example of a trace fossil. They form when animal footprints fill with sediment.
Tracks can reveal size and speed of an animal, and whether it traveled in groups.

37. Other Types of Fossils, continued
Section 2 Looking at Fossils
Other Types of Fossils, continued
Burrows: another trace fossil.
Burrows are shelters made by animals that bury themselves in sediment, such as clams.
Coprolite: Another type of trace fossil. Preserved animal dung.

38. Other Types of Fossils, continued
Section 2 Looking at Fossils
Other Types of Fossils, continued
Molds and casts are two more examples of fossils.
Mold: A cavity in rock where a plant or animal was buried and decomposed
Cast: An object that is created when sediment fills a mold and becomes rock.

39. Using Fossils to Interpret the Past
Section 2 Looking at Fossils
Using Fossils to Interpret the Past
The fossil record gives only a rough sketch of the history of life on Earth.
Most organisms never become fossils.
Many fossils have yet to be discovered.

40. Section 2 Looking at Fossils
Organisms with hard body parts have left more fossils than those with soft body parts.
Soft body parts decay more rapidly
Organisms that lived in areas that favored fossilization (much sedimentary rock) have also left more fossils.

41. But fossils can show a history of environmental change.
Section 2 Looking at Fossils
But fossils can show a history of environmental change.
Example: finding marine fossils on mountaintops in Canada can mean that these mountains formed at the bottom of the ocean.
Or that these areas stayed in place and were covered by water.
Marine fossils can also help scientists reconstruct ancient coastlines and detect the presence of ancient seas.

42. Section 2 Looking at Fossils
Scientists can use fossils of plants and land animals to reconstruct past climates.
By examining fossils, scientists can tell whether the climate of an area was cooler or wetter than that climate is now.

43. History of Changing Organisms
Section 2 Looking at Fossils
History of Changing Organisms
Scientists study the relationships between fossils to interpret how life has changed over time.
The fossil record is incomplete, so paleontologists look for similarities between fossils over time to try to track change.

44. Using Fossils to Date Rocks
Section 2 Looking at Fossils
Using Fossils to Date Rocks
Particular types of fossils appear only in certain layers of rock.
By dating rock layers above and below these fossils, scientists can determine the time span in which the organism lived.
If the organism lived for a relatively short period of time, its fossils would show up in limited layers.

45. To be index fossils, these fossils must be found worldwide.
Section 2 Looking at Fossils
Index fossils: fossils of organisms that lived for a relatively short, well-defined geologic time span.
To be index fossils, these fossils must be found worldwide.

46. Using Fossils to Date Rocks, continued
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
Ammonites of the genus Tropites are index fossils.

47. Using Fossils to Date Rocks, continued
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
These ammonites were marine mollusks similar to modern squids.
Tropites lived between 230 million and 208 million years ago.
Fossils of these ammonites are index fossils for that time period.

48. Using Fossils to Date Rocks, continued
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
Trilobites of the genus Phacops are also index fossils.
Trilobites are extinct. Their closest living relative is the horseshoe crab.

49. Using Fossils to Date Rocks, continued
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
Phacops lived about 400 million years ago.
So, when scientists find fossils of trilobites anywhere on Earth, they assume the rock layers they are in are also approximately 400 million years old.

50. Section 3 Time Marches On
Learning Targets
Explain how geologic time is recorded in layers of sedimentary rock.
Explain how the geologic time scale illustrates the occurrence of processes on Earth.
Explain how the fossil record provides evidence of changes that have taken place in organisms over time.

51. Section 3 Time Marches On
Geologic Time
Earth is estimated by many scientists to be about 4.6 billion years old.
Paleontologists find a record of Earth’s history in rock formations and fossils around the world.
Dinosaur National Monument in Utah: remains of thousands of dinosaurs that lived there about 150 million years ago.

52. Geologic Time, continued
Section 3 Time Marches On
Geologic Time, continued
The Colorado River has cut the Grand Canyon nearly 2 km deep in some places. (1 km = .6 mi.)
Over the course of 6 million years, the river has eroded countless layers of rock.
These layers represent almost half, or nearly 2 billion years, of Earth’s history.
Did the Colorado River do this while flowing uphill? Several theories have been proposed.

53. Geologic Time, continued
Section 3 Time Marches On
Geologic Time, continued
The Fossil Record and Geologic Time Sedimentary rocks in the Green River formation can be found in parts of Wyoming, Utah, and Colorado.
These rocks are thousands of meters thick, and were once part of a system of ancient lakes that existed for millions of years.
Fossils of plants and animals are common in these rocks, and very well preserved.

54. Section 3 Time Marches On

55. The Geologic Time Scale
Section 3 Time Marches On
The Geologic Time Scale
The geologic column represents the billions of years that have passed since the first rocks formed on Earth.
Geologists study a total of 4.6 billion years of Earth’s history!
To make their job easier, geologists have created the geologic time scale, a scale that divides Earth’s history into distinct intervals of time.

56. Section 3 Time Marches On

57. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
The boundaries between geologic time intervals represent shorter intervals in which visible changes took place on Earth.
Some changes are marked by the disappearance of index fossil species.
Other changes can be recognized only by detailed paleontological studies.

58. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
The Appearance and Disappearance of Species At certain times during Earth’s history, the number of species has increased or decreased dramatically.
A sudden increase in species is often a result of a relatively sudden increase or decrease in competition between species.

59. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
The number of species can dramatically decline over a relatively short period of time as a result of a mass extinction event.
Extinction is the death of every member of a species.
Gradual events such as global climate change and changes in ocean currents can cause mass extinctions.

60. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
The Paleozoic era came to an end with the largest mass extinction in Earth’s history.
Some scientists believe that ocean changes were a likely cause of this extinction.
The event killed nearly 90% of all species.

61. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
At the end of the Mesozoic era, about 15% to 20% of all species on Earth became extinct.
This mass extinction event is thought to have wiped out the dinosaurs.
Global climate change may have caused this extinction, but the exact cause is not known.

62. The Geologic Time Scale, continued
Section 3 Time Marches On
The Geologic Time Scale, continued
After the mass extinction at the end of the Mesozoic era, mammals flourished.
Unique traits may have helped these mammals survive.
These traits include the ability to regulate internal body temperature and to develop young inside the mother.