1. Outine 17-1: The Fossil Record
Photo credit: Jackie Beckett/American Museum of Natural History
Copyright Pearson Prentice Hall

2. Fossils and Ancient Life
I. Fossils and Ancient Life
A. Paleontologists are scientists who collect and study fossils.
B. Fossil record.
1. Includes information about:
a. structure of organisms
b. what they ate
c. what ate them
d. what environment they lived in
e. sequence of organisms from past
f. how organisms change over time
Copyright Pearson Prentice Hall

3. Fossils and Ancient Life
C. Fossil record provides incomplete information.
1. Over 99% of all species that have lived on Earth have become extinct.
Copyright Pearson Prentice Hall

4. Copyright Pearson Prentice Hall
How Fossils Form
II. How Fossils Form
A. Most fossils form in sedimentary rock.
1. Erosion causes existing rock to break into small particles of sand, silt, and clay.
2. These particles fall to bottom of lakes, rivers, or the ocean.
3. Then sediments pile up in layers over time
Copyright Pearson Prentice Hall

5. Copyright Pearson Prentice Hall
How Fossils Form
Fossil Formation
Water carries small rock particles to lakes and seas.
The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.
Copyright Pearson Prentice Hall

6. Copyright Pearson Prentice Hall
How Fossils Form
Dead organisms are buried by layers of sediment, which forms new rock.
The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.
Copyright Pearson Prentice Hall

7. Copyright Pearson Prentice Hall
How Fossils Form
The preserved remains may be later discovered and studied.
The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.
Copyright Pearson Prentice Hall

8. Interpreting Fossil Evidence
III. Dating Fossil Evidence
A. Paleontologists determine age of fossils using relative dating or radioactive dating.
B. Relative Dating
1. Look at placement of fossils within the layers of rock
a. Oldest layers at bottom
b. More recent layers on top
2. Can estimate a fossil’s age compared to other fossils & rock layers
Copyright Pearson Prentice Hall

9. Interpreting Fossil Evidence
Relative Dating
In relative dating, a paleontologist estimates a fossil’s age in comparison with that of other fossils. Each of these fossils is an index fossil. It enables scientists to date the rock layer in which it is found. Scientists can also use index fossils to date rocks from different locations.
Photo credit: l. ©David Hanson/Stone; r. ©CORBIS
Copyright Pearson Prentice Hall

10. Interpreting Fossil Evidence
C. Radioactive Dating 
1. Used to assign an absolute age to rocks.
2. Some elements are radioactive and steadily break down into nonradioactive elements. They are said to decay.
3. Uses the half-lives of radioactive elements to determine the age of a rock or fossil
a. Half-life is the length of time required for half of the radioactive atoms in a sample to decay.
Copyright Pearson Prentice Hall

11. Interpreting Fossil Evidence
Radioactive dating involves measuring the amounts of radioactive isotopes in a sample to determine its actual age. Such measurements enable scientists to determine the absolute age of rocks and the fossils they contain.
Copyright Pearson Prentice Hall

12. Interpreting Fossil Evidence
4. Scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains compared to non- radioactive elements.
a. They have tables of ratios that they can compare their data to get a more absolute age for the fossil.
●Compare amount of C-14 to
C-12
Copyright Pearson Prentice Hall

13. Interpreting Fossil Evidence
5. Examples of radioisotopes: a. Carbon-14
●Changes into N-14 as it decays
●Half-life is 5,730 years
●Useful for dating fossils less than 50,000 years old
b. Potassium-40
●Changes into Ar-40 & Ca-40
●Half-life is 1.3 billion years
c. Uranium-238
●Half-life is 4.5 billion years
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Geologic Time Scale
IV. Geologic Time Scale
A. First developed by studying rock layers and fossils worldwide. Later assigned dates with radioactive dating.
B. Geologic time is divided into Eras.
1. Oldest to youngest: Precambrian, Paleozoic, Mesozoic, Cenozoic
C. Eras are divided into Periods.
1. Mesozoic Era divided into:
Triassic, Jurassic, Cretaceous
Copyright Pearson Prentice Hall

15. Copyright Pearson Prentice Hall
Geologic Time Scale
D. Geologic time begins with Precambrian Time, which covers about 88% of Earth’s history.
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
Vendian
650–544
DON’T COPY THE TABLES!
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Geologic Time Scale
E. The Paleozoic began about 544 million years ago.
1. Many vertebrates and invertebrates lived during this time.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Geologic Time Scale
Permian
290–245
Carboniferous
360–290
Devonian
410–360
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
Silurian
440–410
Ordovician
505–440
Cambrian
544–505
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Geologic Time Scale
F. The Mesozoic began about 245 million years ago.
1. Dinosaurs lived during this time.
2. Mammals began to evolve during this era.
Copyright Pearson Prentice Hall

19. Copyright Pearson Prentice Hall
Geologic Time Scale
Cretaceous
145–65
Jurassic
208–145
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
Triassic
245–208
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
Geologic Time Scale
G. The Cenozoic began about 65 million years ago and continues to the present.
1. Mammals became common during the Cenozoic.
Copyright Pearson Prentice Hall

21. Copyright Pearson Prentice Hall
Geologic Time Scale
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
Copyright Pearson Prentice Hall

22. Copyright Pearson Prentice Hall
Geologic Time Scale
Clock Model of Earth’s History
First humans
Radiation of mammals
First land plants
First prokaryotes
First multicellular organisms
Cenozoic Era Mesozoic Era Paleozoic Era Precambrian Time
Earth’s history is often compared to a familiar measurement, such as the twelve hours between noon and midnight. In such a comparison, notice that Precambrian Time lasts from noon until after 10:30 pm.
First eukaryotes
Accumulation of
atmospheric
oxygen
Copyright Pearson Prentice Hall

23. Copyright Pearson Prentice Hall
17-1
Copyright Pearson Prentice Hall

24. Copyright Pearson Prentice Hall
17-1
Which of the following statements about fossils is NOT true?
Most fossils form in sedimentary rock.
Fossils occur in a particular order.
Only a small portion of fossils are from extinct organisms.
Fossils can be used in relative dating of rock formations.
Copyright Pearson Prentice Hall

25. Copyright Pearson Prentice Hall
17-1
The fossil record consistently shows evidence that
all forms of life have existed in all geologic eras.
living organisms have only been on Earth for a short time.
living things have changed over time.
ancient life-forms are much the same as forms found living today.
Copyright Pearson Prentice Hall

26. Copyright Pearson Prentice Hall
17-1
Index fossils assist paleontologists in dating rocks because they represent species that
were widely distributed and existed for a very long time.
existed in a single location for a short period of time.
were widely distributed and existed for a short time.
existed in a single location for a very long time.
Copyright Pearson Prentice Hall

27. Copyright Pearson Prentice Hall
17-1
Determining the age of a fossil by comparing its placement with fossils in other layers of rock is called
carbon-14 dating.
fossil-indexing.
relative dating.
absolute dating.
Copyright Pearson Prentice Hall

28. Copyright Pearson Prentice Hall
17-1
According to the geologic time scale, geologic time begins with
Precambrian Time.
the Paleozoic Era.
the Quaternary Period.
the Cambrian Era.
Copyright Pearson Prentice Hall