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17-1 The Fossil Record
Photo credit: Jackie Beckett/American Museum of Natural History
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2. Fossils and Ancient Life
What is the fossil record?
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3. Fossils and Ancient Life
The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.
Fossils and Ancient Life
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4. Fossils and Ancient Life
The fossil record provides incomplete information about the history of life.
Over 99% of all species that have lived on Earth have become extinct, which means that the species has died out.
Those species that still exist are referred to as being extinct.
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How Fossils Form
How Fossils Form
Most fossils form in sedimentary rock.
Sedimentary rock forms when exposure to the elements breaks down existing rock into small particles of sand, silt, and clay.
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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.
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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.
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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.
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How Fossils Form
Fossils can form in several ways.
Permineralization occurs when minerals carried by water are deposited around a hard structure.
The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.
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How Fossils Form
A natural cast forms when flowing water removes all of the original tissue, leaving an impression.
The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.
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How Fossils Form
The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.
Trace fossils record the activity of an organism including nests, burrows and footprints.
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How Fossils Form
Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.
Preserved remains form when an entire organism becomes encased in material such as ice, ash or immersed in a bog.
The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.
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13. Interpreting Fossil Evidence
Paleontologists determine the age of fossils using relative dating or radioactive dating.
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14. Interpreting Fossil Evidence
Relative Dating
In relative dating, the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock.
Rock layers form in order by age—the oldest on the bottom, with more recent layers on top.
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15. 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
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16. Interpreting Fossil Evidence
Radioactive Dating 
Scientists use radioactive decay of unstable isotopes to assign an absolute age to rocks.
Radioactive dating is a technique that uses decay rate of unstable isotopes found in materials in order to calculate the age of the material.
neutrons
protrons
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17. Interpreting Fossil Evidence
Radioactive Dating 
A half-life is the length of time required for half of the radioactive atoms in a sample to decay.
Isotopes are atoms of an element that differ in their number of neutrons.
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18. Interpreting Fossil Evidence
Index fossils are used to compare the relative ages of fossils.
An index fossil is a species that is recognizable and that existed for a short period but had a wide geographic range.
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Index fossils include fusulinids and trilobites, ammonites.
Index fossils are not rare finds or fossils only found in a certain region.
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The basic divisions of the geologic time scale are eras and periods.
Geologic Time Scale
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Extinction Rate
Millions of years ago
600
500
400
300
200
100
Permian mass
extinction 80 60 40 20
Extinction rate
Cretaceous
mass extinction
2,500
2,000
1,500
1,000
Neogene
Proterozoic eon
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Paleogene
Paleozoic
Mesozoic
Ceno-
zoic
Number of families ( )
Extinction rate (
Number of
taxonomic
families
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Geologic Time Scale
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
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Geologic Time Scale
The geologic time scale organizes Earth’s history.
Eras last tens to hundreds of millions of years and consist of two or more periods.
Geologists divide the time between Precambrian time and the present into three eras:
Paleozoic Era
Mesozoic Era
Cenozoic Era
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Geologic Time Scale
Periods are the most commonly used units of time on time scale and last tens of millions of years.
associated with rock systems.
Epochs are the smallest units of time and last several million years.
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26. Cenozoic Era Mesozoic Era Paleozoic Era Evolution of primates
Description of Eras
Cenozoic Era
Evolution of primates
Diversification of mammals
Flowering plants
Mesozoic Era
Evolution and extinction of dinosaurs
Evolution of ferns and cycads
Evolution of mammals
Paleozoic Era
All existing animal phyla developed
Earliest plants arose

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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
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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
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Geologic Time Scale
The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.
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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
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17-1
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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.
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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.
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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.
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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.
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17-1
According to the geologic time scale, geologic time begins with
Precambrian Time.
the Paleozoic Era.
the Quaternary Period.
the Cambrian Era.
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