Post on 30-Mar-2015
Chapter 17:The History of Life
Calif. Science standards 8e
17-1 The Fossil Record
Fossils and Ancient Life
Paleontologists are scientists who collect and study fossils.
All information about past life is called the fossil record.
The fossil record includes information about the structure of organisms, what they ate, what ate them, in what environment they lived, and the order in which they lived.
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Fossils are useful because . . .
We can compare ancient organisms with today’s to see if organic evolution has occurred.
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.
How Fossils Form
Fossils can be as large as a complete, preserved animal, or as small as a fragment.
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
How Fossils Form
1. Petrified wood1. Petrified wood17-1
2. Imprint
Impressions made in mud which then hardens into rock.
Examples:Leaves, footprints
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Dinosaur footprints
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Interpreting Fossil Evidence
Paleontologists determine the age of fossils using relative dating or radioactive dating.
What information do relative dating and radioactive dating provide about fossils?
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Relative Dating
NO, not dating a relative.
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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Relative Dating
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Index Fossils
• Are used to compare the relative ages of fossils.–e.g. Trilobites, marine animals
which were distant relatives of insects.
–Only lived between 500 and 600 million years ago.
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Absolute Dating
=A method to determine A method to determine how how
long agolong ago an event occurred and an event occurred and to assign an absolute age to to assign an absolute age to
rocks.rocks.
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Radioactive dating
• Some elements are radioactive and Some elements are radioactive and steadily break down into steadily break down into nonradioactive elements. nonradioactive elements.
• They decay at a fixed rate over They decay at a fixed rate over timetime
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17-1E=mcE=mc22 tells us that a very small mass can be tells us that a very small mass can be converted into lots of energy!converted into lots of energy!
Carbon 14 Carbon 14 Nitrogen 14
Radioactive dating: the use of half-lives to determine the age of a sample.
• Half-life: time required for 1/2 the radioactive atoms in a sample to decay.
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Carbon 14 Nitrogen 14
Half life = 5730 years
Example:Example:
CC1414 5730 yrs CC1414 + NN1414
Can date objects up to 50,000 yrs oldCan date objects up to 50,000 yrs old
Uranium 238 Lead
Half life = 4.5 billion yearsHalf life = 4.5 billion years
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What if What if something is something is
older than this?older than this?
Interpreting the Fossil Record
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Through dating of rocks, scientists have been able to
construct a timetable of earth’s history.
Geologic Time Scale
• Major divisions = Eras–(e.g. “Age of Reptiles”)
• Eras divided into: Periods/Epochs–(e.g. “Jurassic Period”)
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• There is good evidence that earth is 4.5 to 5 billion years old.
Way older than your teacher.
ErasGeologists 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
•The Paleozoic began about 544 million years ago.
•Many vertebrates and invertebrates lived during this time.
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Geologic Time Scale
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
290–245
360–290
410–360
440–410
505–440
544–505
Geologic Time Scale
•The Mesozoic began about 245 million years ago.
•Dinosaurs lived during this time.
•Mammals began to evolve during this era.
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Cretaceous
Jurassic
Triassic
145–65
208–145
245–208
Geologic Time Scale
•The Cenozoic began about 65 million years ago and continues to the present.
•Mammals became common during the Cenozoic.
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Geologic Time Scale
Periods
Eras are subdivided into periods, which range in length from tens of millions of years to less than two million years.
•Many periods are named for places around the world where geologists first discovered the rocks and fossils of that period.
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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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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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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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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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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Patterns of evolution• Many fossils are from species no longer Many fossils are from species no longer
living (extinct).living (extinct). Of all species that ever lived, < 1% Of all species that ever lived, < 1%
exist today!exist today!
•Evidence of mass Evidence of mass extinctions:extinctions:
Floods, comets, Floods, comets, volcanic activityvolcanic activity
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