The fossil record



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THE FOSSIL RECORD
Key Concepts

  • How do most fossils form?

  • How can scientists determine a fossil’s age?

  • What is the Geologic Time Scale?

  • What are some unanswered questions about evolution?


Background Knowledge

  • How do we know so much about dinosaurs if none of them are alive now?


How Do Fossils Form?

  • The formation of a fossil is a rare event.

  • Usually on the hard parts of an organism form fossils.

bones

shells

Particles of soil and rock


PETRIFIED FOSSILS

  • Remains buried in sediment change to rock.

Dissolved minerals in water soak into the buried remains.

Gradually, the minerals replace the remains.


MOLDS & CASTS

  • Shells or other hard parts buried in sediment gradually dissolve.

  • An empty space remains in the hard part once occupied.

  • Mold

hollow space

  • Cast

mold filled with hardened minerals
PRESERVED REMAINS

  • Organisms can be preserved in substances other than sediments.

Resin

Ice
Determining a Fossil’s Age

understand how living things have changed through time.

determine the order in which past events occurred.

construct and reconstruct the history of life on Earth.

RELATIVE DATING

  • Used to determine which of two fossils is older.

  • Relative dating cannot tell a fossil’s actual age.

  • Layers of sedimentary rock pile up one on top of another over time.

top layers – youngest

bottom layers – oldest


RADIOACTIVE DATING

  • Used to determine the actual age of fossils.

  • Fossil rock contains radioactive elements.

Elements that decay and break down into different elements.

  • The half-life of the radioactive element is measured.

Time it takes for half of the atoms to decay.


What Do Fossils Reveal?

  • Fossils help scientists piece together information about Earth’s past.

  • Scientists have learned about the history of life on Earth.


EXTINCT ORGANISMS

No members of that species are still alive.

  • Most of what is known about extinct species is based on the fossil record.


GEOLOGIC TIME SCALE

  • A “calendar” of Earth’s history.

  • History of life on Earth dating back 4.6 billion years.

  • Time span divided into smaller units called eras and periods.


UNANSWERED QUESTIONS

  • The fossil record is incomplete.

Most organisms die without leaving fossils behind.

  • There are gaps in the fossil record.

What causes mass extinction?

  • Major climate changes?

An asteroid hitting Earth?

Dust clouds blocking the sun preventing plant growth?

Volcanic eruptions?

At what rate does evolution occur?



  • Gradualism?

  • Punctuated equilibrium?

Learning about the past . . .

Gives us a glimpse into the future!


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