|THE FOSSIL RECORD
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?
How do we know so much about dinosaurs if none of them are alive now?
How Do Fossils Form?
Most fossils form when organisms that die become buried in sediments.
Particles of soil and rock
Layers of sediments from rivers and streams may cover dead organisms.
Over millions of years, the layers may harden to become sedimentary 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 filled with hardened minerals
Organisms can be preserved in substances other than sediments.
Determining a Fossil’s Age
By determining the age of a fossil, scientists can . . .
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.
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
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.
Scientists use this information to calculate the age of the rock, and thus the age of the fossil.
What Do Fossils Reveal?
Fossils help scientists piece together information about Earth’s past.
Scientists have learned about the history of life on Earth.
Almost all of the species preserved as fossils are now extinct.
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.
Most organisms die without leaving fossils behind.
There are gaps in the fossil record.
What causes mass extinction?
An asteroid hitting Earth?
Dust clouds blocking the sun preventing plant growth?
At what rate does evolution occur?
Learning about the past . . .
Gives us a glimpse into the future!
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