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?
  

• 
  The formation of a fossil is a rare event.
  
• 
  Usually on the hard parts of an organism form fossils.
  

• 
  Most fossils form when organisms that die become buried in sediments.
  

• 
  Layers of sediments from rivers and streams may cover dead organisms.
  
• 
  Over millions of years, the layers may harden to become sedimentary rock.
  

• 
  Remains buried in sediment change to rock.
  

Gradually, the minerals replace the remains.

• 
  Shells or other hard parts buried in sediment gradually dissolve.
  
• 
  An empty space remains in the hard part once occupied.
  
• 
  Mold
  

• 
  Cast
  

• 
  Organisms can be preserved in substances other than sediments.
  

• 
  By determining the age of a fossil, scientists can . . .
  

• 
  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.
  

bottom layers – oldest

• 
  Used to determine the actual age of fossils.
  
• 
  Fossil rock contains radioactive elements.
  

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

• 
  Scientists use this information to calculate the age of the rock, and thus the age of the fossil.
  

• 
  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.
  

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

• 
  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.
  

• 
  The fossil record is incomplete.
  

• 
  There are gaps in the fossil record.
  

• 
  Major climate changes?
  

Dust clouds blocking the sun preventing plant growth?

Volcanic eruptions?

At what rate does evolution occur?

• 
  Gradualism?
  
• 
  Punctuated equilibrium?