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The Fossil Record
Ch. 6, section 3
HOW DO FOSSILS FORM
• MOST FOSSILS FORM WHEN
ORGANISMS THAT DIE BECOME
BURIED IN SEDIMENTS.
• USUALLY BONES OR SHELLS OF
ANIMALS FORM FOSSILS
Petrified Fossils
• Fossils that are buried in sediment and
turned to rock.
Molds and Casts
• Shells or other hard parts buried
gradually dissolve and leave a blank
space called a mold. (footprint)
• When minerals fill the space that was
left a casts is formed.
Preserved remains
• Animals that have been found in ice and
have not changed since the day they
died.
Relative Dating
• Scientists look at the layers of sediment
and rock to determine how old a fossil
is.
• If a fossil is found closer to the surface it
is usually younger.
• If it is found in an older layer (deeper) it
is usually older.
Radioactive Dating
• Scientists can compare the amount of a
radioactive element in a sample to the
amount of the element into which it
breaks down.
• Which can be used to determine the
actual age of the fossil.
Precambrian Time
•
•
•
•
4.6 billion - 544 million years ago.
Formation of Earth
Bacteria forms (1st living thing)
Animals first appeared 600 million years
ago.
Paleozoic Era
• 544 million - 245 million years ago
• Sponges, jawless fish, jawed fish,
sharks, and amphibians lived during this
time.
Mesozoic Era
• 245 - 66 million years ago.
• Dinosaurs, mammals, birds appeared.
• Mass extinction happened during this
time.
Cenozoic Era
• 66 millions years ago - present
• Large mammals (mammoths, sabertooth tigers, etc)
• Humans appeared
Unanswered Qestions
• How did animals become extinct?
• Climate change- asteroid or volcanoes.
We are exactly sure.