The Fossil Record and Geologic Time Scale
KEY CONCEPT
Specific environmental conditions are necessary in
order for fossils to form.
The Fossil Record and Geologic Time Scale
Fossils can form in several ways.
• Permineralization occurs when minerals carried by water
are deposited around a hard structure.
The Fossil Record and Geologic Time Scale
• A natural cast forms when flowing water removes all of the
original tissue, leaving an impression.
The Fossil Record and Geologic Time Scale
• Trace fossils record the activity of an organism.
The Fossil Record and Geologic Time Scale
• Amber-preserved fossils are organisms that become
trapped in tree resin that hardens after the tree is buried.
The Fossil Record and Geologic Time Scale
• Preserved remains form when an entire organism becomes
encased in material such as ice.
The Fossil Record and Geologic Time Scale
• Specific conditions are needed for fossilization.
• Only a tiny percentage of living things became fossils.
The Fossil Record and Geologic Time Scale
Radiometric dating provides an accurate way to estimate
the age of fossils.
• Relative dating estimates the time during which an
organism lived.
– It compares the placement
of fossils in layers of rock.
– Scientists infer the order in
which species existed.
The Fossil Record and Geologic Time Scale
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
neutrons
protrons
The Fossil Record and Geologic Time Scale
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
– A half-life is the amount of time it takes for half of the
isotope to decay.
The Fossil Record and Geologic Time Scale
KEY CONCEPT
The geologic time scale divides Earth’s history based
on major past events.
The Fossil Record and Geologic Time Scale
Index fossils are another tool to determine the age of rock
layers.
• Index fossils can provide the relative age of a rock layer.
– existed only during specific spans of time
– occurred in large geographic areas
• Index fossils include fusulinids and trilobites.
The Fossil Record and Geologic Time Scale
The geologic time scale organizes Earth’s history.
• The history of Earth is
represented in the geologic time
scale.
100
250
550
1000
2000
PRECAMBRIAN TIME
This time span makes up the
vast majority of Earth’s history.
It includes the oldest known
rocks and fossils, the origin of
eukaryotes, and the oldest
animal fossils.
Cyanobacteria
The Fossil Record and Geologic Time Scale
• Eras last tens to hundreds of millions of years.
– consist of two or more periods
– three eras: Cenozoic, Mesozoic, Paleozoic
The Fossil Record and Geologic Time Scale
• Periods last tens of millions of years.
– most commonly used units of time on time scale
– associated with rock systems.
• Epochs last several
million years.