Today’s Objective
(learning goal)
To identify how fossils are used as
evidence in changes within a species.
Clues to the Past
• About 95 percent of the species that have existed are
EXTINCT—they no longer live on Earth.
• The oldest rocks that have been found on Earth
formed about 3.8 billion years ago.
• Scientists study fossils to learn about ancient species.
• Fossils are evidence of organisms that lived long ago
that are preserved in Earth’s rocks.
TYPES OF FOSSILS
Fossils Types
Formation
A trace fossil is any indirect evidence
Trace fossils
Casts
left by an animal and may include a
footprint, a trail, or a burrow.
When minerals in rocks fill a space
left by a decayed organism, they make
a replica, or cast, of the organism.
A mold forms when an organism is
Molds
Petrified
fossils
Amber-Preserved
or
frozen fossils
buried in sediment and then decays,
leaving an empty space.
Petrified-minerals sometimes penetrate
and replace the hard parts of an organism
At times, an entire organism was
quickly trapped in ice or tree sap that
hardened into amber.
• Paleontologists, scientists who study ancient life, are
like detectives who use fossils to understand events
that happened long ago.
• They use fossils to determine the kinds of organisms
that lived during the past and sometimes to learn
about their behavior.
• For fossils to form, organisms
usually have to be buried in mud,
sand, or clay soon after they die.
• Fossils are not usually found in other types of rock because of the ways
those rocks
form.
• Most
fossils
are found in sedimentary rocks.
rocksthe
form
at relatively
low temperatures
and
pressures
• These
For example,
conditions
under which
metamorphic rocks
form
often
that
may
damage
to the
organism.
destroy
anyprevent
fossils that
were in the
original
sedimentary rock.
• Few organisms become fossilized because, without
burial, bacteria and fungi immediately decompose
their dead bodies.
• Occasionally, however, organisms do become fossils
in a process that usually takes many years.
How fossils form
• A Protoceratops drinking at
a river falls into the water and
drowns
• Sediments from upstream rapidly cover
the body, slowing its decomposition.
Minerals from the sediments seep into the
body.
• Over time, additional layers of
sediment compress the sediments
around the body, forming rock.
Minerals eventually replace all the
body’s bone material.
• Earth movements or
erosion may expose the
fossil millions of years after it
formed.
• Scientists use a variety of methods to determine the
age of fossils.
• Relative dating
• This method basically indicates that the fossils found
closer to the surface are younger, and the fossils
found in deeper strata are more primitive (older).
(in undisturbed ground)
• This method does not
give a specific age of
the fossil or rock.
• Using this principle, scientists can determine relative
age and the order of appearance of the species that
are preserved as fossils in the layers.
• To find the specific ages of rocks, scientists use
absolute dating, also called Radiometric Dating.
• In this type of dating scientists use radioactive atoms
to determine the absolute age of objects.
As specific atoms decay they emit radiation and lose
electrons, which causes them to turn into a different
element.
So- these atoms change from one thing into something else
over time- and they do this at a constant rate.
• If you know what the unstable atom is (Carbon 14)
and you know what it turns into as it decays
(Nitrogen 14).
• And you know the rate at which the unstable atom
decays/turns into something else
It takes 5,730 years for ½ of Carbon 14 to
turn into Nitrogen 14
• You can measure the amount of the unstable atom
in the rock….and compare that to the amount of the
new atom
• This will tell you the age of the sample rock.
• The half-life of a radioactive atom is the time it takes
for half of that atom in a sample to decay and turn into
another element.
Because Carbon 14 has such a short half-life, it can only
be used to date things that died recently in geologic
history (under 70,000 years ago).
• You can only date organic remains using carbon 14
If you need to date rocky
material from farther back in
our geologic past, you must
use another method, such as
Potassium/Argon, which has a
much longer half-life.
Most radioactive isotopes have rapid rates of
decay (that is, short half-lives) and lose their
radioactivity within a few days or years.
Some isotopes, however, decay slowly, and several of these
are used as geologic clocks. The parent isotopes and
corresponding daughter products most commonly used to
determine the ages of ancient rocks are listed below:
Parent Isotope
Stable Daughter
Product
Currently Accepted
Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147
Neodymium-143
106 billion years