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Lesson Overview
19.1 The Fossil Record
THINK ABOUT IT
Fossils, the preserved remains or traces of
ancient life, are priceless treasures. They tell
of life-and-death struggles and of mysterious
worlds lost in the mists of time.
Taken together, the fossils of ancient
organisms make up the history of life on Earth
called the fossil record.
How can fossils help us understand life’s
history?
Fossils and Ancient Life
What do fossils reveal about ancient life?
From the fossil record, paleontologists learn
about the structure of ancient organisms,
their environment, and the ways in which
they lived.
Fossils and Ancient Life
Fossils are the most important source of
information about extinct species, ones that
have died out.
Fossils vary enormously in size, type, and
degree of preservation. They form only under
certain conditions.
For every organism preserved as a fossil, many
died without leaving a trace, so the fossil record
is not complete.
Types of Fossils
Fossils can be as large and perfectly
preserved as an entire animal, complete
with skin, hair, scales, or feathers.
They can also be as tiny as bacteria,
developing embryos, or pollen grains.
Types of Fossils
Many fossils are just fragments of an
organism—teeth, pieces of a jawbone, or bits
of leaf.
Types of Fossils
Sometimes an organism leaves behind trace
fossils—casts of footprints, burrows, tracks, or
even droppings.
Types of Fossils
Although most fossils are preserved in
sedimentary rocks, some are preserved
in other ways, like in amber.
Fossils in Sedimentary Rock
Most fossils are preserved in sedimentary rock.
Sedimentary rock usually forms when small particles
of sand, silt, clay, or lime muds settle to the bottom of
a body of water.
As sediments build up, they bury dead organisms that
have sunk to the bottom.
Fossils in Sedimentary Rock
As layers of sediment continue to build up over time,
the remains are buried deeper and deeper.
Over many years, water pressure gradually
compresses the lower layers and turns the sediments
into rock.
Fossils in Sedimentary Rock
The preserved remains may later be discovered and
studied.
Fossils in Sedimentary Rock
Usually, soft body structures decay quickly
after death, so usually only hard parts like
wood, shells, bones, or teeth remain. These
hard structures can be preserved if they are
saturated or replaced with mineral
compounds.
Fossils in Sedimentary Rock
Sometimes, however, organisms are
buried so quickly that soft tissues are
protected from aerobic decay. When this
happens, fossils may preserve imprints of
soft-bodied animals and structures like
skin or feathers.
This fish fossil was formed in sedimentary
rock.
What Fossils Can Reveal
The fossil record contains an enormous amount of
information for paleontologists, researchers who
study fossils to learn about ancient life.
By comparing body structures in fossils to body
structures in living organisms, researchers can infer
evolutionary relationships and form hypotheses about
how body structures and species have evolved.
Bone structure and trace fossils, like footprints,
indicate how animals moved.
What Fossils Can Reveal
Fossilized plant leaves and pollen suggest whether
the area was a swamp, a lake, a forest, or a desert.
When different kinds of fossils are found together,
researchers can sometimes reconstruct entire ancient
ecosystems.
Dating Earth’s History
How do we date events in Earth’s history?
Relative dating allows paleontologists to
determine whether a fossil is older or
younger than other fossils.
Radiometric dating uses the proportion of
radioactive to nonreactive isotopes to
calculate the age of a sample.
Relative Dating
Lower layers of sedimentary rock, and fossils they
contain, are generally older than upper layers.
Relative dating places rock layers and their fossils
into a temporal sequence.
Relative Dating
To help establish the relative ages of rock layers and
their fossils, scientists use index fossils. Index fossils
are distinctive fossils used to establish and compare
the relative ages of rock layers and the fossils they
contain.
Relative Dating
If the same index fossil is found in two widely
separated rock layers, the rock layers are probably
similar in age.
A good index fossil species must be easily
recognized and will occur in only a few rock layers
(meaning the organism lived only for a short time).
These layers, however, will be found in many places
(meaning the organism was widely distributed).
Trilobites, a large group of distinctive marine
organisms, are often useful as index fossils.
Radiometric Dating
Relative dating is important, but provides no
information about a fossil’s absolute age in years.
One way to date rocks and fossils is radiometric
dating.
Radiometric dating relies on radioactive isotopes,
which decay, or break down, into nonradioactive
isotopes at a steady rate.
Radiometric Dating
Radiometric dating compares the amount of
radioactive to nonreactive isotopes in a sample to
determine its age.
A half-life is the time required for half of the
radioactive atoms in a sample to decay.
After one half-life, half of the original radioactive
atoms have decayed.
After another half-life, another half of the remaining
radioactive atoms will have decayed.
Radiometric Dating
Different radioactive elements have different halflives, so they decay at different rates.
Radiometric Dating
The half-life of
potassium-40 is 1.26
billion years.
Radiometric Dating
Carbon-14, which has a short half-life, can be used to
directly date very young fossils.
Elements with long half-lives can be used to indirectly
date older fossils by dating nearby rock layers, or the
rock layers in which they are found.
Radiometric Dating
Carbon-14 is a radioactive form of carbon naturally
found in the atmosphere. It is taken up by living
organisms along with “regular” carbon, so it can be
used to date material that was once alive, such as
bones or wood.
After an organism dies, carbon-14 in its body begins
to decay to nitrogen-14, which escapes into the air.
Researchers compare the amount of carbon-14 in a
fossil to the amount of carbon-14 in the atmosphere,
which is generally constant. This comparison reveals
how long ago the organism lived.
Radiometric Dating
Carbon-14 has a half-life of only about 5730 years, so
it’s only useful for dating fossils no older than about
60,000 years.
For fossils older than 60,00 years, researchers
estimate the age of rock layers close to fossil-bearing
layers and infer that the fossils are roughly same age
as the dated rock layers.
A number of elements with long half-lives are used for
dating very old fossils, but the most common are
potassium-40 (half-life: 1.26 billion years) and
uranium-238 (half-life: 4.5 billion years).
Geologic Time Scale
How was the geologic time scale established, and what
are its major divisions?
The geologic time scale is based on both relative
and absolute dating. The major divisions of the
geologic time scale are eons, eras, and periods.
Geologic Time Scale
Geologists and
paleontologists have built
a time line of Earth’s
history called the
geologic time scale.
The basic divisions of the
geologic time scale are
eons, eras, and periods.
Establishing the Time Scale
By studying rock layers and index fossils, early
paleontologists placed Earth’s rocks and fossils in
order according to their relative age.
They noticed major changes in the fossil record at
boundaries between certain rock layers.
Establishing the Time Scale
Geologists used these
boundaries to
determine where one
division of geologic time
ended and the next
began.
Years later, radiometric
dating techniques were
used to assign specific
ages to the various rock
layers.
Divisions of the Geologic Time Scale
The time scale is based on
events that did not follow a
regular pattern.
The Cambrian Period, for
example, began 542 million
years ago and continued
until 488 million years ago,
which makes it 54 million
years long.
The Cretaceous Period was
80 million years long.
Divisions of the Geologic Time Scale
Geologists now recognize
four eons of unequal
length.
The Hadean Eon, during
which the first rocks
formed, began about 4.6
billion years ago.
The Archean Eon, when life
first appeared, began about
4 billion years ago.
Divisions of the Geologic Time Scale
The Proterozoic Eon began
2.5 billion years ago and
lasted until 542 million
years ago.
The Phanerozoic Eon
began at the end of the
Proterozoic and continues
to the present.
Divisions of the Geologic Time Scale
Eons are divided into eras.
The Phanerozoic Eon, for
example, is divided into
the Paleozoic, Mesozoic,
and Cenozoic Eras.
Eras are subdivided into
periods, which range in
length from nearly 100
millions of years to just
under 2 million years. The
Paleozoic Era, for
example, is divided into six
periods.
Naming the Divisions
Geologists started to
name divisions of the
time scale before any
rocks older than the
Cambrian Period had
been identified. For this
reason, all of geologic
time before the
Cambrian is simply
called Precambrian
Time.
Naming the Divisions
The Precambrian actually covers about 90 percent of
Earth’s history.
In this figure, the history of Earth is depicted as a 24hour clock. Notice the relative length of Precambrian
Time—almost 22 hours.
Life on a Changing Planet
How have our planet’s environment and living things
affected each other to shape the history of life on Earth?
Building mountains, opening coastlines, changing
climates, and geological forces have altered habitats
of living organisms repeatedly throughout Earth’s
history. In turn, the actions of living organisms over
time have changed conditions in the land, water, and
atmosphere of planet Earth.
Life on a Changing Planet
Earth and its climate has been constantly changing, and
organisms have evolved in ways that responded to
those new conditions.
The fossil record shows evolutionary histories for major
groups of organisms as they have both responded to
changes on Earth and how they have changed Earth.
Climate is one of the most important aspects of Earth’s
physical environment.
Physical Forces
Earth’s climate has undergone dramatic changes over
time. Many of these changes were triggered by fairly
small shifts in global temperature.
During the global “heat wave” of the Mesozoic Era,
Earth’s average temperatures were only 6°C to
12°C higher than they were during the twentieth
century.
During the ice ages, world temperatures were only
about 5°C cooler than they are now.
These relatively small temperature shifts changed the
shape of life on Earth.
Physical Forces
Geological forces have transformed life on Earth,
producing new mountain ranges and moving
continents.
Volcanic forces have altered landscapes and even
formed entire islands.
Local climates are shaped by the interaction of wind
and ocean currents with geological features such as
mountains and islands.
Physical Forces
The theory of plate tectonics explains how solid
continental “plates” move slowly above Earth’s molten
core—a process called continental drift.
Over the long term, continents have collided to form
“supercontinents.” Later, these supercontinents have
split apart and reformed.
Physical Forces
Where landmasses collide, mountain ranges often
rise.
When continents change position, major ocean
currents change course.
All of these changes affect both local and global
climate.
Geological Cycles and Events
Continental drift has affected the
distribution of fossils and living
organisms worldwide. As continents
drifted apart, they carried organisms with
them.
For example, the continents of South
America and Africa are now widely
separated. But fossils of Mesosaurus, a
semiaquatic reptile, have been found in
both South America and Africa.
The presence of these fossils on both
continents, along with other evidence,
indicates that South America and Africa
were joined at one time.
Physical Forces
Evidence indicates that over millions of years, giant
asteroids have crashed into Earth.
Many scientists agree that these kinds of collisions
would toss up so much dust that it would blanket
Earth, possibly blocking out enough sunlight to cause
global cooling. This could have contributed to, or even
caused, worldwide extinctions.
Biological Forces
The activities of organisms have affected global environments.
For example, Earth’s early oceans contained large amounts of
soluble iron and little oxygen.
During the Proterozoic Eon, however, photosynthetic organisms
produced oxygen gas and also removed large amounts of carbon
dioxide from the atmosphere.
The removal of carbon dioxide reduced the greenhouse effect and
cooled the globe. The iron content of the oceans fell as iron ions
reacted with oxygen to form solid deposits.
Organisms today shape the landscape by building soil from rock,
and sand and cycle nutrients through the biosphere.
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