Lesson Overview
The Fossil Record
Lesson Overview
19.1 The Fossil Record
Lesson Overview
The Fossil Record
Lesson Overview
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?
Lesson Overview
The Fossil Record
Fossils and Ancient Life
What do fossils reveal about ancient life?
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
Types of Fossils
Many fossils are just fragments of an organism—teeth, pieces of a
jawbone, or bits of leaf.
Lesson Overview
The Fossil Record
Types of Fossils
Sometimes an organism leaves behind trace fossils—casts of footprints,
burrows, tracks, or even droppings.
Lesson Overview
The Fossil Record
Types of Fossils
Although most fossils are preserved in sedimentary rocks,
some are preserved in other ways, like in amber.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
The preserved remains may later be discovered and studied.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
Dating Earth’s History
How do we date events in Earth’s history?
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
If the same index fossil is found in two widely separated rock layers, the
rock layers are probably similar in age.
Lesson Overview
The Fossil Record
Relative Dating
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.
Lesson Overview
The Fossil Record
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 compares the amount of radioactive to nonreactive
isotopes in a sample to determine its age.
Lesson Overview
The Fossil Record
Radiometric Dating
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.
Lesson Overview
The Fossil Record
Radiometric Dating
Different radioactive elements have different half-lives, so they decay at
different rates.
Lesson Overview
The Fossil Record
Radiometric Dating
The half-life of potassium-40 is
1.26 billion years.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
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.
Lesson Overview
The Fossil Record
Radiometric Dating
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).
Lesson Overview
The Fossil Record
Geologic Time Scale
How was the geologic time scale established, and what are its major
divisions?
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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 24-hour clock. Notice
the relative length of Precambrian Time—almost 22 hours.
Lesson Overview
The Fossil Record
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?
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
Physical Forces
Climate is one of the most important aspects of Earth’s physical
environment.
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.
Lesson Overview
The Fossil Record
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.