Chapter 5
The Fossil Record
Preview
Section 1 Geologic History
Section 2 Looking at Fossils
Mr. Hall’s thoughts are in
Orange.
Chapter 5
Section 1 Geologic History
Objectives
• Compare uniformitarianism with catastrophism.
• Describe how the science of geology has changed
over the past 200 years.
• Contrast relative dating with absolute dating.
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• In Theory of the Earth (1788), James Hutton
introduced the idea of uniformitarianism.
• Uniformitarianism assumes that geologic
processes that are shaping the Earth today have
been at work throughout Earth’s history.
Key word is “assumes”.
Science shouldn’t assume.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Uniformitarianism Versus Catastrophism
During Hutton’s time, most scientists supported
catastrophism, the principle that all geologic
change occurs suddenly.
• Supporters of catastrophism thought that Earth’s
mountains, canyons, and seas formed during rare,
sudden events called catastrophes.
Empirical evidence supports this!
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Most people also believed that Earth was only a
few thousand years old.
• Hutton’s work suggested a very different reality.
Really another “belief”
• According to his theories, Earth had to be much
older, because gradual geologic processes would
take much longer than a few thousand years.
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• A Victory for Uniformitarianism Catastrophism
remained the guiding principle of geology in the
early 19th century.
• But uniformitarianism became geology’s guiding
principle after Charles Lyell reintroduced the
concept in his Principles of Geology (1830-1833).
Arguable in a BIG way!
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Using Hutton’s notes and evidence of his own, Lyell
successfully challenged the principle of
catastrophism.
• He saw no reason to doubt that major geologic
change happened at the same rate in the past as it
happens in the present—gradually.
HUGE assumption! =
Bad Science!
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium
• During the late 20th century, scientists such as
Stephen J. Gould challenged the principle of
uniformitarianism.
• They believed that catastrophes sometimes play an
important role in shaping Earth’s history.
• Neither theory completely accounts for all geologic
change.
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium,
continued
• Most geologic change is gradual and uniform. This
is not able to be empirically verified!
• But catastrophes that cause geologic change have
occurred during Earth’s long history.
• Asteroid and comet strikes to Earth, for example,
have caused rapid change. So do volcanoes,
hurricanes, tsunamis, etc…
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium,
continued
• Some scientists think an asteroid strike 65 million
years ago caused the dinosaurs to become extinct.
Chapter 5
Section 1 Geologic History
Relative Dating
• Scientists can use two methods to determine the
age of objects in sedimentary rocks.
• One of those methods is known as relative dating.
• Relative dating examines a fossil’s position within
rock layers to estimate its age.
This is a highly speculative
way to date!
Chapter 5
Section 1 Geologic History
Relative Dating, continued
• The bottom layers of rock are usually the oldest, and
the top layers are usually the youngest.
• Scientists can use the order of these rock layers to
determine the relative age of objects within the
layers.
• For example, fossils in the bottom layers are usually
older than fossils in the top layers.
Chapter 5
Section 1 Geologic History
Relative Dating, continued
• The Geologic Column To make relative dating
easier, geologists combine data from all of the
known rock sequences around the world.
• From this information, geologists create the
geologic column—an ideal sequence of rock layers
that contains all of the known fossils and rock
formations on Earth.
• These layers are arranged from oldest to youngest.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
Absolute Dating
• Scientists can use absolute dating to more
precisely determine the age of a fossil or rock.
• In absolute dating, scientists examine atoms to
measure the age of fossils or rocks in years.
• Atoms are the particles that make up all matter.
This method requires a
HUGE assumption!
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• Some atoms are unstable, and will decay over time.
• When an atom decays, it becomes a different and
more stable kind of atom.
• Each kind of unstable atom decays at its own rate.
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• The time it takes for half of the unstable atoms in a
sample to decay is known as the half-life of that
atom.
• Scientists can examine a sample of rock or fossil,
and look at the ratio of unstable to stable atoms.
• Since they know the half-life, they can determine the
approximate age of the sample.
Only if they know the size of the original sample. And
assuming there weren’t any daughter elements present.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• Uranium-238 has a half-life of 4.5 billion years. Scientists
can use uranium-238 to date rocks or fossils that are
millions of years old.
• Assuming none of the daughter elements were present at the
origin of the sample.
• Carbon-14 has a half-life of only 5,780 years.
• Only works on organic material – doesn’t work on rock
• Scientists use carbon-14 to date fossils and other objects
that are less than 50,000 years old, such as human
artifacts.
• There is a LOT of Carbon-14 on the Earth. Meaning……?
Chapter 5
Section 1 Geologic History
Paleontology—The Study of Past Life
• Paleontology is the science involved with the
study of past life.
• Scientists who study past life are called
paleontologists.
• Paleontologists collect data by studying fossils, the
remains of organisms preserved by geological
processes.
Chapter 5
Section 1 Geologic History
Paleontology—The Study of Past Life,
continued
• Vertebrate and invertebrate paleontologists study
the remains of animals.
• Paleobotanists study fossils of plants.
• Other paleontologists reconstruct past ecosystems,
study the traces that animals left behind, and piece
together the conditions under which fossils formed.
Chapter 5
Section 2 Looking at Fossils
Objectives
• Describe five ways in which different types of
fossils form.
• List three types of fossils that are not part of
organisms.
• Explain how fossils can be used to determine the
history of changes in environments and organisms.
• Explain how index fossils can be used to date rock
layers.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms
• The remains or physical evidence of an organism preserved
by geologic processes is called a fossil.
• Rock in the shape of what was once alive.
• Fossils in rocks can form when an organism dies and is
quickly covered by sediment.
• The key is “quickly covered by sediment” not slow deposition
over time.
• When the sediment becomes rock, hard parts of the
organism are preserved.
• Soft parts have also been preserved through fossilization.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• If an insect is caught in sticky tree sap, the sap covers
its entire body and hardens quickly.
• Fossils in amber are entire organisms preserved
inside hardened tree sap, called amber.
• Most species trapped in Amber are identical to current
species.
• Some of the best insect fossils, as well as frogs and
lizards, have been found in amber.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• Organisms can also be preserved by petrifaction.
• Petrifaction is a process in which minerals replace
the organism’s tissues.
• Also called mineral replacement – where a living
organism becomes an inorganic shape in rock.
• Permineralization and replacement are forms of
petrifaction.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• In the process of permineralization, pore space in
an organism’s hard tissue (like bone or wood) is
filled up with mineral.
• In the process of replacement, minerals
completely replace the tissues of the organism.
• Some samples of petrified wood are composed
completely of minerals.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• In some places, asphalt wells up and forms thick,
sticky pools at Earth’s surface.
• These asphalt pools can trap and preserve many
organisms.
• The La Brea asphalt deposits in Los Angeles,
California have preserved organisms for at least
38,000 years.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• Frozen Fossils In 1999, scientists removed a
20,000-year-old woolly mammoth that was frozen
in the Siberian tundra.
• These mammoths became extinct about 10,000
years ago.
• This is highly arguable!
• Because cold temperatures slow down decay, the
mammoth was almost perfectly preserved.
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils
• Trace fossils are any naturally preserved evidence of
animal activity.
• Tracks are an example of a trace fossil. They form
when animal footprints fill with sediment.
• Tracks can reveal size and speed of an animal, and
whether it traveled in groups.
Chapter 5
Huh!
?
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils, continued
• Burrows are another trace fossil.
• Burrows are shelters made by animals that bury
themselves in sediment, such as clams.
• Another type of trace fossil is coprolite, or
preserved animal dung.
• Dino-doo doo
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils, continued
• Molds and casts are two more examples of fossils.
• A cavity in rock where a plant or animal was buried
is called a mold.
• A cast is an object that is created when sediment
fills a mold and becomes rock.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past
• The Information in the Fossil Record The fossil
record gives only a rough sketch of the history of life
on Earth.
• Argumentative point on the “history of life on Earth”.
• Most organisms never become fossils.
• But there are more than we ever thought could be fossils.
• Many fossils have yet to be discovered.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• Organisms with hard body parts have left more
fossils than those with soft body parts.
• Organisms that lived in areas that favored
fossilization have also left more fossils.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• But fossils can show a history of environmental
change.
• For example, the presence of marine fossils on
mountaintops in Canada means that these
mountains formed at the bottom of the ocean.
• Marine fossils can also help scientists reconstruct
ancient coastlines and detect the presence of
ancient seas.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• Scientists can use fossils of plants and land
animals to reconstruct past climates.
• By examining fossils, scientists can tell whether
the climate of an area was cooler or wetter than
that climate is now.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• History of Changing Organisms Scientists study
the relationships between fossils to interpret how
life has changed over time.
• Since the fossil record is incomplete,
paleontologists look for similarities between fossils
over time to try to track change.
• This, of course, is requiring a HUGE assumption to be
made again. In organic material cannot be linked together
due to the lack of DNA to prove a relationship.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks
• Scientists have found that particular types of fossils
appear only in certain layers of rock.
• More assumptions to accept & be made.
• By dating rock layers above and below these
fossils, scientists can determine the time span in
which the organism lived.
• More assumptions to accept & be made.
• If the organism lived for a relatively short period of
time, its fossils would show up in limited layers.
• More assumptions to accept & be made.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Index fossils are fossils of organisms that lived for
a relatively short, well-defined geologic time span.
• Well defined? By what definition? Defined by
assumption?
• To be index fossils, these fossils must be found
worldwide.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Ammonites of the genus Tropites are index
fossils.
Hmm look very similar don’t they?
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• These ammonites were marine mollusks similar to
modern squids.
• Or nautilus
• Tropites lived between 230 million and 208 million
years ago.
• How can we be so sure?
• Fossils of these ammonites are index fossils for that
time period.
• How can we be so sure?
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Trilobites of the genus
Phacops are also index
fossils.
• Trilobites are extinct. Their
closest living relative is the
horseshoe crab.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Phacops lived about 400 million years ago.
• How can we be so sure?
• When scientists find fossils of trilobites anywhere on
Earth, they assume the rock layers are also
approximately 400 million years old.
• How can we be so sure?