Unit 6: Evidence of Evolution

Lesson 6.1
Fossil Formation and Classification

Contents
Introduction 1

Learning Objectives 2

Warm Up 2

Learn about It! 3

Fossils 4
The Process of Fossil Formation 5
Permineralization 5
Carbonization 5
Replacement 5
Types of Fossils 6
Mold Fossils 6
Cast Fossils 6
Trace Fossils 7
True Form Fossils 7
Fossil Dating Methods 8
Relative Dating 8
Absolute Dating 10
Index Fossils 11
Fossils and the Study of Evolution 12

Key Points 14

Check Your Understanding 15

Challenge Yourself 16

Photo Credits 16

Bibliography 16
Unit 6: Evidence of Evolution

Lesson 6.1

Fossil Formation and Classification

Introduction
Why is it that scientists know so much about dinosaurs even though these animals went
extinct long ago? Apart from knowing about the dinosaurs themselves, scientists have even
deduced how these long-extinct creatures are related to many of the modern animals that
we have today. You may have already heard or read about fossils, which are the remains of
organisms in the Earth’s past. It is through these fossils that scientists know so much about
animals and plants that have long been extinct.

The features that are left behind in fossils are extremely important in the study of
evolution. These allow us to determine how the organisms left behind in fossils are related
to the organisms that we have today. This lesson will allow you to learn what the types of
fossils are, how they form, and how they are used in the study of evolutionary
relationships.

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Learning Objectives DepEd Competency

In this lesson, you should be able to do the

Explain fossil records as evidence of
following: evolution (STEM_BIO11/12-IIIc-g-12).
● Describe how fossils are formed.
● Identify the different types of fossils.
● Explain how fossils provide evidence of
evolution.

Warm Up
Cast Fossils and Mold
25 minutes
Fossils

Fossils come in different shapes and forms. Among the most notable are the cast fossils and
the mold fossils. This activity will allow you to differentiate the two.

Materials
● 2 cups of flour
● 1 cup of baking soda
● 1 ½ cups of water
● 1 baking sheet
● 1 bivalve shell (oysters, mussels, and scallops are all acceptable)
● 1 bowl
● Oven or a pan and stove

Procedure
1. Form groups of three for this activity.
2. Mix the baking soda, water, and flour together in one bowl. You may share this
mixture with your classmates.
3. Stir the mixture until a thick paste forms.

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4. Place two separate lumps of the mixture on a baking sheet. Shape these into
cookie-like shapes.
5. On one of the shapes, press the convex end of the shell. This will later on become
something similar to a mold fossil.
6. On the other shape, press the concave end of the shell. Scrape the inside of the shell
to remove an intact figure of the shell made of the mixture. This will, later on,
become something similar to a cast fossil.
7. Use the oven or the pan to heat your fossils until they completely solidify.
8. Fill up Table 6.1.1. with descriptions of the fossils that you formed.
9. Answer the guide questions below.

Data Table
Table 6.1.1. Descriptions of the fossils.

Mold Fossil Cast Fossil

Guide Questions
1. Based on what you know, what is a fossil?
2. How do the products of this activity relate to how fossils are formed?
3. How are cast and mold fossils different, based on what you have seen in this activity?
4. What type of information do you think scientists can gain from fossils?

Learn about It!

How do fossils provide evidence of evolution?

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Unit 6: Evidence of Evolution

Fossils
Earth formed 4.54 billion years ago, giving it a very long history that humans want to
account for. The oldest evidence of life on Earth, on the other hand, has been dated to have
existed 3.5 billion years ago. Scientists, however, believe that life began even before that
period of Earth’s history.

How do scientists know so much about the history of Earth? Much of this knowledge can be
attributed to the marks that natural phenomena leave behind on rocks, the oceans, and
other geologic structures. Our knowledge of past life forms can be attributed to fossils or
the preserved remains of organisms from earlier periods of Earth’s history.

Ancient Earth looked vastly different from Earth of today. The landscape, animals, and
plants present in ancient Earth are not the ones that we are accustomed to.

Fossils are not just indicators of life on Earth. Another important use that scientists have
gained out of them is that they provide very important pieces of evidence for evolution.
Fossils give a glimpse of how organisms that have long since become extinct have lived and
changed over very long periods of time.

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The Process of Fossil Formation

There are many different types of remains that organisms can leave behind as fossils and
each of these types form a certain way. In general, however, fossils may form through the
enclosure of the organism in a preserving substance, through carbonization,
permineralization, or through replacement.

In certain cases, an organism may be encased in a substance that leads it to become

preserved even after very long periods of time. This process usually results in the organism
leaving behind an intact corpse that we can observe even millions of years later. Examples
of preserving agents are tar, sap, or resin.

Permineralization
Some organisms have spaces in their bodies that are usually filled with gases or fluids in life
but become empty spaces in death. The process of permineralization involves
groundwater filling up these spaces. Groundwater leaves behind minerals in these spaces
which results in the formation of a fossil in the image of the organism.

Carbonization
Another process, known as carbonization, involves organisms leaving behind the residual
carbon while many other elements are shed off. This usually happens when the remains of
the organism are subjected to heat and pressure. This results in a highly detailed carbon
print of the organism on rocks. Common examples of fossils that form through
carbonization are the imprints of ancient plants on rocks.

Replacement
Organisms, especially ones that have hard body parts, have significant amounts of minerals
in their bodies. The fossilization process of replacement involves the minerals in these
organisms’ bodies being replaced by other minerals. For example, when minerals replace
the organic parts of a tree trunk to leave behind a fossil.

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A photo of a petrified wood Imprints of land snails on Petrified tree trunk due to
fossil which also involves rocks as an example of replacement.
permineralization. carbonized fossils.

Why are the minerals in fossils left behind but the

tissues of organisms are not?

Types of Fossils
There are many different types of fossils that form through different means. Some of the
major types of fossils that are useful for scientific studies are cast fossils, mold fossils,
trace fossils, and true form fossils. Summary of the similarities and differences of each
type of fossil is presented in Table 6.1.2.

Mold Fossils
Mold fossils form when substances like minerals or sediments press over the body of a
dead organism. The corpse decomposes over time and this leaves behind a space in the
exact shape of the organism. This imprint of the organism is the mold fossil.

Cast Fossils
Cast fossils are formed when sediments and minerals fill up the body cavities of an
organism. These may also form when minerals fill up the spaces of mold fossils. A cast fossil
formed this way is the inverted form of a mold fossil.

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Trace Fossils
Trace fossils are exactly as the name implies. These are traces left behind by organisms in
the past. In most cases, these fossils are not part of the body of the organism that left them
behind. Examples of trace fossils are fossilized animal tracks, eggs, and coprolites which are
fossilized animal feces.

True Form Fossils

True form fossils are among the most useful fossils in studying organisms since these are
entire organisms whose bodies have been preserved.

An example of a mold fossil Coprolites are an example A true form fossil of a wasp
of trace fossils. preserved in amber

Table 6.1.2. Summary of the different types of fossils.

Types Process of Formation Primary Substrate

Mold Varies, may be through carbonization, Silt, mud, etc.

permineralization, and more

Cast Varies, may be through carbonization, Silt, mud, etc.

permineralization, and more

True form Varies, may be through carbonization, Silt, mud, amber,

permineralization, trapping, petrification resin, etc.

Trace Varies, examples include petrification and Varies

pressing of sediments

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Did You Know?

The assemblages of fossils in an area can indicate the type of
ecosystem that was present in that area in Earth’s early history.

For example, the entire assemblages of fossils of ancient marine

animals can be found outside the ocean. This may indicate that the
area was once underwater.

Fossil Dating Methods

Estimating the age of a fossil allows us to determine when the organism lived in Earth’s
history. This allows scientists to reconstruct the history of places, ecosystems, or the
evolutionary history of organisms and how they changed over time. Among the most useful
fossil dating methods are relative dating and absolute dating.

Relative Dating
Relative dating methods involve
estimating the sequence of events
that are involved in Earth’s history.
This is commonly done through the
science of stratigraphy which
involves determining the age
sequence of rock layers and
geological formations since these are
very telling in terms of
environmental conditions in the past.

Stratigraphy is a very important way

of determining the age of fossils. This
is because a vast majority of fossils
are found embedded within rock layers. Therefore, determining the relative age of these
rock layers can also reveal much about the age of the fossils embedded within them.

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Stratigraphy follows a certain set of laws for determining the relative age of rock layers
within a sequence. Among the major laws in stratigraphy are the following:

● The law of superposition states that the lower rock layers in a sequence are older.

The green layer was first deposited, followed by the yellow layer and finally the purple layer.

● The law of cross-cutting states that intrusions are younger than the rock layers that
they intrude into.

The intrusion is younger than the layers it cuts.

● The law of original horizontality states that rock layers are deposited horizontally
because of gravity.

Sediments are deposited horizontally following the surface of deposition.

Rock deposits usually correlate with the age of Earth that they were deposited in. Therefore,
similar rock layers in different areas can indicate that these may possibly be of the same
age. Knowing the relative ages of these layers can allow scientists to cross-check the ages
between different areas and extrapolate data based on similarities between these layers.

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Absolute Dating
Absolute dating methods give a numerical estimate of the age of rock layers and fossils.
Possibly the most common method used in the study of fossils is radiometric dating which
uses the decay of radioactive isotopes in order to determine the age of a subject.

Fig. 6.1.1 The half-life of the radioactive element uranium.

Fig. 6.1.1. shows a method involving

the concept of a radioactive isotope’s
half-life which is the time it takes for
that isotope to be reduced to half of its
amount. Suppose that an isotope’s
half-life is 250 000 years. This means
that 250 000 years from now, the
amount of isotope that will be left is
half the original amount. 250 000 years
from that date (500 000 years from
now), only a fourth of the original
amount will be left, which is half the
amount between this date and ten
years past. 750 000 years from now,
there will be an eighth of the original
amount, and so on.

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Unit 6: Evidence of Evolution

One of the most commonly-used radioactive isotopes in dating fossils is carbon-14 which
has a half-life of 5 730 years. Carbon-14 (as shown by Fig. 6.1.2 )is taken in by organisms
and the uptake stops after the organism’s death. Measuring the amount of carbon-14 in a
rock or fossil sample allows scientists to determine how long ago in Earth’s history the
organism died.

Another radioactive isotope that is useful for these studies is potassium-40 which is used in
potassium-argon dating. Potassium-40 has a half-life of 1.3 billion years.

Index Fossils
Some fossils are more useful in giving information about the time when the organism was
alive compared to other fossils. The presence of these fossils, known as index fossils, in a
rock layer is an instant indicator of a rock layer’s age.

Fig. 6.1.3. The identification of index fossils in rock layers.

As such, there are certain criteria that fossils must fulfill before they are considered as good
candidates to become index fossils. Among the criteria (as shown in Fig. 6.1.3.) are the
following:
● Widespread geographic range: An index fossil needs to be spread out
geographically. This is for the fossil to be more easily cross-checked with other areas
in terms of age.

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● Short temporal range: The fossil needs to have had a narrow time wherein it
existed on Earth. This is for the fossil to be able to precisely pinpoint the period of
Earth where it existed.
● Easily identifiable: Many organisms share similarities in terms of appearance. An
index fossil needs to be easily identifiable for it to not cause confusion regarding
species identification.

Examples of index fossils classified based on era and period.

Fossils and the Study of Evolution

Along with the evidence shown above, fossils provide crucial information regarding
Earth’s earlier life forms that have not been observed by humans. By using fossils,
scientists can observe how different today’s life forms are from the life forms found in
the past.

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Unit 6: Evidence of Evolution

In addition to observing past life forms, scientists can also recreate the evolutionary history
of organisms through the use of fossils (as shown in Fig. 6.1.4 ). As you will learn later on in
this unit, recreating the evolutionary history of organisms involves morphological and
molecular means. Fossils provide many morphological data that scientists can use in order
to deduce the evolutionary linkages between organisms of the past and organisms today.

Fig. 6.1.4 The use of fossils in constructing Earth’s geologic timeline and diversification of
flora and fauna

This endeavor is further aided by the dating methods used on fossils. Using dating methods,
scientists can get an idea of how long ago past species existed and place them in the
correct chronological order regarding the evolutionary history of similar or related species.

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Key Points
_____________________________________________________________________________________________

● Our knowledge of past life forms can be attributed to fossils or the preserved
remains of organisms from earlier periods of Earth’s history.
● Fossilization involves many different processes, examples of which are
permineralization, replacement, and carbonization.
● Some of the major types of fossils that are useful for scientific studies are cast
fossils, mold fossils, trace fossils, and true form fossils.
● Relative dating methods involve estimating the sequence of events that are
involved in Earth’s history.
● Absolute dating methods give a numerical estimate of the age of rock layers and
fossils.

Concepts related to fossils and fossil formation

_____________________________________________________________________________________________

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Check Your Understanding

A. Answer the following questions or identify the term being

described by the statements below.

1. Animal tracks are an example of this type of fossil.

2. Mud filled the inside of a shell. The shell crumbled away over time but the mud
inside was left behind. Identify the type of fossil formed.
3. An entire insect was trapped in amber. Identify the fossil formed.
4. This process involves the groundwater leaving behind minerals when it fills up an
organism, leading to the formation of a fossil.
5. This process involves the shedding off of other elements while carbon is left
behind, forming a fossil.
6. This is a field of geology that involves dating rocks by looking at the sequences of
rock layers.
7. This is the half-life of Carbon 14.
8. A widespread geographic range, short temporal span, and easy identifiability are
categories for this type of fossils.
9. This states that the bottom rock layers are the oldest layers in a sequence.
10. These are the fossilized feces of animals.

B. Write true if the statement is accurate and false otherwise.

1. All fossils are the remains of animals.

2. Permineralization leaves behind an image of the organism because of mineral
residues.
3. Only plants can become carbonized.
4. True form fossils are restricted to small organisms like insects.
5. Coprolites are examples of trace fossils.
6. Stratigraphy is a method of absolute dating.
7. Fossil records allow scientists to rebuild evolutionary histories.
8. Cross-cuts in rocks is older than the layers that they intrude into.

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Unit 6: Evidence of Evolution

9. Sediments are usually deposited in a horizontal manner.

10. The uptake of carbon-14 begins after the death of an organism

Challenge Yourself

Answer the following questions.

1. How do fossils provide evidence of evolution?

2. How does carbonization leave behind a fossil?
3. You found a fossil of Neptunea tabulata in one rock layer and the fossil of Leptodus
americanus in another rock layer. Which among these rock layers is older and why?
4. Why do index fossils need to be easily identifiable?
5. Suppose that you have a sample whose potassium-40 that is an eighth of its original
amount. How old is the sample?

Photo Credits
Leptofoenus_pittfieldae_(male)_rotated by Michael S. Engel and modified by Kevmin, is
licensed under CC BY 3.0 via Wikimedia Commons.

Petrified_wood_closeup_2.jpg by Daniel Schwen, is licensed under CC BY-SA 3.0 via

Wikimedia Commons.

Bibliography

Alberts, Bruce, Alexander Johnson, Julian Lewis, David Morgan, Martin C. Raff, Keith Roberts,
Peter Walter, John H. Wilson, Tim Hunt, and Bruce Alberts. Molecular Biology of the
Cell. New York, NY: Garland Science, Taylor and Francis Group, 2015.

Boyer, Rodney F. Concepts in Biochemistry. Hoboken, NJ: Wiley, 2006.

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Unit 6: Evidence of Evolution

Hickman, Cleveland Pendleton, Susan L.. Keen, Allan Larson, William C.. Ober, and Claire W..
Garrison. Animal Diversity. New York: McGraw Hill Education., 2018.

Hickman, Cleveland P. Integrated Principles of Zoology. New York, NY: McGraw-Hill, 2011.

Miller, Stephen A., and John P. Harley. Zoology. New York, NY: McGraw-Hill, 2010.

Russell, Peter J. Biology: The Dynamic Science. Student Ed. Belmont, CA:
Thomson/Brooks/Cole, 2008.

Starr, Cecie, Ralph Taggart, Christine A. Evers, and Lisa Starr. Biology: the Unity and Diversity
of Life. Boston, MA: Cengage, 2019.

6.1. Fossil Formation and Classification 17