Fossils
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Printed: July 15, 2015
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C HAPTER
Chapter 1. Fossils
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Fossils
• Define fossil.
• Explain radiometric dating.
• Discuss the significance of the fossil record as evidence for evolution.
What’s on this rock?
This rock contains a portion of a fossilized tree fern. Scientists study fossils of plants, animals, and other organisms
in order to better understand what life was like on Earth many years ago and how it has changed over time. Fossils
are important evidence for the theory of evolution.
The Fossil Record
Fossils are the preserved remains of animals, plants, and other organisms from the distant past. Examples of fossils
include bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed. Paleontologists are
scientists who study fossils to learn about life in the past. Paleontologists compare the features of species from
different periods in history. With this information, they try to understand how species have evolved over millions of
years ( Figure below).
Until recently, fossils were the main source of evidence for evolution ( Figure 1.2). Through studying fossils, we
now know that today’s organisms look much different in many cases than those that were alive in the past. Scientists
have also shown that organisms were spread out differently across the planet. Earthquakes, volcanoes, shifting seas,
and other movements of the continents have all affected where organisms live and how they adapted to their changing
environments.
Rock Layers and the Age of Fossils
There are many layers of rock in the Earth’s surface. Newer layers form on top of the older layers. Therefore,
you can tell how old a fossil is by observing which layer of rock it was found. The fossils and the order in which
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FIGURE 1.1
Evolution of the horse.
Fossil evi-
dence, depicted by the skeletal fragments, demonstrates evolutionary milestones in this process.
Notice the 57
million year evolution of the horse leg
bones and teeth. Especially obvious is
the transformation of the leg bones from
having four distinct digits to that of today’s
horse.
fossils appear is called the fossil record. The fossil record provides evidence for when organisms lived on Earth,
how species evolved, and how some species have gone extinct. Geologists use a method called radiometric dating
to determine the exact age of rocks and fossils in each layer of rock. This technique measures how much of the
radioactive materials in each rock layer have broken down ( Figure 1.3).
Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion
years old. The oldest fossils are between 3 and 4 billion years old. Most of these are just traces of an organism’s
presence in a layer of rock that was once the bottom of an ocean or shallow sea. Remember, as recently as 150 years
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Chapter 1. Fossils
FIGURE 1.2
About 25 to 40 million years ago these insects were trapped in a gooey
substance, called resin, that comes from trees. The fossils in the movie
Jurassic Park were trapped in resin.
FIGURE 1.3
This device, called a spectrophotometer,
can be used to measure the level of
radioactive decay of certain elements in
rocks and fossils to determine their age.
ago people believed the Earth was about 6,000 years old. The fossil record and new "dating" technologies proves
that Earth is much older than people once thought.
Vocabulary
•
•
•
•
fossil: Preserved parts of animals, plants, and other organisms from the distant past.
fossil record: Complete set of fossils that has been discovered, and the order in which the fossils appear.
paleontologist: Scientist who studies fossils to learn about life in the past.
radiometric dating: Procedure used to determine the age of rocks or fossils by measuring how much of the
radioactive materials in each sample were broken down.
Summary
• Fossils, or preserved parts of organisms from the distant past, have shown that species existed and changed
over time.
• Radiometric dating can be used to determine the age of fossils by measuring the how much of the radioactive
materials in each rock layer have broken down.
Explore More
Use the resources below to answer the questions that follow.
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Explore More I
• James Hagadorn, Paleontologist: Traces of Early Animal Life at http://shapeoflife.org/video/scientist/j
ames-hagadorn-paleontologist-traces-early-animal-life (6:11)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57326
1. Why is it believed the first animals left no fossilized bones?
2. What do paleontologists look for when they search for evidence of early organisms?
3. How old are the geologic deposits Dr. Hagadorn is searching? How old is the evidence he has found for the
first mobile organisms?
4. What does the ability to hunt others do to the fossil record? Why?
Explore More II
• Jenny Clack, Paleontologist: The First Vertebrate Walks on Land at http://shapeoflife.org/video/scienti
st/jenny-clack-paleontologist-first-vertebrate-walks-land (7:04)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57483
1. What is a "tetrapod"? What question do paleontologists hope they can answer by studying them?
2. What is special about "Boris"?
Explore More III
• Richard Dawkins: Show me the intermediate fossils! at http://www.youtube.com/watch?v=o92x6AvxCFg
(2:34)
1. What evidence is there that modern whales once had hind limbs?
2. Pakicetus and Rodhocetus are considered to be ancestors of modern whales. Scientists still argue about how
aquatic Pakicetus was, but Rodhocetusis was not, considered to be a largely aquatic animal.
a. Where is the nostril located on Pakicetus?
b. Where is the nostril located on Rodhocetus?
c. What is the relationship in time between these two species?
3. What modern animal is most closely related to modern whales? What is the evidence? In what kind of
environment does this modern relation live?
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Chapter 1. Fossils
Review
1.
2.
3.
4.
What is a fossil? Give three examples.
What has the fossil record revealed about life on Earth?
How does radioactive dating work?
Why is radioactive dating important to evolution?
References
1. LadyofHats. The evolution of the horse. CC BY-NC 3.0
2. Vassil. Insects trapped in tree resin, or amber. Public Domain
3. Tim Vickers. Image of a spectrophotometer. Public Domain
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