19.1 WS

Lesson Overview

The Fossil Record

Lesson Overview

19.1 The Fossil Record

Lesson Overview

The Fossil Record

19.1 Learning Targets

1. Students will identify what fossils reveal about ancient life.

2. Students will explain the difference between relative dating and radiometric dating.

3. Students will name the four divisions of the geologic time scale and explain how we use it to organize and understand time.

Open Ended

What could fossils reveal about ancient life?

Open Ended

What is the difference between relative dating and radiometric dating?

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 history of life on Earth depicted by the fossils of
ancient organisms is called the fossil record.

How can fossils help us understand life’s history?

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.

Many fossils are just fragments of an organism—teeth,
pieces of a jawbone, or bits of leaf.

Sometimes an organism leaves behind trace
fossils—casts of footprints, burrows, tracks, or even
droppings.

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.

There are three main steps on how fossils are formed:

1. As sediments build up, they bury dead organisms that have sunk to the
bottom.

2. 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.

3. 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.

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.

These fish fossils were 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.

From the fossil record, paleontologists learn about the structure of ancient
organisms, their environment, and the ways in which they lived.

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,
including their common ancestors.

Bone structure and trace fossils, like footprints, indicate how animals
moved. 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

Relative Dating

How do we date events in Earth’s history?

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 and allows paleontologists to determine whether a fossil is older
or younger than other fossils.

Fossil F is most likely the oldest as it is the deepest in the ground

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 fossils’
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

The half-life of potassium-40 is 1.26
billion years.

By comparing the amount of decay to
the half-life of a substance, scientists
can calculate the age of a sample.

For example, if ¾ of a substance has
decayed leaving only ¼ left, we can
assume that 2 half-lives have passed,
meaning the potassium-40 is 1.26 x 2
= 2.52 billion years old

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).

Reorder

How do most fossils form? Arrange the steps in order.

• The organism dies

• They are buried in sediment

• 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.
  

• The preserved remains may later be discovered and studied

1

2

3

4

5

• 1
• 2
• 3
• 4
• 5

Multiple Choice

What do fossils reveal about ancient life?

• the structure of ancient organisms

• ancient organism's environment

• the ways in which ancient organisms lived

• all of the above

Multiple Choice

_____________ dating places rock layers and their fossils into a temporal sequence and allows paleontologists to determine whether a fossil is older or younger than other fossils.

• Relative

• Half-life

• Index

• Radiometric

Multiple Choice

_____________ dating compares the amount of radioactive to nonreactive isotopes in a sample to determine its age.

• Relative

• Half-life

• Index

• Radiometric

Lesson Overview

The Fossil Record

Geologic Time Scale

Geologists and paleontologists
have built a timeline of Earth’s
history called the geologic time
scale. 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.

The basic divisions of the geologic
time scale are eons, eras, periods,
and epochs.

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.

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.

There is a fourth division, called epoch, that
we separate time into, usually by ice ages or
extinctions. We are currently in the holocene
epoch, while wooly mammoths and other ice
age animals lived in the pleistocene epoch.

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.

Multiple Choice

What is the order of the different sections of time from largest to smallest?

• periods, eons, epochs, eras

• eons, eras, periods, and epochs

• eras, periods, eons, epochs

• epochs, periods, eras, eons

Multiple Choice

Which epoch are we currently in?

• Jurassic

• Holocene

• Cenozoic

• Phanerozoic

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.

Where land masses 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 semi aquatic 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 from asteroids 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.

Open Ended

What do you think? Based on the physical forces, geologic cycles, and biological forces that have happened to Earth over billions of years, do you think we have or will find a fossil of every organism that lived on Earth? Why or why not?