Fossils provide evidence about Earth's ancient past. They form under certain conditions, such as when organisms are quickly buried after death. Fossils come in many forms, including preserved remains, carbon films, molds, and casts. Paleontologists use the principle of uniformitarianism to study fossils and learn about extinct organisms and past environments. Fossils indicate that conditions on Earth have changed over millions of years, with climates ranging from tropical to icy. They provide clues about ancient oceans, forests, and climate changes.

1. Fossils
Chapter 16 Lesson 1
Pages 564 - 572

2. Vocabulary
• Fossil – the preserved remains or evidence of ancient living things
• Catastrophism – the idea that conditions and organisms on Earth
change in quick, violent events
• Uniformitarianism – a principle that states that geologic processes
that occur today are similar to those that have occurred in the past
• Carbon Film – the fossilized carbon outline of an organism or part
of an organism
• Mold – the impression in a rock left by an ancient organism
• Cast – a fossil copy of an organism made when a mold of the
organism is filled with sediment of mineral deposits
• Trace Fossil – the preserved evidence of the activity of an organism
• Paleontologist – scientists who study fossils

3. Evidence of the Distant Past
• Just like old photos, fossils provide us with
clues about Earth’s past
• Fossils are the preserved remain or evidence
of ancient living things

4. Catastrophism
• Many fossils represent plants and animals that are extinct
• In the past many scientists believed that a single, sudden,
catastrophic disaster killed the organisms that become fossil
• Catastrophism is the idea that conditions and organisms on Earth
change in quick, violent events
– These include large volcanic eruptions and widespread flooding

5. Catastrophism
• Scientists eventually disagreed with
catastrophism because Earth’s history is full of
violent events
• Most people who supported catastrophism
thought that Earth was only a few thousand years
old

6. Uniformitarianism
• In the 1700s, James Hutton rejected the idea of
catastrophism
• Hutton thought that the processes responsible for changing
the landscape could also shape Earth’s surface
– For example, he thought that erosion caused by streams could
also wear down mountains
– He realized that these processes would take a long time and
proposed that Earth was much older than a few thousand years

7. Uniformitarianism
• Hutton’s ideas were eventually included in a
principle called uniformitarianism.
– The principle of uniformitarianism states that
geological processes that occur today are similar to
those that have occurred in the past
– According to this view, Earth’s surface is constantly
being reshaped in a steady, uniform manner

8. Uniformitarianism
• Today, uniformitarianism is the basis for understanding
Earth’s past
• But scientists also know that catastrophic event do
sometimes occur
– Huge volcanic eruptions and giant meteorite impacts can
change Earth’s surface quickly and can be explained by
natural processes

9. Fossil Formation
• Not all dead organisms become fossils. More
than likely, you will not become a fossil.
• Fossils form only under certain conditions.

10. Conditions for Fossil Formation
• Most plants and animals are eaten or decay when they die, leaving
no trace that they ever lived
– Consider an apple. More than likely an apple will decay into a soft
lump and be decomposed by bacteria and insects
• Some conditions increase the chances that a fossil will form.
1. If the organism has hard parts, such as shells, teeth, or bones
2. If the organism is buried quickly after it dies.
• This way the layers of sand and mud slow or stop decay

11. Fossils Come in All Sizes
• Many of the fossils we think of are large dinosaur fossils.
• However, not all fossils are large.
• Microfossils are tiny fossils each about the size of a speck
of dust.
– Details of microfossils can be seen only under a microscope

12. Types of Preservation
• Fossil are preserved in many different ways:
– Preserved Remains
– Carbon Films
– Mineral Replacement
– Molds
– Casts
– Trace Fossils

14. Preserved Remains
• Sometimes the actual remains of organisms are
preserved as fossils
• For this to happen, an organism must be completely
enclosed in some material over a long period of time.
– This would prevent it from being exposed to air or
bacteria
– Generally this fossils are less than 10,000 years old.
However insects preserved in amber can be millions of
years old.

15. Carbon Films
• Sometimes when an organism is buried, exposure to
heat and pressure forces gases and liquids out of the
organism’s tissues.
• This leaves only the carbon behind
• A carbon film is the fossilized carbon outline of an
organism or part of an organism

16. Mineral Replacement
• Replicas, or copies, of organisms can form
from minerals in groundwater.
• They fill in the pore spaces or replace the
tissues of dead organisms
• Petrified wood is an example

17. Molds
• Sometimes all that remains of an organism is its fossilized imprint or
impression.
• A mold is the impression in a rock left by an ancient organism
1. First, sediment must harden around a buried organism
2. As the organism decays over time an impression of its shape remains
in the sediment
3. Eventually the sediment turns into rock

18. Casts
• Sometimes, after a mold forms, it is filled with
more sediment,.
• A cast is a fossil copy of an organism made when
a mold of the organism is filled with sediment or
mineral deposits.
Mold
Cast

19. Trace Fossils
• Some animals leaves fossilized traces of their movement or activity
• A trace fossil is the preserved evidence of the activity of an organism
• These include
– Tracks
– Footprints
– Nest
– Droppings
• These fossils help scientists learn about characteristics and behaviors of
animals
– They can reveal clues about an organism’s size, speed, and whether they
traveled alone or in groups

20. Ancient Environments
• Scientists who study fossils are called paleontologists
• Paleontologists use the principle of uniformitarianism to
learn about ancient organisms and their environment
• They often compare fossils of ancient organisms to
organisms living today
– For example, trilobite fossil and horseshoe crabs look alike
– Horseshoe crabs today live in shallow water on the ocean floor
– Partly because trilobite fossils look so much like horseshoe
crabs, paleontologists infer that trilobites also lived in shallow
ocean waters

21. Shallow Seas
• Today, Earth’s continents are mostly above sea level.
• But, sea level has risen, flooding Earth’s continents, many times in
the past
• For example, a shallow ocean covered much of North America 450
million years ago.
• Fossils of organisms that lived in that shallow ocean help scientists
reconstruct what the seafloor looked like at that time.

22. Past Climates
• Evidence indicates that Earth's present-day climate is
warming.
• Fossils show that Earth's climate has warmed and
cooled many times in the past
• Plant fossils are especially good indicators of climate
change
• For example, fossils of ferns and other tropical plants
dating to the time of the dinosaurs reveal that Earth
was very warm 100 million years ago
• Tropical forests and swamps covered much of the land
• Millions of year later, the swamps and forests were
gone, but coarse grasses grew in their place

23. Past Climates
• Huge sheets of ice called glaciers spread over
parts of N. America, Europe, and Asia.
• Fossils suggest that some species that lived
during this time, such as the woolly mammoth
were able to survive in the colder climate