FossilsChapter 16 Lesson 1

Pages 564 - 572

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

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

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

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

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

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

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

Fossil Formation

• Not all dead organisms become fossils. More than likely, you will not become a fossil.

• Fossils form only under certain conditions.

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 bones2. If the organism is buried quickly after it dies.

• This way the layers of sand and mud slow or stop decay

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

Types of Preservation

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

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.

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

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

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 organism2. As the organism decays over time an impression of its shape remains

in the sediment3. Eventually the sediment turns into rock

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

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

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

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.

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

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