FOSSILSFOSSILSBook GBook G

Chapter 4 – Section 1Chapter 4 – Section 1

OVERVIEWOVERVIEW Explain what fossils are and how most fossils Explain what fossils are and how most fossils

form.form. Describe what fossils tell about how organisms Describe what fossils tell about how organisms

have changed over time.have changed over time. Identify and describe different types of fossils.Identify and describe different types of fossils.

FORMATION OF FORMATION OF FOSSILSFOSSILS

Fossils are preserved remains or traces of Fossils are preserved remains or traces of living things.living things.

Most fossils form when living things die and Most fossils form when living things die and are buried by sediments.are buried by sediments.

The sediments slowly harden into rock and The sediments slowly harden into rock and preserve the shapes of the organisms.preserve the shapes of the organisms.

Paleontologists are scientists who study Paleontologists are scientists who study fossils.fossils.

What do fossils tell us?What do fossils tell us?

Fossils provide evidence of how Fossils provide evidence of how life has changed over time.life has changed over time.

Fossils help scientists infer how Fossils help scientists infer how Earth’s surface has changed.Earth’s surface has changed.

Fossils are clues to what past Fossils are clues to what past environments were like.environments were like.

The Fossil Record and lifeThe Fossil Record and life

The fossil record provides evidence about the The fossil record provides evidence about the history of life on Earth. The fossil record also history of life on Earth. The fossil record also shows that different groups of organisms have shows that different groups of organisms have changed over time.changed over time.

Evolution is the gradual change in living Evolution is the gradual change in living things over long periods of time.things over long periods of time.

Extinct is if an organism no longer exists and Extinct is if an organism no longer exists and will never again live on Earth.will never again live on Earth.

Types of fossilsTypes of fossils Petrified fossilsPetrified fossils Molds and castsMolds and casts Carbon filmsCarbon films Trace fossilsTrace fossils Preserved remainsPreserved remains

Petrified FossilsPetrified Fossils Fossils in which minerals replace all or part of Fossils in which minerals replace all or part of

an organism.an organism. How does this happen?How does this happen?

Water rich in dissolved minerals seeped into Water rich in dissolved minerals seeped into spaces, evaporated, leaving the hardened minerals spaces, evaporated, leaving the hardened minerals behind.behind.

Example – petrified woodExample – petrified wood

Molds and castsMolds and casts Most common type of fossil.Most common type of fossil. Both copy the shape of the organism.Both copy the shape of the organism. A mold is a hollow area of sediment in the A mold is a hollow area of sediment in the

shape of the organism.shape of the organism. A cast is a copy of the shape of an organism.A cast is a copy of the shape of an organism.

Carbon FilmsCarbon Films Carbon film is an extremely thin coating of Carbon film is an extremely thin coating of

carbon on rock.carbon on rock. How does this happen?How does this happen?

All organisms are made of carbon. When they are All organisms are made of carbon. When they are buried, the materials that make up the organism buried, the materials that make up the organism evaporates. These gases escape leaving carbon evaporates. These gases escape leaving carbon behind.behind.

Trace FossilsTrace Fossils Trace fossils provide evidence of the activities Trace fossils provide evidence of the activities

of ancient organisms.of ancient organisms. ExamplesExamples

A footprint provide clues about the size and behavior, A footprint provide clues about the size and behavior, the speed, how many legs it walked on, lived alone or the speed, how many legs it walked on, lived alone or with others.with others.

A trail or burrow can give clues about the size and A trail or burrow can give clues about the size and shape of the organism, where it lived, and how it shape of the organism, where it lived, and how it obtained food.obtained food.

Preserved remainsPreserved remains Preservation of remains with little or no Preservation of remains with little or no

change. change. Preservation materialPreservation material

TarTar The sticky oil that seeps from Earth’s surface. Tar soaks into The sticky oil that seeps from Earth’s surface. Tar soaks into

the organisms bones, preserving the bones from decay.the organisms bones, preserving the bones from decay.

AmberAmber The hardened resin, or sap, of trees. The amber seals the The hardened resin, or sap, of trees. The amber seals the

organism from the air protecting it from decay.organism from the air protecting it from decay.

Ice Ice

Relative Age of RockRelative Age of Rock

The age of the rock compared with other The age of the rock compared with other rocks. It is not exact.rocks. It is not exact.

Absolute Age of RockAbsolute Age of Rock

The number of years since the rock was The number of years since the rock was formed.formed.

Geologists use different methods to determine Geologists use different methods to determine the absolute age of rock, usually within a few the absolute age of rock, usually within a few million years.million years.

Law of SuperpositionLaw of Superposition

This states that in horizontal sedimentary rock This states that in horizontal sedimentary rock layers the oldest layer is at the bottom.layers the oldest layer is at the bottom.

Each higher level is younger than the layers Each higher level is younger than the layers below it.below it.

This is used to determine the relative age of This is used to determine the relative age of sedimentary rock layers.sedimentary rock layers.

ExtrusionExtrusion

Made of magma the flows out onto the surface Made of magma the flows out onto the surface and hardens. An extrusion is always younger and hardens. An extrusion is always younger than the rock or other extrusions below it.than the rock or other extrusions below it.

IntrusionIntrusion

Made of magma that pushes into the cracks of Made of magma that pushes into the cracks of rock under the surface and that cools and rock under the surface and that cools and hardens – intrusions are always younger than hardens – intrusions are always younger than the rock around and beneath it.the rock around and beneath it.

By studying where extrusions and intrusions By studying where extrusions and intrusions are found in rock layers scientists can are found in rock layers scientists can determine the relative ages of surrounding determine the relative ages of surrounding rocks.rocks.

FaultsFaults

Faults are always younger than the rock that it Faults are always younger than the rock that it cuts through – to determine the relative age of cuts through – to determine the relative age of a fault, scientists find the age of the youngest a fault, scientists find the age of the youngest layer of rock cut by the fault.layer of rock cut by the fault.

UnconformityUnconformity

A gap in the geologic record in which new A gap in the geologic record in which new rock layers meet a much older rock surface rock layers meet a much older rock surface beneath them – caused by the loss of some beneath them – caused by the loss of some rock layers by erosion which are then covered rock layers by erosion which are then covered over by younger rock. over by younger rock.

Index FossilIndex Fossil

Fossils used to determine the relative age of Fossils used to determine the relative age of rock layers – scientists have determined the rock layers – scientists have determined the time when certain species were found on time when certain species were found on Earth. If fossils of these organisms are found Earth. If fossils of these organisms are found in specific rock layers scientists know that the in specific rock layers scientists know that the rock must be the same age as the fossil.rock must be the same age as the fossil.

Index Fossils Cont.Index Fossils Cont.

To be an index fossil a fossil must be widely To be an index fossil a fossil must be widely distributed and represent a type of organism distributed and represent a type of organism that existed only briefly – ex. A ammonite is a that existed only briefly – ex. A ammonite is a type of mollusk with a hard shell that was type of mollusk with a hard shell that was found all over the world between 500 million found all over the world between 500 million and 65 million years ago.and 65 million years ago.

Index Fossils ContIndex Fossils Cont

There were many different variations that There were many different variations that survived for only a few million years – by survived for only a few million years – by determining which type of ammonite the fossil determining which type of ammonite the fossil is, scientists can determine the age of the rock is, scientists can determine the age of the rock layer within a few million years.layer within a few million years.

Radioactive DatingRadioactive Dating

Some forms of elements are not stable and Some forms of elements are not stable and over time decay by releasing particles and over time decay by releasing particles and energy in a process called energy in a process called radioactive dating radioactive dating to form a new, different element – to form a new, different element – radioactive radioactive decay occurs at a constant ratedecay occurs at a constant rate with one half with one half of the total amount of radioactive atoms of the of the total amount of radioactive atoms of the element present decaying in a certain amount element present decaying in a certain amount of time called a of time called a Half lifeHalf life

Radioactive Dating ContRadioactive Dating Cont

As the amount of orginal element decreases, As the amount of orginal element decreases, the new element being formed increases- by the new element being formed increases- by determining the determining the absolute ageabsolute age of the fossil or of the fossil or the rock. the rock.

Radioactive Dating ContRadioactive Dating Cont

Carbon 14 dating is often used as all living Carbon 14 dating is often used as all living organisms contain Carbon -14 and scientists organisms contain Carbon -14 and scientists know it has a half life of 5,730 years, therefore know it has a half life of 5,730 years, therefore it can only be used to identify organisms that it can only be used to identify organisms that are less than 50,000 years old – potassium 40 are less than 50,000 years old – potassium 40 has a very long half-life of 1.3 billion years has a very long half-life of 1.3 billion years and can be used to date most ancient rocks.and can be used to date most ancient rocks.

Radioactive Dating ContRadioactive Dating Cont

Scientists can date igneous rocks with Scientists can date igneous rocks with radioactive dating but they cannot date radioactive dating but they cannot date sedimentary rocks this way because they are sedimentary rocks this way because they are composed of many differenty types of rock, composed of many differenty types of rock, they can be dated by determining the age of they can be dated by determining the age of the igneous intrusions and extrusions found in the igneous intrusions and extrusions found in them.them.

Earth’s FormationEarth’s Formation

Scientists believe that the Earth was formed Scientists believe that the Earth was formed about 4.6 billion years ago- Scientists use a about 4.6 billion years ago- Scientists use a geological time scale to record the life forms geological time scale to record the life forms and geological events that have occurred since and geological events that have occurred since the Earth was formed.the Earth was formed.