Book G Chapter 4 – Section 1 FOSSILS Book G Chapter 4 – Section 1

OVERVIEW Explain what fossils are and how most fossils form. Describe what fossils tell about how organisms have changed over time. Identify and describe different types of fossils.

FORMATION OF FOSSILS Fossils are preserved remains or traces of living things. Most fossils form when living things die and are buried by sediments. The sediments slowly harden into rock and preserve the shapes of the organisms. Paleontologists are scientists who study fossils.

What do fossils tell us? Fossils provide evidence of how life has changed over time. Fossils help scientists infer how Earth’s surface has changed. Fossils are clues to what past environments were like.

The Fossil Record and life The fossil record provides evidence about the history of life on Earth. The fossil record also shows that different groups of organisms have changed over time. Evolution is the gradual change in living things over long periods of time. Extinct is if an organism no longer exists and will never again live on Earth.

Types of fossils Petrified fossils Molds and casts Carbon films Trace fossils Preserved remains

Petrified Fossils Fossils in which minerals replace all or part of an organism. How does this happen? Water rich in dissolved minerals seeped into spaces, evaporated, leaving the hardened minerals behind. Example – petrified wood

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

Carbon Films Carbon film is an extremely thin coating of carbon on rock. How does this happen? All organisms are made of carbon. When they are buried, the materials that make up the organism evaporates. These gases escape leaving carbon behind.

Trace Fossils Trace fossils provide evidence of the activities of ancient organisms. Examples A footprint provide clues about the size and behavior, the speed, how many legs it walked on, lived alone or with others. A trail or burrow can give clues about the size and shape of the organism, where it lived, and how it obtained food.

Preserved remains Preservation of remains with little or no change. Preservation material Tar The sticky oil that seeps from Earth’s surface. Tar soaks into the organisms bones, preserving the bones from decay. Amber The hardened resin, or sap, of trees. The amber seals the organism from the air protecting it from decay. Ice

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

Absolute Age of Rock The number of years since the rock was formed. Geologists use different methods to determine the absolute age of rock, usually within a few million years.

Law of Superposition This states that in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher level is younger than the layers below it. This is used to determine the relative age of sedimentary rock layers.

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

Intrusion Made of magma that pushes into the cracks of rock under the surface and that cools and hardens – intrusions are always younger than the rock around and beneath it. By studying where extrusions and intrusions are found in rock layers scientists can determine the relative ages of surrounding rocks.

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

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

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

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

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

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

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

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

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

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