Transcript Chapter 25.2 The Fossil Record

Chapter 25.2
The Fossil Record
DION KEVIN, LUKE LEARY, CRAYTON BOWIE
A P B I O L O G Y 3 RD/ 4 TH
Overview
 The Fossil Record
 Crayton
 How Rocks and Fossils are Dated
 Dion
 The Origin of New Groups of Organisms
 Luke
The Fossil Record
 Provides a window into the past
 Reveals changes and supports evolutionary ideas
 Fossils are primarily accumulated from
 Sedimentary rocks
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Insects in amber
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Layers are called strata
Fossilized tree sap
Mammals frozen in ice
The Fossil Record, cont.
 The fossil record offers an incomplete chronicle of
evolutionary change
 The fossil record has gaps because of:
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Many didn’t die in the right place @ the right time
Some were destroyed by geologic processes
Only a fraction have been discovered
 As a result, only creatures which were abundant and
alive for a while bias the fossil record
Fossil Record, cont.
How Rocks and Fossils are Dated
 Radiometric Dating- a method for determining the
absolute ages of rocks and fossils, based on the halflife of radioactive isotopes.
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The rate of decay is expressed as a half life, the time required
for 50% of the parent isotope to decay.
 Radiocarbon Dating- a specific method using
carbon-14 to determine fossil age
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All organisms contain carbon, so this is most useful. However,
it is only accurate up to 75,000 years
Uranium-238 half-life: 4.5 billion years
 Carbon-14 half-life: 5730 years
 Potassium-40 half-life: 1.3 billion years

How Rocks and Fossils are Dated, cont.
 Magnetism
 During the formation of volcanic and sedimentary rocks, iron
particles can align themselves with the Earth’s magnetic field.
 The magnetic field of Earth has changed multiple times in its
history, therefore alignment patterns can be matched with
other areas of rock and the age can be determined.
 *It is important to note that
much of fossil dating is with
relation to other rocks*
The Origin of New Groups of Organisms
 Fossils provide evidence of the origin of new groups
of organisms
 Along with amphibians and reptiles, mammals are
considered tetrapods, with four limbs.
 Mammals with unique anatomical features that
fossilize readily have a good fossil record.
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Mammals
Single dentary bone
3 bones in middle ear
(hammer, anvil, stirrup)
Incisors, canines, molars
Other Tetrapods
composed of several bones
single bone (stirrup)
undifferentiated, singlepointed teeth
The Origin of New Groups of Organisms, cont.
 The fossil record shows that the unique features of
mammals evolved in a series of gradual
modifications in a group of tetrapods called
synapsids.
 The features of mammals arose gradually in a
previously existing group, the cynodonts.
Video
 http://www.youtube.com/watch?v=c_DCP4cLVNg