Chapter 25.2 The Fossil Record
Chapter 25.2
The Fossil Record
D I O N K E V I N , L U K E L E A R Y , C R A Y T O N B O W I E
A P B I O L O G Y 3 R D / 4 T H
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
Layers are called strata
Insects in amber
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:
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.
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
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.
Mammals
Single dentary bone
Other Tetrapods composed of several bones
3 bones in middle ear single bone (stirrup)
(hammer, anvil, stirrup)
Incisors, canines, molars 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
