Wednesday 12/16/15
 Learning Goal:
Explain how fossils form, how scientists determine a fossil’s
age, and the Geologic Time Scale.
 Warm-up:
How do most fossils form?
 Homework:
Chapter 6 test on Wednesday!
How do most fossils form?
 A dead organism becomes buried in sediment
The Fossil Record
Chapter 6 Section 3
Fossils
 The preserved remains or traces of organisms that lived in
the past
How Do Fossils Form?
 Hard parts of organisms form fossils
 Bones, Shells
 Most fossils form when organisms that die become buried in
sediments (particles of soil and rock)
Types of Fossils
 Petrified Fossils
 Remains of organisms that are actually changed to rock
 Molds and Casts
 Mold: a hollow space in sediment in the shape of an organism (cake
pan)
 Cast: a copy of the shape of the organism that made the mold (cake)
 Preserved Remains
 Can be preserved in other substances other than sediments
 ICE
Determining a Fossil’s Age
Relative Dating
Radioactive Dating
a technique used to determine which two fossils a technique used to determine the actual age
are older
of fossil on the basis of the amount of a
• Oldest layer at bottom
radioactive element it contains
• Younger layer at top
• Radioactive elements: unstable
elements that decay, or break down,
into different elements
• Half-life: the time it takes for half of
the atoms in a radioactive element to
decay
Radioactive
Decay
What Do Fossils Reveal?
 Fossil record: the millions of fossils that scientists have
collected
Extinct Organisms
 Extinct: no members of that species are still alive
 Most of what scientists know about extinct species is based on
the fossil record
The Geologic Time Scale
 The calendar of Earth’s history
 The largest span of time in the Geologic Time Scale is
Precambrian Time
 This is when living things appeared on Earth
Page 194-195
Mass Extinctions
 Extinction of many types of organisms at the same time
 Cretaceous Period, about 65 million years ago (plants, animals,
dinosaurs)
Gradualism
 A theory that proposes evolution occurs steadily in tiny
changes over long periods of time
 This is how Darwin thought evolution occurred
Punctuated Equilibria
 the theory that species evolve during short periods of
rapid change