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Wednesday 12/16/15Learning 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
Chapter 6 Section 3
The Fossil Record
FossilsThe preserved remains or traces of
organisms that lived in the past
How Do Fossils Form?
Hard parts of organisms form fossilsBones, Shells
Most fossils form when organisms that die become buried in sediments (particles of soil and rock)
Types of FossilsPetrified Fossils
Remains of organisms that are actually changed to rock
Molds and CastsMold: 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 sedimentsICE
Determining a Fossil’s AgeRelative Dating Radioactive Dating
a technique used to determine which two fossils are older
• Oldest layer at bottom• Younger layer at top
a technique used to determine the actual age of fossil on the basis of the amount of a radioactive element it contains
• 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 OrganismsExtinct: no members of that species are
still aliveMost of what scientists know about extinct
species is based on the fossil record
The Geologic Time ScaleThe calendar of Earth’s historyThe largest span of time in the Geologic
Time Scale is Precambrian TimeThis is when living things appeared on Earth
Page 194-195
Mass ExtinctionsExtinction of many types of organisms at
the same timeCretaceous 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