Transcript 25.2-3 - Laurel County Schools

Concept 25.2: Fossil record documents history of life
• The fossil record documents changes in the
history of life on earth
• Sedimentary rocks are deposited into layers
called strata - richest source of fossils
• Law of superposition - the oldest strata will
be at the bottom of the sequence
Why is the record incomplete? Biased?
• Organisms did not die at the right time and place
to become a fossil
• Fossil destroyed by geologic processes
• Fossil still buried in sediments
• We haven’t seen it and picked it up
• Record biased in favor of species that existed a
long time – had hard parts like bones – lived in
environments that favored fossilization
How Rocks and Fossils Are Dated: Relative vs.
Absolute Dating
• Relative Dating – based on position in
sedimentary strata – oldest at the bottom,
youngest at the surface
– Gives sequence of events not when they
occurred
How Rocks and Fossils Are Dated: Relative vs.
Absolute Dating
• Absolute Dating - provides an estimate of a
date in years; a range of values not an exact
date
• Determined by radiometric dating, based on
half-life of radioactive isotopes
– Rate of decay for an isotope is constant –
“clock like” and not affected by
environmental conditions
• half-life - time required for half “parent” isotope
to decay to “daughter” isotope
Fig. 25-5
1/
2
Remaining
“parent”
isotope
1
Accumulating
“daughter”
isotope
1/
4
1/
3
2
Time (half-lives)
8
1/
4
16
• EX. Carbon-12 and carbon-14 accumulate in
living organism; after death C-14 decays but C-12
does not; ratio of C-14 to C-12 is measured in the
fossil
• Radiocarbon dating limit – fossils up 75,000 years
old
• Living things accumulate the lighter elements –
CHNOPS etc.; Rocks are dated using different
isotopes which typically have much longer halflives
• The age of very old fossils can be inferred by
dating the sediments above and below the fossil
• Magnetism of rocks can provide dating
information – iron particles in rocks align
themselves with the north and south magnetic
poles
– The magnetic poles have reversed repeatedly
in the past
The Origin of New Groups of Organisms
• REVIEW Figure 25.6 Origin of Mammals to
better understand how scientists use fossils to
to document the evolution of a group of
organisms
• Key vocabulary: tetrapod
Concept 25.3: Key events in history of life on Earth
• geologic record – divided into Eon, Era,
Period, Epoch, Age
• Major boundaries between geological divisions
correspond to extinction events in the fossil
record
Concept 25.3: Key events in history of life on Earth
EONS
Phanerozoic Eon – current eon ~last half billion
years; “visible life”; begins with Cambrian Era,
notable for sudden increase in animal diversity
~First 4 billion years:
Proterozoic Eon - “earlier life”; accumulation of
oxygen is a key event; origin of eukaryotes to
multicellular algae and invertebrates
Archaean Eon – “beginning or origin”; origin of
prokaryotic cells, first photosynthetic organisms
You completed this topic as a homework
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