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Paintings by Charles Knight
Fossils & Evolution—Chapter 6
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Fossils & Evolution—Chapter 6
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Fossils & Evolution—Chapter 6
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Fossils & Evolution—Chapter 6
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Fossils & Evolution—Chapter 6
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Fossils & Evolution—Chapter 6
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Late Cretaceous
85 million years ago
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Late Cretaceous
75 million years ago
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end-Cretaceous
65 million years ago
Hell Creek Formation
(coastal plain setting)
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Chicxulub crater
Earth History, Ch. 17
Impact
trajectory
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Radar image of Chicxulub crater
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Chicxulub crater
Gravity survey
data
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Iridium layer at Gubbio, Italy
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Iridium layer near Drumheller
(southern Alberta, Canada)
Earth History, Ch. 17
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Chapter 6—Key concepts
• 99.9% of all organisms that have ever lived are
now extinct. “Background” extinction occurs
when a species cannot adapt to a change in its
environment.
• Mass extinctions are episodes when the extinction
rate far exceeds the normal background rate. Mass
extinctions do not occur at predictable intervals,
and each probably was caused by a unique set of
circumstances.
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Ch. 6—Key terms
• Ecologic limiting factors
• Signor-Lipps effect
• Pulse vs. Press extinction
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Extinction!
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Chapter 6—Extinction
• Two categories of extinction:
– Normal (or background) extinction
– Mass extinction (dramatically accelerated)
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Rates of extinction
• Agents of extinction are changes in
ecologic limiting factors
• Also, population size, number of
populations, and geographic range of
populations affect the probability of
extinction
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Limiting factors
• Ecologic limiting factors = physical, chemical and
biologic properties of the environment that limit the
distribution and abundance of a particular species
– Temperature
– Oxygen
– Depth-related variables
• Light, pressure, water chemistry, etc.
–
–
–
–
Salinity
Substratum (nature of the seafloor)
Food
Other biota (competitors, predators, infectious diseases)
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Rates of extinction
• Probability of extinction vs. No. of
populations
– Suppose that, in a given interval of time, every
population has a 50% chance of becoming
extinct
– Species with large numbers of populations are
unlikely to suffer total extinction
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0.5
probability of total extinction
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
number of populations
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Probability of background
extinction
“overspecialization”
model
or
probability of extinction
probability of extinction
• Are species that have been around a long
time more or less resistant to extinction than
newly formed species?
“resistence”
model
duration of species existence
duration of species existence
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Overspecialization model
Resistence model
Most genera do not survive very long. Importantly, though, the probability
of survival does not increase or decrease with a taxon’s longevity.
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Mass extinctions
Sepkoski (1982)
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Mass extinctions
• Causes are poorly understood
–
–
–
–
Global climate change
Volcanism
Asteroid impact
Environmental deterioration
• CO2 & methane poisoning
• Anoxia
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Mass extinctions:
26 Ma periodicity suggests astronomical cause?
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Causes of mass extinctions
• Causes are extremely difficult to determine
• Timing is key to causal analysis
– “Press” (gradual) vs. “pulse” (abrupt) extinction
• Types of organisms affected is also key to causal
analysis
– Marine only vs. terrestrial and marine
– Physiologic selectivity
• e.g., filter feeders only, etc.
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Signor-Lipps effect
• Consider two species, one rarely preserved (occurs in 10%
of samples) and the other commonly preserved (occurs in
80% of samples)
• Assume that both became extinct at “extinction level”
• Where are we likely to find their highest observed
occurrence?
actual extinction level
2m
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Signor-Lipps effect
• Mass extinctions appear gradual when last
observed occurrences of taxa are plotted on
stratigraphic sections
suspected extinction level
2m
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False “gradualness” of mass
extinctions
• Probability of finding abundantly occurring
taxa in a given sample is much greater than
probability of finding rare taxa
• Most taxa whose last observed occurrence
is some distance below an extinction
horizon are rare taxa
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distance of highest observed occurrence below extinction level
False “gradualness” of mass extinctions
Abundance (% of samples in which each species occurs)
Highest occurrence of common species
Likely will be at or near extinction level
“hollow” distribution curve
is consistent with
Signor-Lipps effect
Highest occurrence of rare species might be
at extinction level or much lower
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Taskent Section, southern Turkey
JT-12
JT-11
JT-10
JT-9
JT-8
approx. 8.69m above base of section
7m
JT-7
JT-18
14 cm stromatolitic ls.
P-T boundary
6m
crinoidal grainstn
JT-6
5m
7 cm oolitic ls.
JT-17
possible intraclasts
JT-16
22 cm oolitic ls.
JT-5
JT-4
4m
3m
JT-15
8 cm oolitic ls.
algal wackestn
JT-3
2m
16 cm oolitic ls.
JT-14
JT-2
1m
JT-13
4 cm packstn
dark grey to black wackestn
(8 beds in 1 m)
approx. 7.98m above base of section
JT-1
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18
meters above base of section
16
14
12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
species diversity
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stratigraphic abundance
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
last occurrence below P-T boundary
1
2
3
4
5
6
7
8
9
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