fossils & evolution—chapter 61 paintings by charles knight
TRANSCRIPT
Fossils & Evolution—Chapter 6 1
Paintings by Charles Knight
Fossils & Evolution—Chapter 6 2
Fossils & Evolution—Chapter 6 3
Fossils & Evolution—Chapter 6 4
Fossils & Evolution—Chapter 6 5
Fossils & Evolution—Chapter 6 6
Fossils & Evolution—Chapter 6 7
Fossils & Evolution—Chapter 6 8
Fossils & Evolution—Chapter 6 9
Fossils & Evolution—Chapter 6 10
Late Cretaceous85 million years ago
Fossils & Evolution—Chapter 6 11
Late Cretaceous75 million years ago
Fossils & Evolution—Chapter 6 12
end-Cretaceous65 million years ago
Hell Creek Formation(coastal plain setting)
Fossils & Evolution—Chapter 6 13
Earth History, Ch. 17 14
Chicxulub crater
Impacttrajectory
Fossils & Evolution—Chapter 6 15
Radar image of Chicxulub crater
Fossils & Evolution—Chapter 6 16
Chicxulub crater
Gravity surveydata
Fossils & Evolution—Chapter 6 17
Iridium layer at Gubbio, Italy
Earth History, Ch. 17 18
Iridium layer near Drumheller (southern Alberta, Canada)
Fossils & Evolution—Chapter 6 19
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.
Fossils & Evolution—Chapter 6 20
Ch. 6—Key terms
• Ecologic limiting factors
• Signor-Lipps effect
• Pulse vs. Press extinction
Fossils & Evolution—Chapter 6 21
Extinction!
Fossils & Evolution—Chapter 6 22
Chapter 6—Extinction
• Two categories of extinction:– Normal (or background) extinction– Mass extinction (dramatically accelerated)
Fossils & Evolution—Chapter 6 23
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
Fossils & Evolution—Chapter 6 24
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)
Fossils & Evolution—Chapter 6 25
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
Fossils & Evolution—Chapter 6 26
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
number of populations
pro
ba
bili
ty o
f to
tal e
xti
nc
tio
n
Fossils & Evolution—Chapter 6 27
Probability of background extinction
• Are species that have been around a long time more or less resistant to extinction than newly formed species?
duration of species existence
prob
abili
ty o
f ex
tinct
ion
duration of species existence
prob
abili
ty o
f ex
tinct
ion
or“overspecialization”model
“resistence”model
Fossils & Evolution—Chapter 6 28
Overspecialization model Resistence model
Most genera do not survive very long. Importantly, though, the probabilityof survival does not increase or decrease with a taxon’s longevity.
Fossils & Evolution—Chapter 6 29
Mass extinctions
Sepkoski (1982)
backgroundextinction levels
Fossils & Evolution—Chapter 6 30
Mass extinctions
• Causes are poorly understood– Global climate change– Volcanism– Asteroid impact– Environmental deterioration
• CO2 & methane poisoning
• Anoxia
Fossils & Evolution—Chapter 6 31
Mass extinctions: 26 Ma periodicity suggests astronomical cause?
Fossils & Evolution—Chapter 6 32
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.
Fossils & Evolution—Chapter 6 33
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
Fossils & Evolution—Chapter 6 34
Signor-Lipps effect
• Mass extinctions appear gradual when last observed occurrences of taxa are plotted on stratigraphic sections
suspected extinction level
2m
Fossils & Evolution—Chapter 6 35
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
Fossils & Evolution—Chapter 6 36
False “gradualness” of mass extinctionsd
ista
nce
of
hig
he
st o
bse
rve
d o
ccu
rre
nce
be
low
ext
inct
ion
leve
l
“hollow” distribution curveis consistent with
Signor-Lipps effect
Highest occurrence of rare species might beat extinction level or much lower
Abundance (% of samples in which each species occurs)
Highest occurrence of common speciesLikely will be at or near extinction level
Fossils & Evolution—Chapter 6 37JT-1
1m
2m
3m
4m
5m
6m
7m
dark grey to black wackestn(8 beds in 1 m)
JT-2
JT-3
algal wackestn
JT-4
JT-5
possible intraclastsJT-6
crinoidal grainstn
JT-7
JT-8JT-9JT-10JT-11JT-12
4 cm packstn
16 cm oolitic ls.
14 cm stromatolitic ls.
JT-13
approx. 7.98m above base of section
approx. 8.69m above base of section
JT-14
8 cm oolitic ls.JT-15
22 cm oolitic ls.
JT-16
7 cm oolitic ls.JT-17
JT-18
P-T boundary
Taskent Section, southern Turkey
Fossils & Evolution—Chapter 6 38
0
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 18
species diversity
met
ers
abo
ve b
ase
of
sect
ion
Fossils & Evolution—Chapter 6 39
0
1
2
3
4
5
6
7
8
9
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
stratigraphic abundance
last
occ
urr
ence
bel
ow
P-T
bo
un
dar
y