fossils & evolution—chapter 61 paintings by charles knight

Fossils & Evolution—Chapter 6 1

Paintings by Charles Knight


Late Cretaceous85 million years ago


Late Cretaceous75 million years ago


end-Cretaceous65 million years ago

Hell Creek Formation(coastal plain setting)

Earth History, Ch. 17 14

Chicxulub crater

Impacttrajectory


Radar image of Chicxulub crater


Chicxulub crater

Gravity surveydata


Iridium layer at Gubbio, Italy

Earth History, Ch. 17 18

Iridium layer near Drumheller (southern Alberta, Canada)


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.


Ch. 6—Key terms

• Ecologic limiting factors

• Signor-Lipps effect

• Pulse vs. Press extinction


Extinction!


Chapter 6—Extinction

• Two categories of extinction:– Normal (or background) extinction– Mass extinction (dramatically accelerated)


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


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)


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


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

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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


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.


Mass extinctions

Sepkoski (1982)

backgroundextinction levels


Mass extinctions

• Causes are poorly understood– Global climate change– Volcanism– Asteroid impact– Environmental deterioration

• CO2 & methane poisoning

• Anoxia


Mass extinctions: 26 Ma periodicity suggests astronomical cause?


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.


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


Signor-Lipps effect

• Mass extinctions appear gradual when last observed occurrences of taxa are plotted on stratigraphic sections

suspected extinction level

2m


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


False “gradualness” of mass extinctionsd

ista

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of

hig

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st o

bse

rve

d o

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rre

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be

low

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inct

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leve

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“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


0

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0 2 4 6 8 10 12 14 16 18

species diversity

met

ers

abo

ve b

ase

of

sect

ion


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1

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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

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fossils & evolution—chapter 61 paintings by charles knight

Documents

italyfossils evolutionchapter

extinction rate

formed species

normal background rate

total extinction10

limiting factorsecologic

iridium layer

number of populations