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Mass Extinctions
Tim Bradshaw
December 7, 2005
Extinction
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“ceasing of existence of a species or
group of taxa”
Classic cause: failure to adapt to
changes in environment
How do you define a species?
Defining species
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Biological species concept – species are groups of
actually/potentially interbreeding natural
populations reproductively isolated from other
such groups (Mayr 1942)
Evolutionary species concept – a species is a
single lineage of populations that maintains an
identity separate from other such lineages and
has its own evolutionary tendencies (Wiley 1978)
Phylogenetic species concept – a species is an
irreducible cluster of organisms that is
diagnosably distinct from other such clusters, and
within which there is a parental pattern of
ancestry and descent (Cracraft 1989)
Species Lifespan
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Mammals ~ 1million year lifespan
5000 currently alive
Background rate ~ 1 species/200
years
89 species extinct in last 400 years
Geologic Time Scale
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Era
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Cenozoic
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Mesozoic
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Paleozoic
Time (mya)
1.8 – 0.01
Period
Quaternary
65 – 1.8
144 – 65
206 – 144
248 – 206
290 – 248
354 – 290
417 – 354
443 – 417
490 – 443
543 – 490
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Ordovician
Echinoderm Time
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490 – 443 mya
great diversification of phyla, including 21
classes of echinoderms
formation of reefs
evolution of agnathans and trilobites
plants invade land
Gondwanaland centered in southern
hemisphere
Gondwanaland (~ 400 mya)
End-Ordovician Extinction
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57% of marine genera wiped out
• Trilobites take a dirt nap
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Probable cause: Global cooling
Glaciation over Gondwanaland led to
cooling and drop in sea levels
Time span: 2 my
Result: Paved the way
for ostracoderms
Devonian
Conquest of Land
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417 – 354 mya
Osteichthyes develop (Sarcopterygii
and Actinopterygii)
Terrestrial arthropods evolve
Land plants diversify (ferns,
horsetails)
Ichthyostegid amphibians evolve
from rhipidistians
Late-Devonian Extinction
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22% of marine families and 57% of
marine genera
• Loss of coral reefs and ostracoderms
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Once again: global cooling from
glaciation,
Time span: 4 million
years
Permian
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First amniotes - captorhinomorphs
Fish/Insects/Reptiles continue to
diversify
Amphibians decline
Formation of
Pangea
Permian-Triassic Extinction
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Mother of Mass Extinctions
Say goodbye to 54% of all marine species
Lost 75% of all land vertebrate families
Estimated that 84% of all genera on Earth
became extinct
Time span: 1 million years
Results: so long frogs,
hello dinos
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Cause:
Continental drift created a non-fatal but
precariously balanced global environment,
a supernova weakened the ozone layer,
and then a large meteor impact triggered
the eruption of the Siberian Traps. The
resultant global warming eventually was
enough to melt the methane hydrate
deposits on continental shelves of the
world-ocean.
Causes
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Plate Tectonics – Pangea formation
Supernova – destroy ozone layer
Asteroid impact – trigger volcanoes
Siberian traps – 200,000 cubic
kilometers
Methane hydrate released from
oceans
Oxygen depleted from oceans
Triassic
Age of the Reptiles
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Pangea would soon break up into
Laurasia and Gondwanaland
Marine groups diversified
• Forams, modern corals, bony fish
Gymnosperms dominate on land
First dinosaurs and first mammals arrive
End-Triassic Extinction
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Loss of 25% of all families
• Decimated sponges, marine reptiles and
conodonts
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Cause: largest volcanic event from
Pangea rift
Results: Paved the way for more
dinosaurs in more niches
Cretaceous
Age of the Dinosaurs
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Continents have fully separated
Angiosperms have developed and
taken over
Birds evolve from theropod dinosaurs
=
Cretaceous-Tertiary Extinction
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Death to the Dinosaurs
50% of all genera
• Pterosaurs, dinosaurs, sea reptiles
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Cause: Celestial impact reduced
photosynthesis
Results: Welcome bug eaters
(aka mammals)
Future Extinctions
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Probable Cause: Us
Habitat destruction - estimated that
5-10% of species will be extinct in
thirty years
Global warming – estimated that
~35% of species will become
committed to extinction in the next
fifty years
Evolving past Extinction
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Possible to become resistant to
extinction?
Probably not
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Natural selection cannot plan ahead
Extinction causes are numerous, thus
there is no carryover of extinction
resistance
Just as likely for old taxa to go
extinct as for new taxa
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Why do we see a decline in the
background extinction rate during
the Phanerozoic (Cambrian – today)?
Two Theories
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1. average # of species per family
has increased over time
(Flessa and Jablonski 1985)
2. Taxa more likely to go extinct went
extinct early in the Phanerozoic
(Erwin et al. 1987)
Importance of Mass Extinctions
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Adaptive radiation
Release from competition, predation