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What causes extinction?
• Old explanations:– Maladaptation -
organisms evolved poorly-adapted features
– Racial senescence - species became “weak over time”
– Discuss the likelihood of these explanations with your table
Adaptation, not maladaptation
• No mechanism for maladaptation
• Natural selection increases, not decreases, fitness
• Species aren’t organisms - no programmed “species death” and no mechanism for “weak genes”
Is everyone equally vulnerable to extinction?
• Generalists v. specialists
• Relationship between extinction and speciation rates– Organisms that speciate readily also tend to
have short species duration - high speciation and high extinction rates go together
– Talk at your table about what would cause high speciation rates
Examples
• High extinction and speciation:– Mammals (E=0.71/my, S=0.93/my)– Also trilobites, ammonites, graptolites
• Low extinction and speciation– Clams (E=0.09/my, S=0.15/my)– Nautiloids
Mass Extinction Causes
• Coincidence: lots of organisms happened to die at the same time. Can be ruled out statistically.– Need to be especially cautious if the species
that go extinct are unstable groups– More persuasive if stable groups also suffer
extinction
Mass Extinction Causes
• Coincidence
• Physical causes: – changes in climate– Salinity– living space: reduction in continental shelf
space due to plate motions or regression
Mass Extinction Causes
• Coincidence
• Physical causes
• Biological causes
• Catastrophe: impact, volcanoes
Permo-Triassic extinction
• Coincidence? No
• Physical: Continental configuration and regression– Reduced continental shelf space– Glaciation– Severe climate
Permo-Triassic extinction
• Coincidence? No• Physical:
– Continental configuration and regression
• Biological: Appearance of biological “bulldozers”: – Shallow burrowers– Earlier life was immobile bottom dwellers
(brachiopods, bryozoans, crinoids, etc.)
Permo-Triassic extinction
• Coincidence? No• Physical: Continental configuration and
regression• Biological: Appearance of biological
“bulldozers”• Catastrophe:
– Impact? Probably not• Lack of tektites, shocked quartz• Some iridium, but not enough
Permo-Triassic extinction
• Coincidence: No• Physical: Continental configuration and
regression• Biological: Appearance of biological
“bulldozers”• Catastrophe:
– Impact? Probably not– Volcanoes (Methane hydrates)
Flood Basalt effects
• Increased carbon dioxide and global warming
• Acid rain from sulphur
• Release of methane hydrates from ocean floor
Testing the volcanic hypothesis
• For volcanic hypothesis to be credible:– Eruptions must predate the extinction– Extinctions must not be instantaneous– Expect to see pulses of extinction as disaster
intensifies
Permian
• Sooo…– It’s complicated - plenty of instability: physical,
biological and disaster– Insufficient evidence yet to point to a single
cause
Cretaceous-Tertiary Extinction
• Coincidence:85% species extinction, so it’s real• No big physical changes - many small continents
with lots of shelf space, mild climate• No big biological changes preceding the
extinction, no big change in ecological structure of the oceans after the extinction
• That only leaves catastrophe
Impact scenario
• Asteroid about 10 km (6 mi.) struck, probably in Yucatan at Chicxulub
• Immediate heat shock and wildfires near impact site
Impact scenario
• Asteroid about 10 km (6 mi.) struck, probably in Yucatan at Chicxulub
• Immediate heat shock and wildfires near impact site
• Particulates of gypsum (Ca2SO4) cause acid rain, killing plankton
Impact scenario
• Asteroid about 10 km (6 mi.) struck, probably in Yucatan at Chicxulub
• Immediate heat shock and wildfires near impact site
• Particulates of gypsum (Ca2SO4) cause acid rain, killing plankton
• Particulates create clouds, block sun, killing plants
Impact scenario
• Asteroid about 10 km (6 mi.) struck, probably in Yucatan at Chicxulub
• Immediate heat shock and wildfires near impact site
• Particulates of gypsum (Ca2SO4) cause acid rain, killing plankton
• Particulates create clouds, block sun, killing plants • Temperature drops, killing organisms with no
tolerance for cold
Evidence
• Crater at Chicxulub• Iridium spike
Asteroids have higher iridium abundance than Earth’s crust. Iridium of Earth is mostly in the mantle and core.
Evidence
• Crater at Chicxulub• Iridium spike• Shocked quartz
Two directions of lamellae typical of
impacts
Evidence
• Crater at Chicxulub• Iridium spike• Shocked quartz• Tektites
Glass globules from melting of surface and
striking object
Evidence
• Crater at Chicxulub• Iridium spike• Shocked quartz• Tektites• Soot
Carbon in boundary clay from wildfires
Evidence
• Crater at Chicxulub• Iridium spike• Shocked quartz• Tektites• Soot• C-13 indicates
catastrophic extinction
Biological effects: predictions
• Who dies?– Planktonic orgs.
– Ocean surface ecosystem
– Orgs. with poor thermoregulation
• Who lives?
Biological effects: predictions
• Who dies?– Planktonic orgs.
– Ocean surface ecosystem
– Orgs. with poor thermoregulation
• Who lives?– Bottom dwellers who
eat dead things
– Orgs. with dormancy capability
Biological effects
• Who actually dies?– Planktonic forams– Marine reptiles– Ammonites– Dinosaurs– Birds– Non-flowering
plants– Marsupials
Biological effects
• Who actually lives?– Bottom communities:
clams, snails, crustaceans, etc.
– Placental mammals
– Angiosperms
– Amphibians
– Turtles
– Insects