What causes extinction? Old explanations: –Maladaptation - organisms evolved poorly-adapted features –Racial senescence - species became “weak over time”

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


• Coincidence

• Physical causes: – changes in climate– Salinity– living space: reduction in continental shelf

space due to plate motions or regression


• Coincidence

• Physical causes

• Biological causes: competition, predation


• Coincidence

• Physical causes

• Biological causes

• Catastrophe: impact, volcanoes

Permo-Triassic extinction

• Coincidence?: Over 90% of life dies, so definitely real


• Coincidence? No

• Physical: Continental configuration and regression– Reduced continental shelf space– Glaciation– Severe climate


• Coincidence? No• Physical:

– Continental configuration and regression

• Biological: Appearance of biological “bulldozers”: – Shallow burrowers– Earlier life was immobile bottom dwellers

(brachiopods, bryozoans, crinoids, etc.)


• 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


• 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

K/T Catastrophe

• Impact hypothesis

• Volcanic hypothesis

Impact scenario

• Asteroid about 10 km (6 mi.) struck, probably in Yucatan at Chicxulub

Impact scenario


• Immediate heat shock and wildfires near impact site

Impact scenario



• Particulates of gypsum (Ca2SO4) cause acid rain, killing plankton

Impact scenario




• Particulates create clouds, block sun, killing plants

Impact scenario




• Particulates create clouds, block sun, killing plants • Temperature drops, killing organisms with no

tolerance for cold

Evidence

• Crater at Chicxulub

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

Volcanic hypothesis

• Huge volcanic eruption produces climatic change, acid rain

• Volcanoes bring up iridium• BUT:

– Problems demonstrating that the eruption is the right age

– Basaltic eruptions usually produce little ash, so little climate change

Documents

What causes extinction? Old explanations: –Maladaptation - organisms evolved poorly-adapted features –Racial senescence - species became “weak over time”