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: competition, predation

Mass Extinction Causes Coincidence Physical causes Biological causes Catastrophe: impact, volcanoes

Permo-Triassic extinction Coincidence?: Over 90% of life dies, so definitely real

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

K/T Catastrophe Impact hypothesis Volcanic hypothesis

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

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 (Ca 2 SO 4 ) 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 (Ca 2 SO 4 ) 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 (Ca 2 SO 4 ) 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

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