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Transcript
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 (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 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