Transcript Lecture 23: Mass Extinctions

Lecture 23: Mass Extinctions
5.) Cretaceous (144 - 65 mya)
• Mesozoic : (Permian → Cretaceous) huge ↑ in
biodiversity
• End Cretaceous (65 mya): 85% of all spp. gone
• All non-avian dinosaurs & pterosaurs gone
• No terrestrial organisms >25 kg survive!
• Relatively unaffected: crocodiles, lizards,
turtles, mammals, birds
Cause: Iridium??
Alvarez et al. (1980)
• spike in levels of
Iridium in sediments
~ 65 mya
• Ir in core; rare in
surface rock
• common in space dust
• 3 Ho for IR spike :
1) volcanic activity?
2) supernova?
3) meteor impact?
Evidence?
• Vulcanism in India, Pakistan
- enough to cause global devastation?
• Supernova: no evidence of other materials
(e.g. plutonium) common in space dust
• Meteor: shocked quartz
(impact pressure)
Effects of a Meteor Impact
• Shock Wave: fires, earthquakes
• Debris Cloud: years of acid rain, dust,
“nuclear winter”
• Lowered solar radiation reaching earth:
cold & dark →  1 productivity
• Nickel Poisoning : effectively kills PSIS
Yucatan Peninsula (Chicxulub)
• “Smoking Gun” :
impact crater
• ~ 10 km meteor
(crater ~ 150 km diam.)
• cause or “coup de
grace?”
Cyclical Nature of Mass Extinctions
Raup and Sepkoski (1984): 26 my cycle?
Controversial : depends on time scale, taxa used in
analysis
Extra-terrestrial Cause?
Suggested by periodicity:
• Planet X (“Nemesis”):
• Brown Dwarf (star that didn’t reach ignition
mass) or Planet from another system
• Passes through Oort comet cloud
• Sends meteors to earth every 26 my
• Controversial!!
Background vs. Mass Extinctions
Signor - Lipps effect:
• Mass extinction may appear gradual:
• Orgs. disappear from the fossil record at diff.
times
• Rare orgs less likely to be found in latest
deposits even if they survived as long as
common ones
Converse:
• Gradual extinction may appear mass
• Pseudoextinctions etc.
Characteristics of Survivors
• characteristics of taxa affect speciation &
extinction rate
e.g. Marine Gastropods:
a) Planktonic larva: shallow, warm seas
↓ extinction rate (wide dist’n)
b) Direct development: polar, deep seas
↑ speciation (isolated pop’ns)
Direct forms predominate (sp. select’n)
Surviving Mass Extinctions
• Surviving spp. more likely to be generalists
- can adjust to Δ’s in conditions
• “Iterative Evolution”: forms “re-evolve” during
Adaptive Radiations after mass extinctions
• B/w mass extinctions: Spp.-Rich Clades survive
background extinction better
• “Bad Genes or Bad Luck?”