1. 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??

2. 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?

3. 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)

4. 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

5. Yucatan Peninsula (Chicxulub)
“Smoking Gun” : impact crater
~ 10 km meteor
(crater ~ 150 km diam.)
cause or “coup de grace?”

6. Cyclical Nature of Mass Extinctions
Raup and Sepkoski (1984): 26 my cycle?
Controversial : depends on time scale, taxa used in analysis

7. 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!!

8. 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.

9. Characteristics of Survivors
characteristics of taxa affect speciation & extinction rate
e.g. Marine Gastropods:
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)

10. 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?”