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 H o 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: 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)

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?”