Lecture 23: Mass Extinctions 5.) Cretaceous (144 - 65 mya)

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Lecture 23 Mass Extinctions5.) 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 gt25 kg survive!
• Relatively unaffected crocodiles, lizards,
  turtles, mammals, birds
• Cause Iridium??

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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?

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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)

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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

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Yucatan Peninsula (Chicxulub)

• Smoking Gun impact crater
• 10 km meteor
• (crater 150 km diam.)
• cause or coup de grace?

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Cyclical Nature of Mass Extinctions

• Raup and Sepkoski (1984) 26 my cycle?
• Controversial depends on time scale, taxa used
  in analysis

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Extra-terrestrial Cause?

• Suggested by periodicity
• Planet X (Nemesis)
• Brown Dwarf (star that didnt reach ignition
  mass) or Planet from another system
• Passes through Oort comet cloud
• Sends meteors to earth every 26 my
• Controversial!!

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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.

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Characteristics of Survivors

• characteristics of taxa affect speciation
  extinction rate
• e.g. Marine Gastropods
• Planktonic larva shallow, warm seas
• ? extinction rate (wide distn)
• b) Direct development polar, deep seas
• ? speciation (isolated popns)
• Direct forms predominate (sp. selectn)

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Surviving Mass Extinctions

• Surviving spp. more likely to be generalists
• - can adjust to ?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?