Lecture 8: February 19

• Chapter 6 textbook.
• Erwin, D., 1994. The Mother of Mass Extinctions. Chapter 9, The Great Paleozoic Crisis, p. 223-258
• Web text on Permo-Triassic (P/T) and Cretaceous/Tertiary (K/T) extinctions

• Mass Extinctions on the web
• Permo-Triassic mass extinction
• K/T mass extinction; drilling Chicxulub Crater
• What is a mastodon?

As discussed earlier: before acceptance of evolution as having occurred, extinction was not thought to have occurred either (benevolent God would not let His creatures become extinct). Major scientists documenting fact of extinction: Buffon (1707-1788), Cuvier (1769-1832).

Theories of extinction became less accepted as Lyell's uniformitarianism ('the present is the key to the past') became more and more accepted (1830s, 1840s), e.g., by Darwin.

Mass extinctions: what are they?

• Rapid events
• Significant part of all life on Earth extinct
• Life forms that become extinct: different phyla, different habitats, whole world
• Outside human experience (non-analog), unless we set of a mass extinction ourselves

• Earth scientists have major difficulties in measuring time intervals that are shorter than about 10,000 to 100,000 years
• Mass Extinction: <10,000 years

How significant is significant?

• Extinction in number of species, genera, families? Is 25, 50 or 75% significant?
• How do we estimate extinction of all biota from the data on biota that can form fossils easily? (shallow oceans)

We would like to know about species (unit of evolution), but usually have not enough data. We infer from numbers of families, or numbers of genera. Note that extinction of a higher category (family higher than genus higher than speciea) is always to a lower %: many species must go extinct to cause a whole family to go extinct; no Phylum has become extinct during the Phanerozoic (as far as we know).

• 'Background extinction': generalists have long species lives, specialists are more 'prone to extinction' (specialists in high-diversity regions)
• 'Mass extinctions': extinction more or less indiscriminate
• After mass extinction: faunas and floras dominated by 'weeds', 'pests': opportunistic species: rapid reproduction, prolific

Survivorship curves:

• Most species have short lives, some have very long lives
• No correlation between length of species life and probability of extinction
• Evolution is a zero-sum game: one's gain is another's loss (Red Queen)

• External (to biota) causes: change in habitat so major, so rapid that evolutionary adaptation and migration are impossible.
• Internal (to biota) causes: evolution of bacteria, viruses. Usually not seen as probable, because many different phyla, on land and in sea, must be vulnerable; most biological agents are specific to secies or groups of species.
• Evolution of humans could be seen as internal cause of extincion

• 1. Terrestrial
• 2. Extraterrestrial

• Moving continents (continents form one land mass) :
  • Plate tectonics: number of continents
  • Influence of number of continents
• Global cooling (ice caps): Volcanoes of the 'Explosive' type: Mt. St. Helens, Pinatubo, Krakatao, Tambora (1816), Mt. Agung (1963).
  • Emit dust and SO₂ fumes, react to small particles in atmosphere.
  • Dust and sulfate particles high in atmosphere scatter sunlight away from Earth: cooling, acid rain
  • Cooling effects: a few years
• Global warming (no ice caps): Volcanoes of the 'Flood basalt' type: lavas as in Hawaii, but much larger: millions of cubic miles. Lakigigar in Iceland (1785); no others in human experience.
  • CO₂ emissions - warming. SO₂-fumes: poisoning.
  • Flood basalts flow out over weeks, but keep repeating over a few millions of years
  • Warming effects long-term (millions of years)
• Low oxygen conditions (oceans)
• Low sea level (not much area with shallow seas)

Note: these causes are not mutually exclusive. Climate warming causes warming of oceans, and in warmer waters less oxygen can dissolve. Formation of large ice caps causes sea level to fall.

2. Causes of mass extinction (extraterrestrial)

• Impact of large asteroids (i.e., very large meteorites, with a diameter of several kilometers; example: Meteor Crater in Arizona)
• Impact of comets: Tunguska, Siberia, 30 June 1908; 2150 square km forest flattened; trees 'fell outward', no meteorite found: comet exploded above ground
• Supernova explosions: occur at the end of a star's lifetime. Nuclear fuel exhausted; if star is massive its core will collapse, releasing a huge amount of energy, causing blast wave, ejecting the star's envelope in space. Sometimes rapidly rotating neutron star (radio pulsar) remains. Would be detectable in sediment records by enrichment in such elements as Plutonium (never been found, although looked for).

Effects of asteroid impact:

• Ground zero: destruction (crater diameter ~ 20 x asteroid)
• Impact: dust blown up (dark, cold). About 100 x material from asteroid blown up from Earth. Vaporized, condenses into small particles. These reflect light and heat from the Sun back into space, so that it gets very cold and dark, for maybe a few months.
  • Note: these small particles fall out, get into sediments, and thus can be detected in the sediment record as chemical tracers of the impact. Reflect high temperatures, pressures, or 'different' (from Earth surface) chemical nature of asteroid.
• Heating of atmosphere: NO₂ made from N₂, reacts to HNO₃ (acid rain; leaves traces in rock record)
• Dependent upon rocks where hit: CO₂ from limestones; SO₂ from gypsum (acid rain, leaves traces in rock record)
• Mega-tsunamis (if impact in ocean): leave marine material thrown on land (can be preserved in rock record), or causes large slumps that throw material from shallow waters into deep waters (can be preserved in rock record)

1. End Ordovician (~445 Ma); ~26% of families, ~ 85% species; glaciation/sea level fall??
2. Late Devonian (~360 Ma); ~ 14% of families, ~ 72% species; impact ( Woodleight Crater??)
3. End Permian (~250 Ma); ~ 52 % families, >90% species; impact (Bedout Crater)?; flood basalts (Siberia); one continent; global warming; low oxygen conditions
4. End Triassic (~210 Ma); ~ 12% families, ~ 65% species; impact (Manicouagan Crater); flood basalts (Central Atlantic)
5. End Cretaceous (65 Ma); ~11% families, ~ 62% species; impact (Chixculub Crater); flood basalts (Deccan, India)