BIO372 : Extinction : Mass Extinctions : Lesson

I. The Death of the Dinosaurs Observations Iridium found in geologic strata at K-T boundary Meteorite crater near India Impact quartz in India Several asteroid craters discovered around the planet

2. Hypotheses
   • Catastrophe theories
     • Giant asteroid struck Earth, spewed up dust and smoke into the atmosphere, climate cooled, vegetation withered, dinosaurs starved
     • Giant volcanic eruptions spewed ash into atmosphere, climate cooled, vegetation withered, dinosaurs starved
     • Giant asteroid struck Earth, triggered massive volcanic eruptions, climate cooled, vegetation withered, dinosaurs starved
   • Noncatastrophe theories
     • Climatic cooling

3. Uncertainty
   • Atmospheric particulates could have originated from either meteorites or volcanoes
   • Impact quartz could have originated from either meteorites or volcanoes
   • Iridium could have originated from either meteorites or volcanoes
   • Gradual climatic changes could account for dinosaur's demise
   • It took 100,000 years for dinosaurs to die out

II. What killed the dinosaurs

1. K-T boundary, 65 million years ago
   • Cretaceous period (K) dominated by conifers and dinosaurs
   • Tertiary period (T) dominated by flowering plants and mammals

2. Hypotheses
   • Impactor: swarm of meteorites pelted the earth, one at least 10 km across
   • Volcanist: extensive volcanic eruptions

3. Indications - correlations
   • Iridium in relatively large concentrations in clay layer in Gubbio (Italy); subsequent discoveries of iridium at K-T boundary around the world
   • Impact quartz at meteorite craters
   • Rock spherules: tektites
   • Impact craters: Haiti, Cuba, Iowa
   • Stepwise extinctions occurred over a few million years

4. Killing mechanism (hypotheses)
   • Oblique impact simulation
     • Meteorite at low angle would break up into a swarm of fragments which would block sunlight and fall back down gradually
   • Acid rain
     • Shock-heated atmosphere caused oxygen and nitrogen to form nitric acid which rained down, caused the three-meter gap
   • Thermal radiation
     • Vaporized rock heated the atmosphere and burned the earth, graphitic carbon
   • Greenhouse effect
     • Impact produced 2 - 5 times CO₂ overnight, inferred climate change from disappearance of marine algae

5. Patterns
   • Groups
     • Certain groups of land and ocean organisms went extinct gradually before K-T boundary
     • Some groups went extinct shortly after the K-T boundary
     • Many groups suffered catastrophically right at the boundary
   • Ocean
     • Tropical groups and plankton feeders most vulnerable
     • Species with resting spores survived best (diatoms and dinoflagellates)
     • Scavengers and detritivores had an advantage
   • Land
     • Large reptiles suffered the most
     • Flowering plants recovered rapidly: protected seeds, spores, rhizomes
     • Insects and insectivores had an advantage

6. Periodicities
   • Other mass extinctions occurred in Earth's history
   • Statistical analysis of extinctions suggested a period of 26 million years
   • Hypotheses: Nemesis, Planet X, solar system oscillations

7. Volcanist alternative
   • Sulfur dioxide clouds would briefly cool the planet and rain acid
   • Carbon dioxide would produce a greenhouse effect for years
   • Hot spot volcanoes could bring up iridium from earth's core and create impact quartz
   • Math and physical models contain thermal boundary layers and magnetic field reversals

8. Scientific process
   • Some erroneous hypotheses die hard
   • Value of a new hypothesis leads to debate which stimulates research to obtain new data

III. Mass extinctions in the oceans

1. Extinctions
   • Background: plant and animal species die out at a low, constant rate
   • Mass extinction: sharp peaks in the number of vanishing species associated with radical changes in regional/global environment

2. Temperature
   • Temperature discontinuity often marks the boundary of a species' geographic range
   • Evidence of changing temperatures reflected in glacial gravels and loss of species adapted to warm water or a narrow temperature range

3. Hypotheses for mass extinctions
   • Climatic cooling
   • Shrinking of the area of the shallow sea floor

4. Observations
   • A narrow continental shelf can harbor an enormous diversity of bottom-dwelling organisms
   • Biotic crises are not correlated with changing sea levels
   • A large number of species vanished during the Ice Age
   • Extinctions concentrated on warm water species
   • Survivors of extinction episodes are adapted to cool temperatures, e.g., globigerines
   • Switch from tropical to temperate forests at end of Eocene

5. K-T boundary
   • Faunas of the Tethyan Sea suffered most
     • Coccolithophores, planktonic foraminifera, reef-building rudists, ammonoids
   • Strata in Denmark and North Dakota showed that many Cretaceous species lived several million years more before going extinct
   • Biotas from cool, northern latitudes shifted southward
   • Ammonoids and inoceramid bivalve molluscs declined gradually and went extinct before the boundary
   • Top of food chain went extinct before the base
   • Time lags in the evolution of species diversity after an extinction episode might account for apparent periodicity

6. Other mass extinctions
   • Late in Precambrian: extinction of acritarchs was correlated with glaciation
   • Ordovician: extinction of trilobites and graptolites, glaciation originating in N. Africa
   • Devonian: tropical marine groups disappeared while cool water species fared well, e.g. glass sponges
   • Permian: complex, with warming first followed by cooling

7. Patterns
   • Oldest marine animal species are those adapted to cold water
   • Tropical animals have come and gone rapidly in geologic time
   • Tropical species are often adapted to a narrow range of warm temperatures; no refuge in times of cooling

When you have completed this lesson, go on to Review Questions