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November 12, 2011: THE FOSSIL RECORD: Mass Extinctions Instructor: Jonathan Hendricks Assistant Professor of Paleontology, SJSU, Dept. Geology Email: BAESI.

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November 12, 2011: THE FOSSIL RECORD: Mass Extinctions


Jonathan Hendricks

Assistant Professor of Paleontology, SJSU, Dept. Geology



Note: If you wish to print these slides, you will need to first delete the image of the Phanerozoic Diversity curve on Slides 3 and 6 (for some reason, they cause PowerPoint to crash when trying to print).

what determines the boundaries
What Determines the Boundaries?
  • Slide shows the geological time scale.

By: “Dragons flight” (Wikimedia Commons)

mass extinctions
Mass Extinctions
  • It is thought that over 4 billion species have lived on planet Earth, but 99% of them are now extinct.
  • Extinction is a fact of life; typically balanced by speciation.
  • Mass extinctions are characterized by the following attributes:
    • >75% of species go extinct.
    • Global events.
    • Extinctions in variety of habitats.
the big 5
The “Big 5”

Data from Barnosky et al. (2011)

the big 5 or 6 mass extinctions
The “Big 5” (Or, 6?) Mass Extinctions
  • Slide shows the geological time scale.

By: “Dragons flight” (Wikimedia Commons)

the end permian mass extinction
90% marine species (80% genera, 50% families) go extinct.

Trilobites,tabulate and rugose corals, many types of brachiopods, many echinoderms, etc.

75% families of land vertebrates.

Many plants.

“Today’s oceans still reflect the winners and losers of events at the end of the Permian” (Erwin, 2006).

Slide shows a photograph of a fossil tabulate coral.

The End-Permian Mass Extinction

Tabulate Coral

what caused it
Short answer: scientists are not sure.

Any hypothesis must explain extinction patterns in the sea and on land.

Many scientists have looked for evidence of an asteroid or comet impact at the Permian-Triassic boundary, but have not produced conclusive results.

Siberian flood basalts (called the “Siberian Traps”) may be a likely culprit.

What Caused It?
siberian flood basalts
Slide shows a map reconstruction of Pangea, detailing the position of the Siberian Traps and their extent.

Siberian Flood Basalts

Erupted in region that is now Siberia from about 252 to 251 Ma – almost coincident with P/T boundary (251 Ma).

Mass of erupted magma equal to extent of continental U.S. – 4 million cubic kilometers.


Source: Ron Blakey, NAU Geology

siberian flood basalts1

Siberian Flood Basalts

Possible consequences:

  • Dust clouds.
  • Acid rain.
  • Massive volcanic release of CO2.
  • Global warming (+ 6º C). As temp. increases, O2 becomes less soluble in water, resulting in anoxic (oxygen-free) conditions in shallow water - may have driven marine extinction.
  • Warming may have also caused melting of frozen gas hydrates in the oceans, releasing substantial amounts of methane, another greenhouse gas.
  • Runaway greenhouse effect?
theories about dinosaur extinction
Theories About Dinosaur Extinction

Many ideas have been put forth to explain the demise of the dinosaurs at the end of the Cretaceous:

  • Global cooling.
  • Global warming.
  • Disease.
  • Inability to digest flowering plants.
  • Mammals ate their eggs.
  • They developed cataracts, went blind, and fell off cliffs.

However, any theory seeking to explain dinosaur extinction must also explain other patterns of extinction at the end of the Cretaceous period.

patterns of end cretaceous extinction
Slide shows photographs of ammonoid fossils and rudist bivalve fossils.

Extinction on land:

Non-avian dinosaurs.


Much angiosperm plant life.

Extinction in the sea:




Most marine plankton.

Many invertebrate groups.

Patterns of End-Cretaceous Extinction


iridium and the end cretaceous boundary

Iridium and the End Cretaceous Boundary

In the late 1970’s, it was discovered that a clay layer (marine) spanning the K-T boundary in Italy had extremely high amounts of the metal iridium.


Rare platinum-group metal.

Does not normally occur in Earth’s crust.

Occurs in trace amounts in extraterrestrial rocks and the interior of the earth.

iridium and the end cretaceous
Iridium and the End Cretaceous

In 1980, Alvarez and colleagues proposed that a bolideimpact was responsible for the iridium anomaly.

Iridium anomaly was subsequently found in over 100 K-T boundary sections (marine and terrestrial) from around the world.

Other K-T boundary evidence for an impact includes:

  • Shocked quartz.
  • Microspherules.
  • As the evidence for a bolide impact gathered, some scientists went in search of an impact crater of the appropriate age (~65 Ma).
meteor crater arizona

Meteor Crater, Arizona

Meteor Crater, AZ:

~ 1.2 km diameter.

Meteorite responsible probably about 50 m in diameter.

Age of crater about 50,000 years old.

Photograph by Shane Torgerson (Wikimedia Commons)

the smoking gun
Slide shows an illustration that detail the position of the Chiczulub crater in Mexico.

Chicxulub Crater, Yucatan Peninsula, Mexico:

180 km diameter.

Close to 65 Ma.

The Smoking Gun?


a bad day for planet earth

A Bad Day For Planet Earth

The bolideimpactor:

Likely a meteor 10 km in diameter.

Impact likely released energy equivalence of about 100,000 gigatons of TNT.

Tsar Bomba = 1/20 of one gigaton (most powerful nuclear weapon ever tested).

Probably about a 13 on the Richter scale (2004 Indian Ocean tsunami earthquake about 9.1 on Richter scale; equivalent to about 100 gigatons of TNT).

Photograph by “Croquant” (Wikimedia Commons)



1 Fires.

  • Worst near impact site.

2. Tsunamis.

3. Dust fills atmosphere.

  • Perpetual night for months.
  • Collapse of photosynthesis.
  • Global refrigeration (nuclear winter).
  • Heat atmosphere by friction during descent.

4. Aerosols fill atmosphere.

  • Trap solar radiation.
  • Greenhouse warming.
  • Release of carbon dioxide.
    • Bolide hit carbonates.
    • Rapid melting!!
    • Temperature increase of 4.5-13.5 °C.
    • Increase lasted few hundred thousand years.
  • Acid Rain.
    • Caused by release of sulfur oxides when bolide hit evaporites.

Important questions remain.

For example, why did non-avian dinosaurs go extinct, but birds, lizards, crocodilians, turtles, amphibians, and mammals survive?

why does extinction matter

Why Does Extinction Matter?

  • Extinction marks the end of evolution for the lineage that goes extinct.
  • From an evolutionary perspective, the consequences of a single species going extinct may be minor for a group with many species.
  • The extinction of a species within a group with only a few species may be very significant, however.
gould 1984

Gould (1984)

“If mass extinctions … [are caused by agents] … so utterly beyond the power of organisms to anticipate, then life’s history either has an irreducible randomness or operates by new and undiscovered rules for perturbations, not (as we always thought) by laws that regulate predictable competition during normal times.”

– Stephen J. Gould (1984)

activity idea
Activity Idea
  • Ask your students (perhaps in groups) to develop hypotheses about what killed the dinosaurs.
  • Ask them how they would test their ideas. In other words, if their hypothesis is true, what observations should they expect to make in the fossil/rock record?
  • Then, present the scientific evidence for a bolide impact.
  • Other ideas?