The Extinction and Survival of Dinosaurs

Learning Goals

• Define Mass Extinction: A mass extinction is characterized by significant global loss of biodiversity within a relatively short geological timeframe. Key criteria for recognition in the fossil record include:

  • A sharp decline in species numbers (typically >75%).
  • A rapid occurrence compared to normal background extinction levels.

• Support for Impact Event: There is substantial evidence supporting a catastrophic impact event at the end of the Cretaceous, marked by:

  • A distinct clay layer rich in iridium found in different parts of the world, notably at the K-T boundary.
  • Geological and fossil records indicating widespread devastation and losses.

• Consequences of Impact Events: Potential consequences of an impact event contributing to mass extinction include:

  • Immediate vaporization of nearby ecosystems.
  • Global forest fires and a 'nuclear winter' effect blocking sunlight for extended durations, which dramatically affects photosynthesis.
  • Tsunami formation impacting coastal habitats.
  • Long-term climatic shifts leading to "cold house" and "hot house" conditions.

• Disappearance of Terrestrial and Marine Vertebrates: Large terrestrial and marine vertebrates could have disappeared due to:

  • Habitat destruction from initial impact events.
  • Severe climate changes reducing viable ecosystems.
  • Competition with surviving fauna, particularly mammals that adapted to the altered environments.

Mass Extinction Classifications

• Major Extinction: 50% of families and 80-95% of species lost.
• Intermediate Extinction: 20-30% of families and 50% of species lost.
• Minor Extinction: 10% of families and 20-30% of species lost.

Cretaceous-Paleogene (K-PG) Extinction Event

• Marks the end of the Mesozoic Era; significant losses observed:
  • Over 50% of all species, with estimates exceeding 75%.
  • Terrestrial survivors generally weighed under 25 kg.
• Marine Losses:
  • Severe impacts on marine fauna, with losses of 80-90% of marine species, including ammonites and marine reptiles.

Extinction Hypotheses

• Diverse hypotheses regarding dinosaur extinction include:
  • Disease spread by biting insects.
  • Genetic issues and competition with mammals.
  • Environmental changes (e.g., falling sea levels, poisonous plant evolution).
  • The asteroid impact hypothesis as a primary cause.

Evidence of Impact Event

1. Iridium Layer:

   • High levels of iridium found in a clay layer just above K-T boundary, consistent with extraterrestrial origin.
   • Observations made by Walter and Luis Alvarez at sites like Gubbio, Italy.

2. Fern Spores:

   • Spike in fern spores indicates rapid colonization of fire-affected areas after the impact, suggesting global fires.

3. Soot Layers:

   • Soot deposits found concurrent with iridium layer evidencing extensive wildfires.

4. Tektites:

   • Natural glass formed from molten rock during impact, further supporting the extraterrestrial hypothesis.

5. Shocked Quartz:

   • Characteristic stress lines in quartz crystals (shock lamellae) serve as evidence of high-pressure impacts.

6. Tsunami Deposits:

   • Indications of widespread tsunami activity at the end of the Cretaceous, linked to the impact event.

Chicxulub Impact Crater

• Characteristics:
  • Measures approximately 180 km across located in the Gulf of Mexico.
  • Identified as a significant impact site linked to the K-PG extinction events.
  • Initial effects included forest fires, vaporization of nearby habitats, and tsunamis.

Long-term Effects of Impact

• Immediate Effects:

  • Uptake of dust and vapor reducing sunlight leading to a sudden halt in photosynthesis (nuclear winter).

• Environmental Changes:

  • Following the clearing of dust, greenhouse gases rise, leading to a long-lasting climate warming effect.
  • Initial cold conditions transitions into prolonged warmth (hot house).

Species Survival and Extinction Rates

• Examining the survival rates by habitat:
  • Aquatic environments saw 100% – 90% extinctions.
  • Terrestrial habitats had lower survival rates (around 50% – 28% for various groups).
• Notably, smaller, ectothermic animals survived better than larger, endothermic dinosaurs.

Other Contributing Factors

• Deccan Traps: While the asteroid impact is a pivotal factor, volcanic activities (Deccan Traps in India) produced acid rain and climatic shifts contributing to extinction dynamics.
• The existing stress on ecosystems may have exacerbated extinction risks, with the impact acting as the final trigger ("nail in the coffin").

Conclusion

• The event leading to the extinction of non-avian dinosaurs was complex, with the K-PG impact event playing a crucial role but not acting alone. Environmental stressors, evolutionary pressures, and biological competition were critical to the extinction narrative.