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.