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Difficulty: HardIdentifying Underlying Assumptions and Premises

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Cretaceous-Paleogene Extinction Theories

Two scientists discuss the primary cause of the Cretaceous-Paleogene (K-Pg) extinction event, which occurred approximately 66 million years ago.

Scientist 1

The extinction of non-avian dinosaurs and many other species was triggered by the impact of a 10-kilometer-wide asteroid. This impact released a massive dust cloud and sulfur aerosols into the stratosphere, blocking sunlight for several years. This caused a global winter and halted photosynthesis, leading to a sudden collapse of terrestrial and marine food webs. The presence of a global iridium-rich clay layer precisely at the K-Pg boundary and the Chicxulub impact crater in Mexico support this theory. The suddenness of the extinction matches the immediate catastrophic aftermath of an impact.

Scientist 2

The extinction was a gradual process driven by the eruption of the Deccan Traps, a massive volcanic province in modern-day India. Over a span of 800,000 years surrounding the boundary, these eruptions released millions of cubic kilometers of lava, along with enormous quantities of carbon dioxide (CO2CO_2) and sulfur dioxide (SO2SO_2). The resulting volatile emissions caused severe climate fluctuations, including periods of intense global warming and cooling, acid rain, and ocean acidification. This prolonged environmental instability degraded habitats, steadily driving species to extinction before the asteroid impact, which was merely a minor factor.

Question

Scientist 1's hypothesis regarding the primary cause of the Cretaceous-Paleogene extinction relies on which of the following implicit assumptions?

  1. A
    Volcanic emissions over hundreds of thousands of years caused severe habitat degradation prior to the impact.
  2. B
    A global layer of clay enriched with iridium exists at the Cretaceous-Paleogene boundary.
  3. Ecosystems and food webs at the end of the Cretaceous period were relatively stable and healthy prior to the asteroid impact.Answer
  4. D
    Dust and sulfur aerosols are the only atmospheric components that can block solar radiation.

Answer

Ecosystems and food webs at the end of the Cretaceous period were relatively stable and healthy prior to the asteroid impact.
The correct answer states that ecosystems and food webs were relatively stable and healthy prior to the asteroid impact. For Scientist 1's hypothesis to hold—that the asteroid impact was the primary trigger of a sudden collapse of food webs—it must be assumed that those food webs were not already collapsing or in severe terminal decline due to another cause, such as the Deccan Traps volcanism. If they were already failing, the asteroid would have been a minor contributing factor rather than the primary cause.

Step-by-Step Solution

1
Identify the core claim of Scientist 1.
Scientist 1 claims that the K-Pg extinction was sudden, triggered primarily by an asteroid impact that caused sunlight blockage and food web collapse.
Understanding the primary claim is necessary to determine what unstated premises must support it.
2
Analyze the role of pre-impact conditions in Scientist 1's argument.
For the asteroid to be the primary cause of a sudden collapse, the ecosystem must not have been already failing due to other long-term factors.
If ecosystems were already terminally declining, the asteroid would only be a minor secondary factor, which directly contradicts Scientist 1's primary assertion.
3
Differentiate between explicit evidence and implicit assumptions.
The iridium layer and crater are explicit evidence, whereas the healthy pre-impact status of the biosphere is an unstated, required premise.
This confirms the correct option represents an assumption rather than stated evidence.

Key Concept

Identifying Implicit Assumptions and Premises
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Topics

Identifying Underlying Assumptions and Premises

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