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Difficulty: HardIdentifying Necessary Assumptions

Paleontologist: The fossilized remains of the newly discovered dinosaur G. antiquus reveal unusually large eye sockets compared to its contemporary relatives. Because exceptionally large eyes are typically an evolutionary adaptation for maximizing visual acuity in low-light conditions, G. antiquus must have been a nocturnal species that hunted exclusively during the night.

Which one of the following is an assumption required by the paleontologist's argument?

  1. A
    Any prehistoric species that possessed exceptionally large eye sockets used them primarily to hunt exclusively at night.
  2. B
    The primary prey species hunted by G. antiquus were active only during the nighttime hours.
  3. The natural habitats occupied by G. antiquus did not feature perpetually low-light conditions during the daytime.Answer
  4. D
    G. antiquus possessed the largest eye sockets of any dinosaur species living during its evolutionary era.
  5. E
    Most dinosaur species that hunt exclusively during the night evolve unusually large eye sockets compared to their diurnal relatives.

Answer

The necessary assumption is that the natural habitats occupied by G. antiquus did not feature perpetually low-light conditions during the daytime.
For the paleontologist's conclusion to be logically sound, it must be true that the 'low-light conditions' requiring large eyes were exclusively nocturnal. If the dinosaur's daytime environment was also dark (such as in a deep oceanic trench or a dense, sun-blocking forest canopy), the large eyes could simply be an adaptation for daytime hunting in a dark environment. The argument fundamentally relies on ruling out these alternative sources of low light, which makes this statement a necessary 'defender' assumption. When negated, it destroys the author's argument.

Step-by-Step Solution

1
Deconstruct the argument to identify the core premises and main conclusion.
Premises: G. antiquus has unusually large eyes. Large eyes are typically an adaptation for low-light conditions. Conclusion: G. antiquus must have been nocturnal.
Understanding the exact structural relationship between evidence and conclusion is the first step in finding the logical gap.
2
Identify the conceptual shift or gap between the evidence and the conclusion.
The author equates 'low-light conditions' strictly with 'the night', ignoring other potential sources of low light.
Necessary assumptions often serve to rule out alternative explanations that could bridge the gap between premises and conclusion differently.
3
Evaluate the answer choices using the Negation Test.
Negating the correct answer yields: 'The habitats occupied by G. antiquus DID feature perpetually low-light conditions during the daytime.'
The Negation Test is the definitive method for identifying necessary assumptions; if the negated version of a choice destroys the argument, that choice is strictly necessary.
4
Determine the impact of the negated correct answer on the argument's validity.
If the daytime environment was dark, the large eyes would be useful during the day. Therefore, large eyes do not prove the dinosaur was nocturnal, and the conclusion collapses.
Confirming that the argument falls apart proves the original statement was a mandatory logical pillar for the author's reasoning.

Key Concept

Identifying Necessary Assumptions (Ruling Out Alternative Explanations)
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Identifying Necessary Assumptions

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