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Difficulty: HardNecessary Assumption

Paleontologist: Some researchers conclude that *Drakosuchus*, a large Late Cretaceous crocodilian, was a fully marine reptile because its fossils were discovered in deep-ocean sedimentary deposits alongside marine fossils. However, *Drakosuchus* must have actually been a freshwater predator whose remains were swept out to sea after death. This is because the *Drakosuchus* bones exhibit severe surface pitting, a specific chemical degradation pattern known to occur when the bones of freshwater reptiles are exposed to high-salinity water for extended periods post-mortem.

The paleontologist's argument relies on which one of the following assumptions?

  1. The specific chemical degradation pattern observed on the *Drakosuchus* bones is not also produced on the bones of marine reptiles after extended post-mortem exposure to saltwater.Answer
  2. B
    No fully marine reptiles from the Late Cretaceous possessed bones with a chemical composition susceptible to any form of surface pitting.
  3. C
    The marine fossils found alongside the *Drakosuchus* remains were not themselves transported into a freshwater deposit by powerful tidal forces.
  4. D
    Most freshwater reptiles whose remains are swept into saltwater environments exhibit this specific chemical degradation pattern on their bones.
  5. E
    The river systems in the region during the Late Cretaceous had currents strong enough to transport the remains of large predators into marine environments.

Answer

The assumption that the specific chemical degradation pattern observed is not also produced on the bones of marine reptiles.
The argument concludes that *Drakosuchus* was a freshwater predator based exclusively on the presence of a specific bone pitting pattern known to occur when freshwater reptiles are exposed to saltwater. However, this evidence only points to a freshwater origin if marine reptiles do NOT also develop this exact same pattern. If marine reptiles also experienced this chemical degradation in saltwater, the pitting would be useless for distinguishing a freshwater reptile from a marine one. Therefore, the argument necessarily assumes that this pattern is not also produced on marine reptile bones. By applying the Negation Test, if we assume the pattern DOES also occur on marine reptiles, the paleontologist's conclusion completely collapses.

Step-by-Step Solution

1
Deconstruct the argument to isolate the conclusion and premises.
Conclusion: *Drakosuchus* was a freshwater predator. Premise: Its bones show a specific pitting pattern known to occur when freshwater reptile bones are exposed to saltwater.
Identifying the structural components is the first step in revealing the logical gap between the evidence and the conclusion.
2
Identify the logical gap in the paleontologist's reasoning.
The author assumes that because the pitting occurs in freshwater reptiles, its presence exclusively proves a freshwater origin.
If the pattern is not exclusive to freshwater reptiles, the evidence fails to support the definitive conclusion.
3
Apply the Negation Test to the correct option.
If the pattern DOES also occur on marine reptiles in saltwater, finding it on *Drakosuchus* provides no proof that the animal was a freshwater predator, completely destroying the argument.
A necessary assumption is one that, if proven false, causes the argument's conclusion to logically collapse.

Key Concept

Identifying Necessary Assumptions using the Negation Test (Defender model)
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Topics

Necessary Assumption

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