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Difficulty: Very hardAssumptions in Causal Arguments

Paleontologists have excavated a dense concentration of Bactrosaurus fossils in a single geological stratum dating to a period immediately following a colossal volcanic eruption. Volcanic ash contains extremely high levels of fluorine, and ingesting large quantities of fluorine causes acute, fatal bone toxicity in large herbivores. Based on this evidence, the paleontologists concluded that these dinosaurs perished from fluorine poisoning contracted by grazing on vegetation blanketed by the volcanic ash.

Which one of the following is an assumption on which the paleontologists' argument depends?

  1. A
    Any large herbivore that grazes on vegetation blanketed by volcanic ash will ingest fatal quantities of fluorine.
  2. B
    The Bactrosaurus had a specialized diet that consisted exclusively of the types of vegetation most heavily affected by the volcanic ash.
  3. C
    The concentration of fluorine in the bones of the excavated Bactrosaurus fossils is positively correlated with the volume of volcanic ash found in the stratum.
  4. The layer of volcanic ash did not completely bury the local vegetation to a depth that rendered it inaccessible to the Bactrosaurus.Answer
  5. E
    The volcanic eruption that deposited the ash was the most severe geological event to occur in the region during the era in which the Bactrosaurus lived.

Answer

The argument assumes that the volcanic ash did not completely bury the vegetation, leaving it accessible for the dinosaurs to graze upon.
The argument concludes that the dinosaurs died specifically by grazing on vegetation covered in volcanic ash. For this causal sequence to be physically possible, the vegetation must have remained accessible after the eruption. If the 'colossal' eruption completely buried the plants under feet of ash (the negation of this option), the dinosaurs could not have grazed on them, and the argument's specific causal mechanism completely fails. Therefore, assuming the plants were not rendered completely inaccessible is strictly necessary.

Step-by-Step Solution

1
Identify the conclusion of the argument.
The Bactrosaurus perished from fluorine poisoning contracted by grazing on vegetation blanketed by the volcanic ash.
Isolating the conclusion is necessary to understand the exact causal mechanism proposed.
2
Identify the premises supporting the conclusion.
Fossils were found after a colossal eruption; ash contains fluorine; large quantities of fluorine cause fatal bone toxicity.
Premises establish the correlation and the biological mechanism of the toxin.
3
Locate the logical gap or vulnerability in the reasoning.
The argument assumes a practical link between the ash falling and the dinosaurs eating the ash-covered plants.
The conclusion relies on the physical act of 'grazing on vegetation blanketed by ash'. If the 'colossal' eruption buried all plants, this act would be physically impossible.
4
Evaluate the options using the Negation Technique.
Negating the correct option yields: 'The ash completely buried the vegetation, rendering it inaccessible.' This destroys the conclusion, confirming it is a necessary assumption.
A necessary assumption must be fundamentally true for the argument to survive; negating it proves its absolute necessity.

Key Concept

Identifying necessary assumptions by verifying the logistical viability of a proposed causal mechanism.
Estimated Time:2m 0s
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Assumptions in Causal Arguments

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