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Difficulty: MediumArgument Analysis and Evaluating Claims

Paleobiologists studying hydrothermal vent communities have long debated when tubeworms established their obligate endosymbiosis with sulfur-oxidizing bacteria. Traditionally, scholars posited that this relationship formed during the late Cretaceous period, relying on fossilized seafloor mineral tubes that exhibit microscopic structural striations characteristic of modern tubeworm tissue. However, recent genomic sequencing reveals that the bacterial metabolic pathways for sulfur oxidation diverged much earlier, during the early Jurassic. To reconcile this, researchers argue that the Cretaceous mineral tubes represent a later evolutionary adaptation to mineralized fluid chimneys, and conclude that deep-sea vent ecosystems were colonized by symbiotic tubeworms during the early Jurassic.

Which of the following, if true, would most seriously weaken the researchers' argument regarding the timing of deep-sea vent colonization?

  1. A
    Modern hydrothermal vents host fluid flows with significantly higher sulfide concentrations than those estimated to have existed during the early Jurassic.
  2. B
    Recent oceanic surveys have discovered several previously uncharacterized species of non-symbiotic tubeworms inhabiting shallow volcanic reefs.
  3. The genomic divergence of sulfur-oxidizing metabolic pathways occurred in free-living ancestral bacteria inhabiting shallow coastal waters long before those bacteria entered into symbiotic relationships with deep-sea tubeworms.Answer
  4. D
    Fossilized Cretaceous seafloor deposits display a higher ratio of heavy sulfur isotopes than early Jurassic marine sediment layers.
  5. E
    Microscopic structural striations in fossilized mineral tubes are occasionally obscured by post-depositional mineral crystallization.

Answer

The argument is most weakened by the finding that the genomic divergence of sulfur-oxidizing metabolic pathways occurred in free-living ancestral bacteria in shallow waters prior to their symbiosis with tubeworms.
The correct answer undermines the fundamental assumption upon which the researchers' conclusion rests. The researchers infer that tubeworms colonized deep-sea vents in the early Jurassic because their bacterial endosymbionts underwent genomic divergence at that time. However, if that bacterial divergence took place while the bacteria were still free-living organisms in shallow coastal waters—prior to forming any symbiotic association with tubeworms—then the timing of the bacterial genetic shift provides no evidence whatsoever regarding when deep-sea vent colonization took place.

Step-by-Step Solution

1
Identify the main conclusion and underlying evidence.
The researchers conclude that deep-sea vent colonization occurred during the early Jurassic based on the genomic divergence timing of sulfur-oxidizing bacteria.
Evaluating an argument requires isolating the gap between the premises (bacterial genomic divergence date) and the conclusion (tubeworm vent colonization date).
2
Determine the unstated assumption linking premise to conclusion.
The researchers assume that the genomic divergence of the bacteria occurred specifically in the context of, or simultaneously with, the establishment of tubeworm vent symbiosis.
If the bacterial trait evolved independently before symbiosis, the bacterial clock cannot be used to date the tubeworms' vent colonization.
3
Evaluate the answer choices for a factor that undermines this assumption.
Showing that the bacteria evolved sulfur oxidation while free-living in shallow waters severes the connection between the genomic date and deep-sea vent colonization.
This provides a direct counter-explanation that invalidates using the early Jurassic bacterial divergence as proof of tubeworm vent colonization.

Key Concept

Evaluating Claims and Unstated Assumptions
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Topics

Argument Analysis and Evaluating Claims

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