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Difficulty: Very hardInferences from the Text

Early paleontologists assumed that the exceptional preservation of soft tissues in the Burgess Shale required catastrophic anoxic (oxygen-depleted) bottom waters that instantly killed and protected the organisms from scavengers. However, modern taphonomic analysis reveals a more complex picture. While anoxia inhibits macroscopic scavengers, it does not arrest microbial decay, which can obliterate soft tissues within days. The true preservation catalyst in Burgess Shale-type deposits was the rapid precipitation of authigenic minerals—specifically clay-mineral films—onto the organism's surface immediately upon burial. This mineralization process was uniquely favored by the specific ocean chemistry of the Cambrian period, which possessed unusually high concentrations of dissolved silica and iron prior to the evolutionary advent of silica-secreting organisms like sponges and radiolarians that later depleted these oceans. Consequently, the later disappearance of Burgess Shale-type preservation in the fossil record is likely attributable not to a cessation of sudden burial events, but rather to a fundamental shift in marine geochemical baselines driven by biological innovation.

Based on the passage, which of the following inferences are most strongly supported? Select all that apply.

  1. The evolutionary proliferation of sponges and radiolarians negatively impacted the potential for soft-tissue fossilization in marine environments.Answer
  2. B
    Catastrophic burial in anoxic bottom waters is insufficient to prevent the destruction of soft-bodied organisms by macroscopic scavengers.
  3. If the evolutionary appearance of silica-secreting organisms had been significantly delayed, Burgess Shale-type deposits might be found in post-Cambrian geological strata.Answer
  4. D
    The unusually high concentrations of dissolved silica and iron in Cambrian oceans were a direct consequence of the lack of oxygen in the bottom waters.
  5. E
    Organisms preserved in the Burgess Shale were fossilized in part because they possessed unique physiological adaptations that rendered their soft tissues unusually resistant to microbial decay.

Answer

The correct statements are that the proliferation of silica-secreting organisms negatively impacted soft-tissue fossilization, and that a delay in their evolution might have allowed such deposits to form in later eras.
The valid inferences synthesize the passage's causal logic: high silica enabled preservation, and the evolution of silica-secreting organisms removed that silica. Therefore, those organisms hindered preservation, and their absence would logically have prolonged the period during which such preservation could occur.

Step-by-Step Solution

1
Analyze the causal chain regarding preservation presented in the text.
Preservation requires rapid mineral precipitation, which requires high dissolved silica/iron. High silica was present before silica-secreting organisms evolved and depleted it.
Mapping causal relationships is essential for evaluating complex inferences in reading comprehension.
2
Evaluate the first correct inference based on the established causal chain.
Since silica-secreting organisms depleted the necessary minerals, their proliferation directly caused the end of this preservation type.
This directly confirms the validity of the inference regarding their negative impact on fossilization.
3
Evaluate the counterfactual inference.
If the cause of the disappearance (silica-secreting organisms) had been delayed, the effect (loss of Burgess-type preservation) would also likely have been delayed into post-Cambrian strata.
Counterfactual reasoning based on stated causal mechanisms is a standard LSAT inference pattern.

Key Concept

Identifying implicit causal relationships and evaluating hypothetical counterfactuals based on passage premises.
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Topics

Inferences from the Text

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