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Difficulty: HardMulti-Sentence Synthesis Inferences

Passage:
For decades, paleontologists hypothesized that the diversification of Early Cambrian metazoans was driven primarily by a sudden surge in atmospheric oxygen concentration, which enabled higher metabolic rates and larger body sizes. Proponents of this oxygen-limitation hypothesis frequently cited geochemical proxy data indicating a pronounced positive carbon isotope excursion near the Proterozoic–Cambrian boundary as evidence of increased organic carbon burial and concomitant oxygen accumulation.

However, recent high-resolution isotopic analyses of sedimentary strata in the Yangtze Platform reveal a crucial structural pivot: the carbon isotope anomaly was not globally uniform, but localized exclusively to shallow continental shelves. Furthermore, trace element ratios in contemporaneous deep-water black shales demonstrate persistent anoxic conditions in benthic environments throughout the interval of explosive metazoan diversification. These findings suggest that while shallow coastal surface waters underwent transient oxygenation, deep marine basins remained severely hypoxic. Consequently, early calcifying metazoans, whose fossil remains appear synchronously in both shallow and deep-water biofacies, could not have relied solely on global atmospheric oxygenation to trigger their morphological complexity. Instead, the spatial heterogeneity of oceanic oxygenation implies that local ecological pressures—such as predator-prey dynamics in shallow refugia—served as the primary catalysts for structural adaptation.

Based on the passage, which of the following statements regarding early calcifying metazoans can be most logically inferred?

  1. Their emergence in deep marine biofacies took place despite the lack of widespread oxygenation in benthic environments during that period.Answer
  2. B
    Their morphological diversification was triggered entirely by the transient surge in oxygen levels in shallow coastal surface waters.
  3. C
    They developed calcified exoskeletons primarily as a physiological mechanism to isolate toxic trace elements found in deep-water black shales.
  4. D
    They experienced rapid evolutionary adaptation because global atmospheric oxygen levels reached modern atmospheric concentrations at the Proterozoic–Cambrian boundary.
  5. E
    They were unable to inhabit shallow continental shelves until after deep marine basin waters achieved stable oxygenation.

Answer

Early calcifying metazoans appeared in deep marine biofacies despite the absence of widespread oxygenation in benthic environments during that interval.
The passage states in the second paragraph that trace element ratios in deep-water black shales prove benthic environments remained severely hypoxic. Two sentences later, the text notes that fossil remains of early calcifying metazoans appeared synchronously in both shallow and deep-water biofacies. Combining these two non-contiguous facts logically proves that these organisms emerged in deep-water environments despite the absence of widespread oxygenation there.

Step-by-Step Solution

1
Identify key premises regarding deep-water environments.
Sentence 4 notes that trace element ratios in deep-water black shales demonstrate persistent anoxic (hypoxic) conditions in benthic environments throughout the metazoan diversification period.
Establishing environmental conditions in deep water is required for synthesizing multi-sentence evidence.
2
Locate explicit passage evidence regarding early calcifying metazoans.
Sentence 6 explicitly states that fossil remains of early calcifying metazoans appear synchronously in both shallow and deep-water biofacies.
Connecting the organism's distribution to the benthic environment conditions allows drawing a valid synthesis inference.
3
Synthesize the non-contiguous premises to form the inference.
Since benthic environments were persistently anoxic/hypoxic, yet early calcifying metazoans appeared synchronously in deep-water biofacies, their emergence in those deep-water settings occurred despite the lack of pervasive benthic oxygenation.
Combines environmental data from paragraph 2 with organism fossil data to evaluate the options strictly within passage bounds.

Key Concept

Multi-Sentence Synthesis Inferences
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Multi-Sentence Synthesis Inferences

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