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Difficulty: Very hardStrengthen

Evolutionary Biologist: Fossils of the ancient mammal *Thalassocnus* reveal unusually dense, heavy bones. Some researchers contend this extreme density evolved as a defensive adaptation against blunt-force impacts from terrestrial predators. However, maintaining such bones requires an immense caloric intake rarely justified by defensive needs alone. Because dense bones drastically decrease buoyancy—a vital advantage for foraging aquatic plants in turbulent coastal waters—this skeletal density likely evolved primarily to facilitate underwater grazing rather than as a defense mechanism.

Which one of the following, if true, provides the strongest additional support for the evolutionary biologist's argument?

  1. A
    Biologists agree that any physical adaptation demanding an immense caloric intake will inevitably precipitate a species' extinction if its primary food source becomes scarce.
  2. B
    Fossilized *Thalassocnus* rib cages frequently exhibit healed fractures characteristic of surviving massive blunt-force trauma, a feature absent in known aquatic grazers.
  3. C
    While the proportion of dense-boned individuals within the *Thalassocnus* genus is uniquely high, the absolute total number of light-boned mammal fossils recovered nearby is significantly greater.
  4. Terrestrial herbivores that shared the exact same ecosystem as *Thalassocnus* and faced identical terrestrial predation pressures possessed remarkably light skeletal structures.Answer
  5. E
    Generating dense skeletal tissue does not actually demand a high caloric intake if an animal's primary diet consists of naturally calcium-rich aquatic vegetation.

Answer

The option stating that terrestrial herbivores sharing the exact same ecosystem and predation pressures possessed remarkably light skeletal structures.
The correct answer strengthens the argument by systematically undermining the competing hypothesis. By demonstrating that other species facing the exact same threat (terrestrial predators) did not evolve dense bones as a defense, it makes it far less likely that predation was the primary evolutionary driver for this extreme trait. Ruling out the primary alternative explanation is a powerful way to strengthen the author's preferred causal hypothesis (aquatic grazing).

Step-by-Step Solution

1
Identify the main conclusion.
The extreme bone density of Thalassocnus evolved primarily to facilitate underwater grazing rather than as a defense mechanism.
Understanding the precise claim being made is necessary to know what needs to be supported.
2
Identify the premises and the alternative explanation.
Premise 1: Dense bones are calorically expensive. Premise 2: Dense bones decrease buoyancy (good for aquatic grazing). Alternative hypothesis: Dense bones evolved as a defense against predators.
Strengthening an argument often involves validating its premises or systematically destroying alternative explanations.
3
Evaluate the options for their impact on the causal relationship.
The correct option shows that the purported alternative cause (predators) existed for other similar species without producing the purported effect (dense bones).
Showing that a competing cause failed to produce the effect in a control group effectively rules out that alternative, leaving the author's proposed cause as the most logical explanation.

Key Concept

Strengthening Causal Arguments by Eliminating Alternative Causes
Estimated Time:2m 0s
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