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Difficulty: EasyStrengthening or Weakening Author Assertions

In marine paleontology, determining the anatomical functions of unpreserved soft tissues presents a recurring challenge. Early ichthyosaurs, sleek marine reptiles of the Mesozoic era, lacked bony supports for a dorsal fin, leading early researchers to assume they swam without one. However, exceptional fossil discoveries in limestone deposits revealed soft-tissue outlines showing a well-developed dorsal fin.

Dr. Elena Vance asserts that this dorsal fin evolved specifically as a hydrodynamic stabilizer to prevent rolling motions during rapid, pursuit-style hunting of fast prey. Vance points to fluid dynamics simulations indicating that a dorsal structure reduces turbulence along the upper trunk at high speeds. Critics, however, argue that thermal management was a more critical evolutionary pressure for large pelagic reptiles operating in surface waters exposed to solar radiation. Vance counters this by highlighting that ichthyosaur skin layers show dense pigmentation that would reflect radiation, rendering internal cooling fins unnecessary. Thus, Vance maintains that hydrodynamic stabilization remains the primary driver explaining dorsal fin development.

Which of the following discoveries, if true, would most weaken Dr. Vance's assertion regarding the primary function of the ichthyosaur dorsal fin?

  1. A
    Hydrodynamic testing of physical models demonstrates that removing the dorsal fin increases rolling instability by forty percent during high-speed swimming maneuvers.
  2. Microscopic analysis of ichthyosaur dorsal fins shows a dense network of blood vessels that expanded exclusively during periods of elevated surface water temperatures.Answer
  3. C
    Limestone formations from the same geological era contain fossilized remains of ancient predatory fish with similar dorsal fin structures.
  4. D
    Fluid dynamics computer models used in Dr. Vance's study were originally created to analyze modern dolphin species.
  5. E
    Chemical analysis confirms that skin pigmentation in Mesozoic marine reptiles provided effective protection against ultraviolet radiation.

Answer

The statement showing that dorsal fins contained dense vascular networks that expanded during ocean warming weakens the claim by establishing thermoregulation as the primary evolutionary driver.
The correct answer directly undercuts the author's main assertion by offering empirical evidence that the dorsal fin operated primarily as a thermoregulatory vessel system. If the fin's tissue structure adjusted specifically to water temperature changes, thermal management—not hydrodynamic stabilization—was the primary evolutionary purpose.

Step-by-Step Solution

1
Identify the author's target assertion
Dr. Vance asserts that the primary evolutionary driver of the ichthyosaur dorsal fin was hydrodynamic stabilization during fast pursuit hunting.
To weaken an assertion, one must isolate the exact conclusion and its underlying cause-and-effect relationship.
2
Evaluate potential counter-evidence
Finding that the fin's internal structure was optimized for heat dissipation during warm periods demonstrates an alternative primary function (thermoregulation).
Demonstrating that an anatomical feature served a different primary adaptive purpose invalidates the claim that stabilization was the sole main driver.

Key Concept

Weakening an Author Assertion via Alternative Primary Functions
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Topics

Strengthening or Weakening Author Assertions

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