Passage Summary: Scientists found evidence that an ancient reptile ancestor had whiskers. They reason that if it had whiskers, it probably had hair all over to stay warm. Since only warm-blooded animals really benefit from that kind of insulation, they conclude the reptile was warm-blooded.

Conclusion: The ancient reptile Thrinaxodon was likely a warm-blooded animal.

Reasoning: Evidence suggests the creature had whiskers, and if it had whiskers, it likely had a full coat of hair for insulation, which is a trait typically reserved for warm-blooded animals.

Analysis: This question asks you to identify the structural role of a specific sentence. The statement in question acts as an intermediate link in a chain of reasoning. It connects the physical evidence (whiskers) to a biological function (insulation), which then serves as the basis for the final conclusion about metabolism. In formal terms, it is a premise used to support the main conclusion by establishing a necessary connection between two physical traits.