Passage Summary: Most experts think these ancient sea monsters were lurkers, but because their fins look like the wings of long-distance birds, they were probably long-distance hunters.

Conclusion: Plesiosauromorphs likely hunted by pursuing their prey over long distances rather than using ambush tactics.

Reasoning: The fins of these marine reptiles were long and thin, which is a physical trait shared by birds that are specialized for long-distance flight.

Analysis: This argument relies on an analogy between the anatomy of birds and the anatomy of marine reptiles. For this comparison to hold weight, the author must assume that similar physical structures (long, thin appendages) serve the same functional purpose (long-distance travel) across different species and environments. To find the necessary assumption, ask yourself: 'If long, thin fins didn't actually help with long-distance swimming, would the argument fall apart?' The answer is yes, so look for a choice that validates that functional link.