LSAT 143 | Section 4 | Logical Reasoning: Q21

LR Question 21 Explanation

QUESTION TEXT: In modern deep-diving marine mammals, such as whales…

QUESTION TYPE: Weaken

CONCLUSION: Ichthyosaurs were deep divers.

REASONING: Modern deep diving mammals have light, porous outer shells on their bones. This makes the mammals light enough to easily swim back up after a dive. And Ichthyosaurs had porous outer shells on their bones.

ANALYSIS: This argument sounds strong, but it’s actually very weak. It’s a cause and effect argument. The author failed to address an important question:

• Are there any known examples of animals with light, porous outer shells on their bones who aren’t deep divers?

The author is trying to imply a always true cause-effect relationship between the shells and deep diving. But it’s possible that the shells actually don’t always lead to deep diving.

___________

1. This would show that it’s possible to be a deep diver without a porous outer shell. This sounds like it weakens the argument.
    
   But that’s not what we’re trying to do. We’re instead trying to show that ichthyosaurs could have had porous shells yet not been deep divers.
2. This tells us nothing: we don’t know whether those reptiles are deep divers, or whether you could have porous shells on your bones and not be a deep diver.
3. CORRECT. This ruins the argument. If many species have a porous shell yet don’t deep dive, then a porous shell can’t be an indication of deep diving.
4. This doesn’t matter. It’s possible that whales are better suited to deep diving than Ichthyosaurs were. But that doesn’t mean Ichthyosaurs were not deep divers.
5. This strengthens the argument slightly. Light bones make an animal suited to deep diving. This fact increases the odds that ichthyosaurs were deep divers.