LSAT 17 | Section 2 | Logical Reasoning: Q20

LR Question 20 Explanation

QUESTION TEXT: Until about 400 million years ago, fishes-the first true…

QUESTION TYPE: Must be True

FACTS: Here’s a summary of what we know.

1. Fishes were jawless until 400 million years ago.
2. Jawless fish could only suck in plankton or suck in food from mud.
3. Jaws allowed fish to pursue prey and bite it.
4. Fish then developed into two groups. The first kept cartilage for its skeleton. Sharks and rays are an example.
5. The other group developed bone.
6. Teleosts developed from the latter group. They are very diverse.

ANALYSIS: It’s worth reading this sort of question twice. If you are clear on what it says then the right answer will jump out at you. If you’re not clear then you’ll waste a lot of time stuck on wrong answer choices.

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1. We have no idea what jawed fish eat. We just know that they can pursue “prey.”
2. CORRECT. This must be true. Jawless fish were limited to plankton or muddy food particles.
3. They might. We know jawless fish had to. But even fish with jaws might like to eat mud. Yum.
4. The stimulus strongly indicates that they did. It says that sharks “retained” cartilage. This implies they had it before.
5. We don’t know if jawless fish ever became extinct. We only know that 400 million years ago is when jawed fish first emerged.

Recap: The question begins with “Until about 400 million years ago, fishes-the first true”. It is a Must be True question. Learn more about LSAT MBT questions in our guide to LSAT Logical Reasoning question types.