LSAT Explanation PT 25, S2, Q12: It has long been thought that

LSAT Question Stem

An assumption made in the argument is that there are no 

Logical Reasoning Question Type

This is a Necessary Assumption question. 

Correct Answer

The correct answer to this question is A. 

LSAT Question Complete Explanation

Let's first summarize and analyze the argument in the passage. The passage states that it has been believed that lizards evolved from anthracosaurs, but no anthracosaur fossils have been found in rocks older than 300 million years. However, a lizard fossil was found that is estimated to be 340 million years old. The conclusion drawn from this is that lizards could not have evolved from anthracosaurs since lizards existed before anthracosaurs.

To simplify this, imagine a group of people thought that cellphones evolved from pagers. However, a cellphone was found that was older than any pager ever discovered. This would lead to the conclusion that cellphones could not have evolved from pagers since cellphones existed before pagers.

The structure of the argument is as follows:

1. Premise: No anthracosaur fossils have been found in rocks older than 300 million years.

2. Premise: A lizard fossil was found that is 340 million years old.

3. Conclusion: Lizards could not have evolved from anthracosaurs.

An "Evaluate" question for this argument could be: "Are there any unknown anthracosaur fossils older than 340 million years?"

Now, let's discuss the question type and the answer choices. The question type is Necessary Assumption, which means we need to find an assumption that is necessary for the argument to be valid.

a) unknown anthracosaur fossils older than 340 million years

This is the correct answer choice. The argument assumes that there are no unknown anthracosaur fossils older than 340 million years. If there were such fossils, it would weaken the conclusion that lizards could not have evolved from anthracosaurs.

b) unknown lizard fossils older than 340 million years

This answer choice is incorrect because it doesn't affect the conclusion. Even if there were older lizard fossils, it would still support the idea that lizards did not evolve from anthracosaurs since lizards would still exist before anthracosaurs.

c) known lizard fossils that predate some anthracosaur fossils

This answer choice is irrelevant. It doesn't provide any information about when the respective species emerged, and whether lizards could have evolved from anthracosaurs.

d) known anthracosaur fossils that predate some lizard fossils

This answer choice is too weak. It leaves the possibility open that there is an unknown anthracosaur fossil that predates some lizard fossil, which would go against the conclusion that lizards could not have evolved from anthracosaurs.

e) known lizard fossils whose age is uncertain

This answer choice would have been correct if it were about anthracosaur fossils whose age cannot be determined. However, since it's about lizard fossils, it doesn't affect the conclusion of the argument.

In conclusion, the correct answer is choice A, as it addresses the necessary assumption that there are no unknown anthracosaur fossils older than 340 million years, which supports the argument's conclusion that lizards could not have evolved from anthracosaurs.