Q12 - It has long been thought

[u2manish]

Can this question be done using formal logic. Since I am in the middle of understanding diagramming, I shall very much appreciate if some could help walk us through using diagramming..!

Cheers,
M

[ManhattanPrepLSAT1]

This question cannot be done with formal conditional logic. This is a Necessary Assumption question, and while some of these work by connecting conditional statements into a chain of reasoning, this one represents the other form of Necessary Assumptions - protecting the argument from something that would be devastating, were it true.

The conclusion is that lizards could not evolve from anthracosaurs. Why? Because the known lizard fossils predate any of the known anthracosaur fossils. Obviously, it doesn't make much sense for lizards to have evolved from anthracosaurs if lizards were roaming the earth before anthracosaurs.

But what if there were unknown anthracosaur fossils out there that predate any of the known lizard fossils? Then it could be the case that lizards evolved from anthracosaurs. Answer choice (A) protects the argument from such a situation, and is the correct answer.

Let's look at the incorrect answers:

(B) is irrelevant. Even if there were lizard fossils that predated 340 million years ago, that would still suggest that lizards did not evolve from anthracosaurs. Since the negation of answer choice (B) would not destroy the argument's reasoning, it's not assumption of the argument.
(C) is irrelevant. There could be some lizard fossils that predate some anthracosaur fossils, but this doesn't tell us anything about when the respective species emerged, and whether lizards could have evolved from anthracosaurs.
(D) again is too weak, because if the lizards did evolve from anthracosaurs, and the two species lived simultaneously, there would be some anthracosaur fossils that predate some lizard fossils. The important thing is that the known fossil records indicate the time when the respective species first emerged.
(E) would have been correct had it been about anthracosaur fossils whose age cannot be determined.

Hope that helps, and sorry if you were looking for a more elegant, "conditional logic" solution. it's just not the concept being tested here.

[cobyerez79]

I understand why A is correct, in just having a really hard time wrapping my head around why D is incorrect.
Any help would be greatly appreciated!

[rickytucker]

I understand why A is correct, in just having a really hard time wrapping my head around why D is incorrect.
Any help would be greatly appreciated!

You can use "one" in place of "some" to test its weakest link.

(D) There are no As that predate one L.

Here's a hypothetical timeline: LALLLLL

This means that ultimately the first L must predate the first A (A cannot be first), which may seem like an assumption that the argument relies upon. However, it's not an absolutely necessary assumption that rises to the level of "unstated premise". I know this because negating (D) says:

There are As that predate one L.

Here's a hypothetical timeline: LLLLLAL

Since the conclusion still stands ("lizards could not have evolved from anthracosaurs"), (D) is incorrect.

[ncesomonu]

Important question to resurrect this thread.

How would we negate answer choice A? More specifically, my question is, how would we negate these answer choices with the question stem that reads "An assumption made in the argument is that there are no.."

I'm a bit thrown off by the "no." Would I negate the statement to read, "An assumption made in the argument is that there are some unknown anth. fossils older than 340 million years?

Or would I negate it to read, "An assumption made in the argument is that there are no unknown anth. fossils that are not older than 340 mill. years?"

Any help would be much appreciated, thanks in advance!

[ManhattanPrepLSAT1]

Great question ncesomonu!

Your first approach is correct. Negate the "no" in the question stem to "some." Generally when you have quantifiers "none," "some," "most," or "all," they're a good place to negate (but you do have to be careful - it's never as simple as following a rule without deviation on the LSAT).

Hope that helps!

[ncesomonu]

thanks so much!

[Chanqvu]

More simply put, isn't answer choice D incorrect because it leaves the possibility open that there is an unknown anthracosaur fossil that predates some lizard fossil? That would go against the conclusion that lizards could move evolved from A's

[civnetn]

Quick question here about the negation test. Is it absolutely necessary? I started off using it, but I stopped and I honestly don't find it very useful. I've noticed a tendency for people to get lost in wrong answers simply because they rely too heavily on the negation test. It's like the blinders go up and all that matters is the negation test.

You only time when I can see the negation test being practical is when you get stuck on a really tough question. But even then, I think in order to successfully apply the test, you need to really understand the subtleties of the question, and if you do...you shouldn't need the negation test.

I mean to each his/her own, and I'm not saying it's useless but there are so many posts for necessary assumption questions where people are asking how to successfully negate answer choices, and why answer choices they did negate are wrong.

Once I stopped devoting so much time to negating answer choices I started to actually understand the stimulus more as well as the logic behind it and the answers came easier. I really think people rely too heavily on this tool and stop learning how to solve these types of problems as a result.

[MeenaV936]

D is wrong because it is too weak and says no "known" anthracosaur fossils that predated lizards -- meaning there could be some unknown anthracosaur fossils that predated lizards, meaning lizards still evolved from anthracosaurs. Also, this just repeats the premise that already says no KNOWN anthracosaur fossils have been found in rocks older than 300 million years.

A is correct because it specifically says there are no "unknown" anthracosaur fossils that are over 340 million years old, which closes the loophole and assures that every anthracosaur fossil ever has come after the lizard who was 340 million years old, ensuring that lizards could not have evolved from anthracosaurs.

[ohthatpatrick]

The author has explicitly stated that no known Anthra fossils are older than 300 million.
So some known Anthra fossils are probably 300 million years old.

That's pretty old. So I'm sure the author assumes that there ARE some known anthra fossils that predate some lizard fossils.

An anthra fossil from 300 millions years ago would predate any lizard fossil we found in the last 299 million years.

Since I assume the author would assume the opposite of (D), that's how I'd get rid of it.
Plus, the potential gap / room for objection in this argument is in the "Unknown", not the "known".