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# Birds have been said to be descended from certain birdlike dinosaur

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Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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AbdurRakib wrote:
Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.

Which of the following is an assumption on which the argument relies?

(A) The birdlike dinosaurs have no living descendants.

(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.

(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.

(D) It could not have been the case that some birds were descended from one of the birdlike dinosaur species and other birds from another.

(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.

ID - CR03272
OG 2017 New Question

Birds and Dinosaurs

Step 1: Identify the Question

The word assumption in the question stem indicates that this is a Find the Assumption question.

Step 2: Deconstruct the Argument

Birds desc from B-like dinos?

C: Fossil = NO b/c B fossils older than B-like dino fossils

The author concludes that a certain theory cannot be true. As evidence, the author states that there are bird fossils even older than the oldest birdlike dinosaur fossils found. So birds must be descended from something other than birdlike dinosaurs?

Not necessarily. The author is assuming that the fact that older birdlike dinosaur fossils haven’t been found means that these fossils don’t exist. It’s possible that there were older birdlike dinosaurs but either their fossils haven’t been found yet or their bones didn’t survive as fossils for some reason.

Step 3: Pause and State the Goal

On Assumption questions, the goal is to find something that the author must believe to be true in order to draw the conclusion.

Step 4: Work from Wrong to Right

(A) The author’s argument does not assume anything about modern-day descendants of the birdlike dinosaurs.

(B) The argument is confined to the possible connection between birds and birdlike dinosaurs. Flightless dinosaurs, another category, are not at issue in the argument.

(C) CORRECT. The author does assume that there aren’t older fossils of the birdlike dinosaurs that just haven’t been found yet. Try the Negation Test: if there are birdlike dinosaur fossils that just haven’t been found yet, then the author’s argument is ruined.

(D) The author’s argument does not assume that all birds descended from the same birdlike dinosaur species. The author’s argument is open to the idea that there could have been multiple birdlike dinosaur species from which modern-day birds evolved.

(E) The argument is confined to rebutting the idea that birds descended from certain birdlike dinosaur species. It does not address whether the birds could have been descended from other dinosaur species with different structural features.

Argument Construction

Situation
Although birds have been said to be descended from birdlike dinosaurs, some bird fossils predate the earliest known birdlike dinosaur fossils.

Reasoning
What must be true in order for the premise that some bird fossils predate the earliest known birdlike dinosaur fossils to support the conclusion that birds are not descended from birdlike dinosaurs? The argument implicitly reasons that since the cited bird fossils predate the earliest known birdlike dinosaur fossils, they must be from birds that lived before the earliest birdlike dinosaurs, and which therefore could not have been descended from birdlike dinosaurs. This reasoning assumes that any birdlike dinosaurs that lived before the first birds would have left fossils that still exist. It also assumes that no undiscovered birdlike dinosaur fossils predate the cited bird fossils.

(A) The argument is only about whether birds are descended from birdlike dinosaurs. Whether birdlike dinosaurs have any living descendants other than birds is irrelevant.

(B) The argument is only about birds and birdlike dinosaurs. It is not about other types of dinosaurs that were not birdlike.

(C) Correct. If any undiscovered birdlike dinosaur fossils predate the cited bird fossils, then the latter fossils' age does not support the conclusion that birds are not descended from birdlike dinosaurs.

(D) The argument purports to establish that the relative ages of bird fossils and birdlike dinosaur fossils show that birds cannot be descended from any of the known birdlike dinosaur species. In doing this, it acknowledges multiple birdlike dinosaur species and leaves open the question of whether some birds may be descended from one such species and other birds from another such species.

(E) The argument does not claim that the known fossil record shows that birds cannot be descended from dinosaurs. It only claims that the record shows that they cannot be descended from the birdlike dinosaurs that shared their distinctive structural features.

Solution
Passage Analysis

Birds have been said to be descended from certain birdlike dinosaur species

Birds are believed to be the descendants of certain birdlike dinosaur species.

with which they share distinctive structural features.
The birds and those birdlike dinosaur species have common distinctive structural features.

The fossil record, however, shows that this cannot be so,
The fossil records show that the birds cannot have been descended from the birdlike dinosaur species.

since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.
And what does this fossil record show? Again a fact: The bird fossils that exist are dated much older than the earliest birdlike dinosaur fossils that have been found.

Conclusion: The birds could not have been descended from birdlike dinosaur species because existing bird fossils are much older than the earliest birdlike dinosaur fossils that have been found.

Pre-thinking
Falsification Question

In what scenario would it be possible for birds to have been descended from certain birdlike dinosaur species?
Given that -> Birds are believed to be the descendants of certain birdlike dinosaur species
-> The birds and those birdlike dinosaur species have common distinctive structural features

-> The bird fossils that exist are dated much older than the earliest birdlike dinosaur fossils that have been found

Thought Process

Let us look at the author’s reasoning. He starts with a general belief that birds are descendants of certain birdlike dinosaur species with the two having common structural features. Then he sets out to disprove it by using comparing the age of the existing fossils of both. The fossils of birds are found to be much older than the fossils of the birdlike dinosaur species. This proves the dinosaur species could not have preceded the birds. And this is how the author disproves the general belief.

Falsification condition

What if there could exist fossils of bird-like dinosaur species that are older than the bird fossils, but they have not been found yet?

In that case, once such fossils are discovered, it will become possible to prove that the birds could have been descended from birdlike dinosaur species.

Assumption

Whatever fossils of bird-like dinosaur species that are older than the bird fossils that can exist have all been found and none remains to be further discovered.

A
Does this option break down my conclusion? Even if there were descendants of the birdlike dinosaurs, the conclusion still holds true that birds were not the descendants of the dinosaurs as older bird fossils have been found.

Hence, this is not the answer.

B
In the argument we were concerned only with birds and the structural features they had in common with the birdlike dinosaur species. Bringing in flightless dinosaur species is irrelevant to the discussion.

Hence, this is not the correct answer.

C
This option means that no birdlike dinosaur fossils that are older than the bird fossils remains to be found.

This is in line with our pre-thinking assumption. It clearly puts the reasoning in place. If there is a possibility of the discovery of birdlike dinosaur fossils that are older than the bird fossils, then our conclusion will break down

Hence, this is the correct answer.

D
This option means that it is not possible that some birds descended from a particular dinosaur species and other birds descended from another dinosaur species.

Let us negate this option to see if it breaks down my conclusion. Say some birds were descended from one of the birdlike dinosaur species and other birds from another. Does this weaken my conclusion? No. Because there are bird fossils that have been found and they are much older than the dinosaur fossils. And in the passage, we are talking about birds in general.

Hence, this is not the correct answer choice.

E
This choice says that it is not possible that birds descended from a species of dinosaur with which the birds did not have distinctive structural features in common.

Again, this does not break down my conclusion. We do not know whether this dinosaur species with which the birds do not share distinctive structural features existed earlier or after the birdlike dinosaurs with which they shared features. If they existed earlier, then only my conclusion would break down but we do not know.

Hence, this is not the correct answer choice.­
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils much older than the earliest birdlike dinosaur fossils that have been found.

Pre- thinking - Because birdlike dinosaur fossils that are older that the bird fossils have not been found , it does not mean such birdlike dinosaur did not exist before birds .

Which of the following is an assumption on which the argument relies?

A) The birdlike dinosaurs have no descendants. Out of scope
B) There are no flightless dinosaur species that have the distinctive structural features shred by birds and birdlike dinosaurs. Out of scope
C) There are no birdlike dinosaur fossils that are older that the bird fossils but have not yet been unearthed. Correct - if we negate this statement , then the argument falls apart .
D) It could not have been the case that some birds were descendant from one of the bird like dinosaur species and other birds from another. Incorrect
E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features. Incorrect

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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils much older than the earliest birdlike dinosaur fossils that have been found.

Which of the following is an assumption on which the argument relies?

PRETHINKING: DINOSAUR CAME LATER TO THOSE BIRDS WHOSE FOSSILS ARE DATED OLDER THAN THE DINOSAUR FOSSILS.

A) The birdlike dinosaurs have no descendants.
B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.
C) There are no birdlike dinosaur fossils that are older that the bird fossils but have not yet been unearthed.
This is one of the versions of our PRETHINKING.
D) It could not have been the case that some birds were descendant from one of the bird-like dinosaur species and other birds from another.
E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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AbdurRakib wrote:
Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils much older than the earliest birdlike dinosaur fossils that have been found.

Which of the following is an assumption on which the argument relies?

A) The birdlike dinosaurs have no descendants.
B) There are no flightless dinosaur species that have the distinctive structural features shred by birds and birdlike dinosaurs.
C) There are no birdlike dinosaur fossils that are older that the bird fossils but have nit yet been unearthed.
D) It could not have been the case that some birds were descendant from one of the bird like dinosaur species and other birds from another.
E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.

OG 2017 (Book Question 607)

nit (noun) = the egg of a louse, which sticks to the fur of an animal or the hair of a person

Birds and Dinosaurs

Step 1: Identify the Question

The word assumption in the question stem indicates that this is a Find the Assumption question.

Step 2: Deconstruct the Argument

Birds desc from B-like dinos?
C: Fossil = NO b/c B fossils older than B-like dino fossils

The author concludes that a certain theory CANNOT be true. As evidence, the author states that there are bird fossils even older than the oldest birdlike dinosaur fossils found. So birds must be descended from something other than birdlike dinosaurs?

NOT necessarily. The author is assuming that the fact that older birdlike dinosaur fossils haven’t been found means that these fossils don’t exist. It’s possible that there were older birdlike dinosaurs but either their fossils haven’t been found yet or their bones didn’t survive as fossils for some reason.

Step 3: Pause and State the Goal

On Assumption questions, the goal is to find something that the author must believe TO BE TRUE in order to draw the conclusion.

Step 4: Work from Wrong to Right

(A) The author’s argument does not assume anything about modern-day descendants of the birdlike dinosaurs.
(B) The argument is confined to the possible connection between birds and birdlike dinosaurs. Flightless dinosaurs, another category, are not at issue in the argument.
(C) CORRECT. The author does assume that there aren’t older fossils of the birdlike dinosaurs that just haven’t been found yet. Try the Negation Test: if there are birdlike dinosaur fossils that just HAVEN'T been found yet, then the author’s argument is ruined.
(D) The author’s argument does not assume that all birds descended from the same birdlike dinosaur species. The author’s argument is open to the idea that there could have been multiple birdlike dinosaur species from which modern-day birds evolved.
(E) The argument is confined to rebutting the idea that birds descended from certain birdlike dinosaur species. It does not address whether the birds could have been descended from other dinosaur species with different structural features.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
I negated D and it destroyed the conclusion for me. Though option C's negation was a stronger destroyer of the argument, I request an expert to explain why option D is certainly not the right answer.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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gumnamibaba wrote:
I negated D and it destroyed the conclusion for me. Though option C's negation was a stronger destroyer of the argument, I request an expert to explain why option D is certainly not the right answer.

Quote:
D) It could not have been the case that some birds were descendant from one of the bird like dinosaur species and other birds from another.

To negate choice (D) is not to say that some birds definitely were descended from multiple dinosaur species. If we do not make the assumption stated in choice (D), we allow for the possibility that some birds were descended from MULTIPLE dinosaur species rather than a single birdlike dinosaur species.

In order for the author's argument to hold, it is not necessary that we first establish that birds could not have descended from multiple dinosaur species. Here is a breakdown of the argument if we do not make the assumption stated in choice (D):

• Birds may have descended from one or more species of dinosaurs
• Whether it was one species or multiple species, if birds were in fact descended from dinosaurs, we would not expect to find birds fossils that are older than dinosaur fossils.
• Since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found, the fossil record shows that birds could NOT have descended from certain birdlike dinosaur species.

Choice (D) is tricky, but, unlike choice (C), it is not a required assumption.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.

Premises: Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features.
Conclusion: The fossil record, however, shows that this cannot be so... (i.e. birds cannot have been descended from certain birdlike dinosaur species with which they share distinctive structural features.)
Support to the conclusion: since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.

Which of the following is an assumption on which the argument relies?
Assumption: Which is the bridge between the conclusion and premises. Without the assumption being true the conclusion falls apart.

Quote:
(A) The birdlike dinosaurs have no living descendants.

They have living dependent or not, doesn't really matters. They do or not, conclusion still stands TALL.

Quote:
(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.

Flightless dinosaur Firstly, aren't of much concern here, as the argument mentions only birdlike dinosaur. Even if they are, they don't hurt the conclusion.

Quote:
(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.

This sounds legitimate, if there are NO birdlike fossils that are older than the bird fossils, then the conclusion stands TALL. As this answer option supports the conclusion.
BUT
If I negate this and say "There ARE birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed."
Then the conclusion falls apart.
KEEP C

Quote:
(D) It could not have been the case that some birds were descended from one of the birdlike dinosaur species and other birds from another.

If this is the case, that all birds descended from same birdlike dinosaur species. This still doesn't affect the conclusion, which says that "birds cannot have been descended from certain birdlike dinosaur species with which they share distinctive structural features"

Quote:
(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.

Even If it says that "Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features." The author still says, whomsoever they descended from, they didn't descend from birdlike dinosaur.

Option C is the best.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
GMATNinja

"Birds have been said to be descended from certain birdlike dinosaur species...."

If according to Option A the birdlike dinosaurs have no descendants at all then it means birds are not their descendents right?

I feel like I am missing something but i dont seem to understand why it is wrong.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
Hello Everyone,

I have a very general question, I see that the tag reads that this question is a 600-700 range question. Is this likely? are these tags accurate in general?

Thanks in advance for giving me this information. GMATNinja
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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blitzkriegxX wrote:
GMATNinja

"Birds have been said to be descended from certain birdlike dinosaur species...."

If according to Option A the birdlike dinosaurs have no descendants at all then it means birds are not their descendents right?

I feel like I am missing something but i dont seem to understand why it is wrong.

Remember that we're asked to identify an assumption on which the argument relies.

Quote:
Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features. The fossil record, however, shows that this cannot be so, since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.

The conclusion is: the fossil record shows that this (birds descended from certain birdlike dinosaur species) cannot be so. The correct answer choice will identify an assumption required by this specific logical conclusion.

Quote:
(A) The birdlike dinosaurs have no living descendants.

The conclusion does not rely on this statement. The presence of bird fossils is the author's primary evidence. And that evidence supports a conclusion about what the fossil record shows.

As a few other folks using negation have pointed out, if the birdlike dinosaurs had any living descendants, the conclusion would still hold true because it is focused on what the fossil record tells us. Even if we discover that birdlike dinosaurs do have descendants that are not birds (for instance, lizards), the author's argument about the fossil record remains unchanged. We still see bird fossils much older than the earliest birdlike dinosaur fossils, and we can still accept that these bird fossils contradict the notion that birds descended from birdlike dinosaur species.

This is why we can safely eliminate (A).

Choice (C), on the other hand, fills in an unstated assumption about other fossils, which could challenge what the record shows if they were found. That's why it remains the best choice.

I hope this resolves any last bones you have to pick with this question. (I know: horrible joke.)
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
venky964 wrote:
Hello Everyone,

I have a very general question, I see that the tag reads that this question is a 600-700 range question. Is this likely? are these tags accurate in general?

Thanks in advance for giving me this information. GMATNinja

The difficulty level tags are determined based on how GMAT Club members perform using the "timers" for each question. Basically, if a lot of people miss a certain question -- and take a long time on it -- the question will be rated as difficult.

The GMAT Club difficulty levels do not necessarily match the "official" difficulty levels for these questions, though. The GMAT Club algorithm has nothing to do with the official GMAC algorithm, and GMAT Club members aren't necessarily representative of GMAT test-takers in general.

So think of the difficulty tags as a helpful guideline, but they may or may not match the "official" difficulty levels of the questions.

I hope this helps!
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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Passage Analysis

• Birds have been said to be descended from certain birdlike dinosaur species with which they share distinctive structural features.
o Birds are thought to have originated from an ancestral dinosaur species which is birdlike.
o This dinosaur species had distinctive similarity with birds in structural features.

• The fossil record, however, shows that this cannot be so,
o But fossil records suggest this cannot be the truth.

• since there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.
o The reason is that fossils of birds that are known to have existed much earlier than the earliest birdlike dinosaur whose fossils are unearthed, have been discovered.

Conclusion: Birds couldn’t have evolved from birdlike dinosaur species.

Question Stem Analysis
This question asks us to identify the assumption behind the argument.

Pre-thinking

Falsification Question
In what scenario is it possible that birds could have evolved from birdlike dinosaur species?
Given that
• there are bird fossils that are much older than the earliest birdlike dinosaur fossils that have been found.
Thought Process
The author first presents a previous belief that birds were thought to have evolved from certain birdlike dinosaur species and proceeds to give the reason for why it can no longer be thought so. The reason for this change in perception is that bird fossils older than the oldest birdlike dinosaur fossil has been discovered. But the possibility of discovering older birdlike dinosaur fossils remain open.

Falsification condition#1
What if there are fossils of birdlike dinosaurs older than the earliest bird fossils yet to be unearthed?
In that case the conclusion can no longer hold.
Assumption#1
There aren’t any fossils of birdlike dinosaurs older than the earliest bird fossils yet to be unearthed.

Falsification condition#2
What if there are methods more conclusive than testing the age of fossils that could prove birds did descend from the birdlike dinosaur species?
Here too, the argument breaks down.
Assumption#2
It cannot be proved by any other methods that birds descended from the birdlike dinosaur species, other than testing the age of fossils.

(A) The birdlike dinosaurs have no living descendants.
INCORRECT
The non-existence of living descendants for birdlike dinosaurs is not a necessary assumption for the conclusion to be true. The species very well could have had descendants which are not birds. Therefore, this is a wrong answer.

(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.
INCORRECT
The existence of flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs is immaterial in the question whether birds descended from the mentioned birdlike dinosaurs or not. Hence this cannot be the correct answer.

(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.
CORRECT
Only if this statement is true will the conclusion hold. Therefore, this is the correct answer.

(D) It could not have been the case that some birds were descended from one of the birdlike dinosaur species and other birds from another.
INCORRECT
This statement is not a necessary assumption for the conclusion to hold; just one of the many possibilities. Therefore, this cannot be the right answer.

(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.
INCORRECT
The negation of this statement does not negate the argument’s conclusion. Therefore, this cannot be the correct choice either.
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
GMATNinja (C) is based on the findings from the fossil records. However, fossil records obviously mean that these exclude the fossils that are yet to be unearthed. Doesn't this make (C) less suited than (E)?
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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GMATNinja (C) is based on the findings from the fossil records. However, fossil records obviously mean that these exclude the fossils that are yet to be unearthed. Doesn't this make (C) less suited than (E)?

The question asks for an assumption on which the argument depends. In other words, what must be true in order for the conclusion to be properly drawn?

With that in mind, let’s take a look at (E):

Quote:
(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.

It’s totally possible that birds descended from a different type of dinosaur species that the birds do not share distinctive structural features. If this were the case, then the author’s conclusion that birds cannot have descended from birdlike dinosaurs could still be true. Maybe birds descended from lizard-like dinosaurs. Even if this is true, birds did not descend from birdlike dinosaurs, and the author’s conclusion stands. So, (E) does not have to be true in order for the author’s conclusion to be properly drawn, and we can eliminate it.

And here’s (C):

Quote:
(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.

(C) indicates that current fossil records accurately represent the fossil records that will be unearthed in the future. In other words, (C) suggests that just as we currently do not have fossils indicating birdlike dinosaurs are older than birds, we will never find birdlike dinosaur fossils that are older than bird fossils.

This must be true because the entire basis of the argument is that, based on existing fossil evidence, birds are older than birdlike dinosaurs. If there were birdlike dinosaur fossils yet to be found that would indicate otherwise, then the argument would fall apart. So, (C) must be true in order for the conclusion to be properly drawn. For that reason, (C) is the best answer choice, and it is correct.

I hope that helps!
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
Hi GMATNinja

I still don't understand why E is wrong option?
If E is true (as an assumption), the case will be that birds are only descended from ancient birds not possibly from bird-like dinosaur species.➢ Support the conclusion that this (birds descended from certain birdlike dinosaur species) cannot be so.

Thank you so much
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
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Well0909 wrote:
Hi GMATNinja

I still don't understand why E is wrong option?
If E is true (as an assumption), the case will be that birds are only descended from ancient birds not possibly from bird-like dinosaur species.➢ Support the conclusion that this (birds descended from certain birdlike dinosaur species) cannot be so.

Thank you so much

The author's argument rules out the idea that birds are "descended from certain birdlike dinosaur species with which they share distinctive structural features." In other words, the author is ruling out one distinct possibility about where birds are descended from.

Notice, however, that the author does not make any claims about where birds ARE descended from. In fact, he/she would be happy with ANY alternative explanation.

Answer choice (E) rules out one of these alternative explanations -- namely, it rules out the possibility that birds are "descended from dinosaur species with which the birds do not share the distinctive structural features." At the same time, it makes no claims about where birds ARE descended from.

So at best, (E) is not relevant to the author's argument, since it tells us nothing about whether birds are descended from certain "birdlike dinosaur species." At worst, it WEAKENS the author's argument, since it rules out an alternative explanation the author might support. So, (E) can't be an assumption on which the argument relies.

I hope that helps!
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Re: Birds have been said to be descended from certain birdlike dinosaur [#permalink]
Hi,

I have some confusion in option C.

If option C only said - "There are no birdlike dinosaur fossils that are older than the bird fossils" - Then, I agree, this is the assumption on which the conclusion "that the birds could not have been descended from birdlike dinosaur species" is true.

However, by including the last bit in this sentence that says "but have not yet been unearthed", so the complete assumption now reads - "There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed." - how can author believe that some of these fossils may not yet have been discovered and still arrive at the conclusion that the birds could not have been descended from birdlike dinosaur species??