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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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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.


GMAT® Official Guide 2017

Practice Question
Question No.: CR 607
Page: 528

Originally posted by AbdurRakib on 15 Jun 2016, 12:57.
Last edited by Bunuel on 16 Jun 2019, 21:56, edited 7 times in total.
Renamed the topic and edited the question.
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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 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 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 New Question


This is an assumption question.

PREMISE: Some believe that birds descended from dinosaurs
PREMISE: There are bird fossils much older than the earliest birdlike dinosaur fossils found.
CONCLUSION: Birds did NOT descend from birdlike dinosaurs


First try to identify an assumption that the author needs in order for the conclusion be valid.
If you can't think of one, you may need to resort to the negation method
So, we'll negate each answer choice.
The answer choice that, when negated, DESTROYS the argument will be the correct answer.

A) The birdlike dinosaurs do have descendants.
Does this destroy the conclusion that Birds did NOT descend from birdlike dinosaur?
No. It makes no difference where the birdlike dinosaurs came from.
ELIMINATE A

B) There ARE flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.
Does this destroy the conclusion that Birds did NOT descend from birdlike dinosaur?
No, the argument is about whether birds have DESCENDED from birdlike dinosaurs.
ELIMINATE B

C) There ARE birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.
Does this destroy the conclusion that Birds did NOT descend from birdlike dinosaur?
YES! The author's argument is based on the fact that there are no discovered birdlike dinosaur fossils that are older than bird fossils.
So, if there ARE birdlike dinosaur fossils that are older than the bird fossils, then this KILLS the author's conclusion.
KEEP C

D) It COULD have been the case that some birds were descendant from one of the bird like dinosaur species and other birds from another.
Does this destroy the conclusion that Birds did NOT descend from birdlike dinosaur?
No. The word COULD doesn't have the certainty required to destroy an argument.
ELIMINATE D

E) Birds MAY have descended from dinosaur species with which the birds do not share the distinctive structural features.
Does this destroy the conclusion that Birds did NOT descend from birdlike dinosaur?
No. The conclusion is all about birds descending from birdlike dinosaurs
ELIMINATE E

Answer: C

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Re: Birds have been said to be descended from certain birdlike dinosaur  [#permalink]

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New post 27 Feb 2017, 18:33
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Simplify the argument -

Bird fossils much older than birdlike dinosaur fossils have been found --> Birds did not descend from certain birdlike dinosaur fossils

Let us look at the answer options -

Option A - Incorrect.
Try to negate this. Even if there were descendants of birdlike dinosaurs, the argument still holds because bird fossils much older than them have been found.

Option B - Incorrect.
We are interested only in birds and birdlike dinosaurs and their fossils. Talking about flightless dinosaurs is not relevant to the argument.

Option C - Correct.
Negate this.
If there are indeed such fossils (but have not yet been unearthed), the conclusion "Birds did not descend from certain birdlike dinosaur fossils" will be significantly weakened.
Hence, the correct answer.

Option D - Incorrect.
Even if that were the case, the argument is not weakened because bird fossils that are much older than those of birdlike dinosaurs have been found.

Option E - Incorrect.
Negate this one.
"Birds could have descended from dinosaur species with which the birds do not share the distinctive structural features." - But we do not know whether these other dinosaur species lived before or after birdlike dinosaurs.
If they did not live before, the argument is not weakened. Hence, 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.

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

Answer C
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Re: Birds have been said to be descended from certain birdlike dinosaur  [#permalink]

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New post 26 Jun 2016, 07:04
The issue with the reasoning is the assumption that there are no birdlike dinosaurs fossils that have not been found.

Answer choice C.
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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]

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New post 28 Sep 2017, 04:11
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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New post 29 Sep 2017, 15:43
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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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New post 06 Sep 2018, 05:36
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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.


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]

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New post 01 Oct 2018, 01:56
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Please help me with Option A. I narrowed it down to A and C.

"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.
Please help me. Thank you.
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Re: Birds have been said to be descended from certain birdlike dinosaur  [#permalink]

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New post 02 Oct 2018, 10:13
Use the negation technique for each of the choices. Only answer choice C breaks the conclusion i.e There are birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed. In this case the conclusion is weakened.
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Re: Birds have been said to be descended from certain birdlike dinosaur  [#permalink]

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New post 09 Oct 2018, 08:08
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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New post 09 Oct 2018, 20:57
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blitzkriegxX wrote:
GMATNinja
Please help me with Option A. I narrowed it down to A and C.

"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.
Please help me. Thank you.

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]

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New post 09 Oct 2018, 21:02
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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New post 26 Nov 2019, 19:37
D options give the possibility that different birds may be descended from a different animal so even if earlier birds whose fossil were found were not descended from dinosaurs we could still say that later birds may be descended from dinosaurs.
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Re: Birds have been said to be descended from certain birdlike dinosaur  [#permalink]

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New post 30 Nov 2019, 00:30
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Conclusion: birds descended from same birdlike species, they share distinctive structural features, fossil record shows opposite

(A) The birdlike dinosaurs have no living descendants. -irrelevant

(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs. -talks about flightless-out of scope

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

(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. -irrelevant

(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.-does not talks about whether dinosaur species descended from dinsosaur species, incorrect
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Re: Birds have been said to be descended from certain birdlike dinosaur   [#permalink] 30 Nov 2019, 00:30
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