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.
GMAT® Official Guide 2017Practice QuestionQuestion No.: CR 607
Page: 528
SolutionPassage Analysis
Birds have been said to be descended from certain birdlike dinosaur speciesBirds 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-thinkingFalsification QuestionIn 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 ProcessLet 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 conditionWhat 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.
AssumptionWhatever 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.
Answer Choice AnalysisADoes 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.
BIn 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.
CThis 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.
DThis 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.
EThis 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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