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.
Simplify the argument -
Bird fossils much older than birdlike dinosaur fossils have been found --> Birds did not descend from certain birdlike dinosaur species.
Let us look at the answer options -
(A) The birdlike dinosaurs have no living descendants.
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.
Negation of option A doesn’t break the conclusion. Eliminate.
(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.
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. Eliminate.
(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.
Option C - Correct.
Negate this.- There are birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.
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.
(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.
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.
(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.
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.