When we're approaching an ASSUMPTION question, we're looking for the logical gap between the premises and conclusion - the thing that MUST be true for the argument to hold.

Premise: there are bird fossils much older than the earliest birdlike dinosaur fossils that have been found.

Conclusion: It cannot be true that birds are descended from these birdlike dinosaur species.

What would indicate a flaw in this logic? Sometimes when looking for an assumption, it helps to think about "what ifs" that would make the conclusion untrue:
- what if some of these birdlike dinosaurs evolved into birds, but others stayed as they were?
- what if an animal species and the type of species they branched off from could be alive at the same time?
- what if the fossil record is incomplete, and doesn't give us enough puzzle pieces to make inferences about chronology?

Assumptions: Here's what's necessary for this premise to support this conclusion:
- we have a fossil record comprehensive enough to give us a complete evolutionary picture
- evolution is linear, not branching. Descendent species cannot be contemporaneous with ancestor species.

As you're going through answer choices, ask yourself - does this HAVE to be true for the conclusion to hold?

A) The birdlike dinosaurs have no descendants.
This does not have to be true for the argument to hold. The birdlike dinosaurs could have had descendants, but just not these bird descendants.

B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs.
Irrelevant. There is no direct connection between flightless dinosaurs and birdlike dinosaurs or birds.

C) There are no birdlike dinosaur fossils that are older that the bird fossils but have nit yet been unearthed.
Yes, this MUST be true for the argument to hold. If the reverse were true - there ARE older birdlike dinosaurs, but we haven't found them yet - then the logic of this argument would fall apart. The fossils we currently have wouldn't reflect the true chronology. Correct.

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.
The idea of evolutionary branching is germane to the argument, but because the conclusion itself talks about "the earliest birdlike dinosaur fossils," we can assume that all such species are included in this group. Distinctions within the group wouldn't matter to the argument.

E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.
This doesn't have to be true. Birds could be very distant descendants of dinosaurs that had very different features. We're specifically looking at the connection to BIRDLIKE dinosaurs, and whether the chronology we see indicates a line of descent. This is irrelevant.

The correct answer is C.

NandishSS 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.

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

Cheers,
Brent