Birds have been said to be descended from certain birdlike dinosaur

The question wants us to understand the conclusion, and purpose in order to identify the assumption. First we'll point out the conclusion, birds share features of dinosaurs but that isn't the case since bird fossils are older. The purpose is to disprove that birds have been said to be descended from birdlike dinosaur species. Now we can start to pick apart the options to find an assumption that points to the conclusion and the evidence that supports it.

A doesn't mention the older fossils found before birdlike dinosaurs. Eliminate.

B, and D mention information that wasn't in the argument.

E doesn't address the issue of birdlike fossils being older than bird fossils.

C addresses the older birdlike fossils and the change that there could be older bird fossils that are undiscovered. Correct!

Conc: ­Birds didn't descend from certain birdlike dinosaur species with whom they share distinctive structural features. 

(A) The birdlike dinosaurs have no living descendants - Do the bird like dinosaurs have living descendants or not? Whatever the answer to the question, this won't impact the conclusion. Drop

(B) There are no flightless dinosaur species that have the distinctive structural features shared by birds and birdlike dinosaurs. Do birds and bird like dinosaurs have common ancestors? Well the answer to this question, doesn't help us support the conclusion. Drop

(C) There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed. This sounds good, i.e., there are no bird-like dinosaur fossils (older than bird fossils) yet to be found buried in the earth. Let's negate to confirm - There are bird-like dinosaur fossils older than bird fossils yet to be extracted from the earth. Sounds like this would break the assumption in the argument. Keep.

(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. An alternative way to state the option  would be "All birds orginated from a single bird-like dinosaur species." The argument has a different standpoint on the origin of birds. Drop

(E) Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features. Does it means bird can descend from bird-like dinosaur species? We know the arugment doesn't support that. Drop