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 feature

(C) There are no birdlike dinosaur fossils that are older that the bird fossils but have nit yet been unearthed

Explanation:

The author's argument is based on the fact that there are no exact discovered birdlike dinosaur fossils that are more aged than bird fossils. 

So, if there are birdlike dinosaur fossils that are older than the bird fossils, then this illustrates the author's conclusion. 

Archeology is an ever evolving field with newer findings often leading to date shifting for artefacts. What if there are fossil remains of this bird-like species which hasn't been unearthed yet. Clearly, the author is assuming all fossil discoveries related to species have already been made.

This is 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