题目选项分析

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?

【选项】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.