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? ['The birdlike dinosaurs have no descendants.', 'Birds cannot have been descended from dinosaur species with which the birds do not share the distinctive structural features.', 'There are no birdlike dinosaur fossils that are older than the bird fossils but have not yet been unearthed.', '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.']