Birds have been said to be descended from certain birdlike dinosaur

Birds and Dinosaurs

Step 1: Identify the Question

The word assumption in the question stem indicates that this is a Find the Assumption question.

Step 2: Deconstruct the Argument

Birds desc from B-like dinos?

C: Fossil = NO b/c B fossils older than B-like dino fossils

The author concludes that a certain theory cannot be true. As evidence, the author states that there are bird fossils even older than the oldest birdlike dinosaur fossils found. So birds must be descended from something other than birdlike dinosaurs?

Not necessarily. The author is assuming that the fact that older birdlike dinosaur fossils haven’t been found means that these fossils don’t exist. It’s possible that there were older birdlike dinosaurs but either their fossils haven’t been found yet or their bones didn’t survive as fossils for some reason.

Step 3: Pause and State the Goal

On Assumption questions, the goal is to find something that the author must believe to be true in order to draw the conclusion.

Step 4: Work from Wrong to Right

(A) The author’s argument does not assume anything about modern-day descendants of the birdlike dinosaurs.

(B) The argument is confined to the possible connection between birds and birdlike dinosaurs. Flightless dinosaurs, another category, are not at issue in the argument.

(C) CORRECT. The author does assume that there aren’t older fossils of the birdlike dinosaurs that just haven’t been found yet. Try the Negation Test: if there are birdlike dinosaur fossils that just haven’t been found yet, then the author’s argument is ruined.

(D) The author’s argument does not assume that all birds descended from the same birdlike dinosaur species. The author’s argument is open to the idea that there could have been multiple birdlike dinosaur species from which modern-day birds evolved.

(E) The argument is confined to rebutting the idea that birds descended from certain birdlike dinosaur species. It does not address whether the birds could have been descended from other dinosaur species with different structural features.

Argument Construction

Situation
Although birds have been said to be descended from birdlike dinosaurs, some bird fossils predate the earliest known birdlike dinosaur fossils.

Reasoning
What must be true in order for the premise that some bird fossils predate the earliest known birdlike dinosaur fossils to support the conclusion that birds are not descended from birdlike dinosaurs? The argument implicitly reasons that since the cited bird fossils predate the earliest known birdlike dinosaur fossils, they must be from birds that lived before the earliest birdlike dinosaurs, and which therefore could not have been descended from birdlike dinosaurs. This reasoning assumes that any birdlike dinosaurs that lived before the first birds would have left fossils that still exist. It also assumes that no undiscovered birdlike dinosaur fossils predate the cited bird fossils.

(A) The argument is only about whether birds are descended from birdlike dinosaurs. Whether birdlike dinosaurs have any living descendants other than birds is irrelevant.

(B) The argument is only about birds and birdlike dinosaurs. It is not about other types of dinosaurs that were not birdlike.

(C) Correct. If any undiscovered birdlike dinosaur fossils predate the cited bird fossils, then the latter fossils' age does not support the conclusion that birds are not descended from birdlike dinosaurs.

(D) The argument purports to establish that the relative ages of bird fossils and birdlike dinosaur fossils show that birds cannot be descended from any of the known birdlike dinosaur species. In doing this, it acknowledges multiple birdlike dinosaur species and leaves open the question of whether some birds may be descended from one such species and other birds from another such species.

(E) The argument does not claim that the known fossil record shows that birds cannot be descended from dinosaurs. It only claims that the record shows that they cannot be descended from the birdlike dinosaurs that shared their distinctive structural features.