Logical reasoning PrepTest 109 · Section 4 · Question 22

Incorrect. The Expert doesn't conclude or imply that the dromeosaurs and birds aren't evolutionarily related, just that the birds didn't descend from the dromeosaurs. The dromeosaurs descending from the birds aligns with her argument.

Incorrect. The Expert's argument sets out to prove only that birds didn't evolve from dromeosaurs. Whether or not there's some other connection between them is thus out of scope.

Incorrect. This is such a great trap answer! It aligns with our anticipation of the correct answer, but it goes a little too far. For the argument to work, it isn't necessary that the fossil records be "complete"—just that there's enough information to be certain about the relative timeline of these species developing.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Paleontologists: The fossil record shows that dromeosaurs are more similar to birds than other dinosaurs are, so dromeosaurs evolved into birds

Expert: The earliest "discovered" bird fossils are from millions of years before the "oldest known" dromeosaur fossils, so birds didn't descend from dromeosaurs

Answer Anticipation:
The two arguments here are interesting in that they both base their conclusions on the same set of evidence—the fossils that have been discovered of both birds and dromeosaurs. However, they focus on different things in order to reach opposing conclusions—the Paleontologists focus on the similarities between the fossils, whereas the Expert focuses on the age.

Since the question stem asks about the Expert's argument we should focus there.

The Expert argues that birds couldn't have evolved from dromeosaurs because we have bird fossils that date from "tens of millions of years" earlier than any of our dromeosaur fossils, which would seem to suggest that the birds were here first. After all, a species has to exist before another if the latter is to evolve out of it!

However, there are important qualifiers on both statements the Expert makes about the fossils. When discussing the bird fossils, she calls them the "earliest bird fossils that have been discovered," and the discussion of the dromeosaur fossils refers to them as the "oldest known" ones. That doesn't establish that either of these sets of fossils actually represents the general (or relative) time period that the species originated. It's possible that bird fossils are much hardier than dromeosaur ones, and so the dromeosaur fossils from before the birds existed are all destroyed by now.

Whatever the specific reason (none of which individually would be necessary), the overall assumption here is that the fossils we have represent the relative ages of the two types of animals, so let's find an answer establishing that timeline.

Answer Explanation:
This answer is a weaker version of (C), and it correctly addresses the question stem and assumption in the argument. The fossils we have don't need to be complete for the argument to work—just complete enough for us to be able to determine the general or relative age of the origin of birds and dromeosaurs. If the known fossils don't indicate the relative dates of origin, then they can't be used to show which animal descended from the other, destroying the argument. This answer is therefore a necessary premise.

Key Takeaway:
Always watch out for qualifying phrases or words! Here, talking about the fossils that are "known" or "have been discovered" established the possibility that those fossils didn't represent a large enough sample to allow the conclusion to be drawn.

Incorrect. The argument doesn't require dissimilarities in order to show that birds aren't descended from dromeosaurs—they could be quite similar! In fact, it could be the case that dromeosaurs descended from birds!