PrepTest 106, Section 3, Question 13
A recent study concludes that prehistoric birds, unlike(73 more words not shown)
Which one of the following,(18 more words not shown)
- ASome modern warm-blooded(13 more words not shown)
- BHaving growth rings(9 more words not shown)
- CModern birds did(7 more words not shown)
- DDense blood vessels(7 more words not shown)
- EIn some cold-blooded(16 more words not shown)
Reading the argument
We have two different readings of the prehistoric birds' bones. Growth rings are offered as evidence that the birds were cold-blooded. Dense blood vessels are offered as evidence that they were active and therefore warm-blooded. I don't yet know which conclusion the evidence supports better.
The discussion of ancestry also makes me pause. An ancient cold-blooded ancestor wouldn't necessarily rule out a later warm-blooded ancestor between it and modern birds. But that is a separate question from whether the prehistoric birds at issue were themselves cold-blooded or warm-blooded.
The first study relies on growth rings being limited to cold-blooded animals. That is described as something thought to be true, so I'd want to know how secure the connection is. The second study relies on its own chain from blood vessels to activity to warm blood. We need evidence that helps decide between those accounts.
What we’re looking for
The question allows us to favor either side. I don't have a precise prediction of what the new fact will be; I'll look for information that helps us interpret the disputed physical evidence. It needn't answer every question about the entire history of birds.
Working the answers
- A
Other warm-blooded species having cold-blooded ancestors would show that such a transition is possible. It doesn't tell us whether these prehistoric birds were warm-blooded or cold-blooded. The dispute concerns the birds' traits, not merely whether a transition could ever occur.
- B
Cold-blooded species having other physical traits gives us no information about whether the prehistoric birds possessed those traits. It might suggest something else to investigate, but it doesn't supply a finding that favors either account of these birds.
- C
Denying that modern birds descended from prehistoric birds would challenge the ancestry discussion. It would still leave open whether the prehistoric birds themselves were warm-blooded or cold-blooded. We need help with that dispute.
- D
This allows a warm-blooded species without dense blood vessels. But the second study is reasoning from the presence of dense vessels to warm blood. Showing that some warm-blooded species lack them doesn't show that an animal with dense vessels could be cold-blooded.
The direction matters: warm-blooded without dense vessels is not the counterexample needed to challenge dense vessels as evidence of warm blood.
- ECorrect
This shows how both observed features can occur in a cold-blooded animal: the same gene can produce growth rings and dense blood vessels. Dense vessels therefore needn't force the warm-blooded conclusion drawn by the second study.
The first study's growth-ring evidence remains, while the apparently conflicting vessel evidence can fit a cold-blooded explanation too. That moves the dispute toward the first study's conclusion. E doesn't prove these particular birds had that gene or settle every biological question, but it helps one account accommodate both findings.
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