PrepTest 109, Section 4, Question 22
Dinosaur expert: Some paleontologists have claimed that birds(66 more words not shown)
The expert's argument depends on(6 more words not shown)
- AHaving similar characteristics(11 more words not shown)
- BDromeosaurs and birds(4 more words not shown)
- CKnowledge of dromeosaur(8 more words not shown)
- DKnown fossils indicate(9 more words not shown)
- EDromeosaurs are dissimilar(6 more words not shown)
Reading the argument
What if the older dinosaur fossils just haven't been found?
I understand the expert's timeline objection. Birds couldn't descend from a group that first appeared after birds already existed. But the evidence concerns the earliest fossils we've discovered, not necessarily the earliest members of each group. A missing part of the fossil record could change which group appeared first.
The initial paleontologists haven't proved descent merely by pointing to similar characteristics, either. Still, the expert's argument against them relies on dates. To conclude that their claim is false, the expert needs the known fossils to tell us something reliable about the order in which the groups originated.
What we’re looking for
We're asked for a necessary assumption. My first way of putting the concern would be, “We need the fossil record to be complete.” Let's test that prediction against the choices.
Working the answers
- A
The expert doesn't need to deny that similarity can be a sign of an evolutionary relationship. Similarity could be relevant evidence while the timeline still ruled out this proposed descent. The expert's stated objection concerns chronology, not a general rejection of resemblance as evidence.
- B
Having a common ancestor is different from one group descending from the other. The expert could reject the proposed bird-from-dromeosaur relationship without taking a position on whether they shared some earlier ancestor. B isn't required by the argument given.
- C
This is tempting because it responds to the missing-fossil concern, but completeness asks for too much. We could lack many details about these fossils and still have evidence of which group originated first. The expert needs the relevant timeline to be reliable, not complete knowledge of everything about the fossils.
For example, additional specimens dating from periods after both groups originated wouldn't necessarily change their order of origin. The mere possibility of more knowledge doesn't by itself undermine that order.
- DCorrect
The known fossils must indicate the groups' relative dates of origin for the expert's timeline argument to work. “Relative” means which came earlier and which came later; it doesn't require assigning an exact year to each group's beginning.
If the known fossil dates didn't tell us that order, older discovered bird fossils wouldn't establish that birds themselves predated dromeosaurs. This is the precise assumption behind our initial concern. It is narrower than C's demand for complete knowledge.
- E
The expert doesn't need additional differences between the groups. Even very similar animals could be ruled out as ancestor and descendant if they appeared in the wrong order. Conversely, listing differences wouldn't establish the timeline the expert relies on. E addresses a different comparison.
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