Logical ReasoningDifficulty: Easy

PT1 S4 Q3 Explanation

In 1860 Bavarian quarry workers

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsNecessary Assumption

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Stimulus

In 1860 Bavarian quarry workers discovered the impression of a feather in a limestone slab dating to the Mesozoic era. It had previously been assumed that birds developed only after the close of the Mesozoic era and after the disappearance of pterosaurs, a species characteristic of that era. But there in limestone must have been the earliest bird—certainly, the earliest found to that date.

What this question is testing

Necessary Assumption

Conclusion

The author finds a feather in old rock and concludes it must have belonged to the earliest bird.

Evidence

Why a bird? Because feathers are bird things — that's the implicit step.

Evaluate

This is where the argument gets interesting. The author treats "feather found" as proof that "a bird existed." But what if some other Mesozoic creature — a pterosaur, a feathered dinosaur, anything — also had aerodynamic feathers? Then the feather doesn't prove a bird at all.

The author needs to assume that during the Mesozoic, only birds had this kind of feather.

Goal

Find the answer that rules out non-bird Mesozoic creatures with similar feathers. Negate it (some non-bird had them) and the argument falls apart.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
3.

The argument assumes which one of

Answer choices

  1. Out of Scope5% picked this

    The creature to which the feather belonged was a descendant of

    The argument doesn't need the bird to descend from pterosaurs. Whether the bird's ancestry traces through pterosaurs, some other lineage, or independent evolution, the argument's claim that this feather belonged to a bird is unaffected. Negation test: if the creature was not descended from pterosaurs, the conclusion still holds.

  2. Out of Scope2% picked this

    Birds with such feathers were preceded by species of birds with

    The argument doesn't require earlier bird species with less-developed feathers. The author claims this feather marks the earliest known bird; whether earlier proto-birds existed (with worse feathers) doesn't affect that claim. Negation test: even if there were no less-developed-feather predecessors, the conclusion that this is the earliest known bird still holds.

  3. Correct87% picked this

    In the Mesozoic era, no creatures other than birds had

    Why this is right

    This locks the argument's key step. To go from "feather found in Mesozoic limestone" to "bird in the Mesozoic," the author needs to know no non-bird creature in the Mesozoic had such a feather. Negation test: if some non-bird (say, a feathered pterosaur or proto-dinosaur) did have such a feather, the impression could be from that creature, not from a bird — and the conclusion collapses. The argument depends on this exclusion.

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

  4. Bad Assumption5% picked this

    The feather belonged to a Mesozoic creature that was neither a pterosaur nor a bird, but

    The argument doesn't need the feather's owner to be an intermediate between pterosaurs and birds — it just needs the owner to be a bird. Negation test: even if the creature was a pure bird (not intermediate), the argument's conclusion still holds. This identifies an assumption the argument doesn't actually rely on.

  5. Out of Scope0% picked this

    The earliest bird flew in an

    How well or awkwardly the bird flew is irrelevant. The argument concludes only that the feather marks the earliest bird; it makes no claim about flight skill. Negation test: even if the bird flew gracefully (not awkwardly), the conclusion stands.

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