Reading ComprehensionDifficulty: Hard

PT156 S1 P4 Q27 Explanation

Sand Fossils

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Passage

`Discovered in 1993, the site known as Ukhaa Tolgod, in the Gobi desert of Mongolia, is one of the world's best sources of fossils from the Late Cretaceous period, which ended about 65 million years ago. The dinosaur, lizard, and mammal fossils from this area, including skeletal structures less than 2 millimeters scientists analyzing the geological formations of Ukhaa Tolgod indicates that this sandstorm hypothesis is probably mistaken.

The scientists found that there are three distinct types of sandstone at the site. The first exhibits a well-defined structure of layers tilted at 25 degrees and sorted by particle size. Such arrangements are typical of deposits created from windblown sand. While this sandstone contains dinosaur footprints, it contains no skeletal remains. this third type of deposit that all the vertebrate skeletal fossils of Ukhaa Tolgod are found.

This third type of sandstone exhibits a structure similar to that caused by a phenomenon in which an otherwise stable sand dune becomes drenched with water from heavy rains, triggering a sudden debris flow. The resulting avalanche of sand can be as powerful and violent as a snow avalanche or mudslide. Such were in its path, resulting in the pristine quality of the remains.

The cause of these sandslides is not well understood. However, there is evidence that clay plays an important role. When windblown clay particles deposited by dust storms are moistened with rain, the wet clay filters into the dune and adheres to individual sand grains. The buildup of clay eventually inhibits drainage of time was not a sterile desert, but a stable dune field with plant life and rain.

What this question is testing

Weaken

Your task

Find the choice that makes the argument's conclusion less likely to be true.

Common trap

Answers that look negative but attack a claim the argument never relied on.

Winning move

Find the assumption the argument depends on, then pick the choice that undermines it.

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

The question
27.

Which one of the following, if true, provides the strongest grounds for doubting that the animals whose fossilized skeletons were found at Ukhaa Tolgod

Answer choices

  1. No Impact3% picked this

    Scientists have never directly observed animals being killed by a sandslide in

    First of all, scientists probably don't spend a ton of time in the desert, watching for animals to be killed by a sandslide. So the lack of observational confirmation is not making us suspicious that the hypothesis was wrong. More importantly, this hypothesis is about what happened 65 million years ago. The last sentence of the passage suggests that the Gobi desert had different climatic conditions 65 million years ago (it wasn't a sterile desert like today; it had plant life and rain). So we certainly wouldn't expect scientists to have observed a phenomenon that wouldn't be possible under today's Gobi desert conditions.

  2. No Impact10% picked this

    The area of the Gobi desert in which Ukhaa Tolgod is located is currently not

    This is similar to (A). We're not saying that nowadays there are sandslides trapping animals and preserving their bones. We're saying 65 million years ago there were. The last sentence of the passage already implies that the Gobi area is not currently a stable dune field, so this answer adds no new information.

  3. No Impact25% picked this

    In areas of the Gobi desert other than Ukhaa Tolgod, pristine vertebrate skeletal fossils have been found in the second type of

    This answer is somewhat tempting because it makes it seem like, "Oh. If the 2nd type can preserve fossil in pristine fashion, then maybe the scientists are wrong to be trying to come up with a story for how these fossils got preserved by the 3rd type." But the passage gave us reason to think that this Ukhaa Tolgod deposit of fossils is definitely not of the 2nd type. After all, it has pebbles in it that are too big to have been blown by wind (1st and 2nd type of sandstone were caused by wind). We know that all the fossils of Ukhaa Tolgod are found in the 3rd type of deposit (end of 2nd paragraph), so talking about what's going on in the 2nd type of deposit is not giving us an alternate explanation for how these 3rd type fossils were preserved.

  4. No Impact8% picked this

    Geologists believe that no sandslides have occurred in the Gobi desert for at least

    The theory we're addressing is about conditions 65 million years ago, so the fact that the Gobi desert doesn't presently look like a stable dune field with periodic sandslides, and hasn't looked that way for at least 5 million years, is not strong counterevidence. (A), (B), and (D) are all trying to say, "This hypothesis doesn't seem like it would be true currently / recently", but the hypothesis is only about what was true 65 million years ago.

  5. Correct54% picked this

    There are several natural processes that can produce the third type of sandstone described in

    Why this is right

    This is an annoying correct answer because it just barely suggests the possibility of alternate explanations. We know that this miraculous deposit of fossils was found in the 3rd type of sandstone. Researchers have hypothesized that the reason we have these fossils is because these animals were killed by a sandslide, which is known to result in a structure that resembles this 3rd type of sandstone. But, according to this answer, there are a few other natural processes that could result in this 3rd type of sandstone, so we need to consider whether any of those processes could better explain the deposit of fossils. Basically, this answer weakens by creating doubt that we've considered as many possible explanations as we need to. Before we settle on this sandslide hypothesis, we need to think through the other ways that this 3rd type of sandstone forms.

    Skill tested: Weaken · how this choice captures the passage's function is the move to repeat next time.

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