Reading Comprehension

PT156 · S1 · P4 · Q27 Sand Fossils

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`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.

Topic

Ukhaa Tolgod fossil site in Mongolia and how its fossils were so well-preserved.

Framework

Answer / Explain

Main Point

Scientists originally thought the fossils at Ukhaa Tolgod were preserved by sudden sandstorms, but evidence now points to rain-triggered debris flows (sandslides) in stable, vegetated dune fields as the real cause. (The Most Valuable Sentence appears at the end of the first paragraph and is echoed in the third: "this sandstorm hypothesis is probably mistaken" and )

P1: Introduction to Ukhaa Tolgod and the Original Sandstorm Hypothesis

This paragraph introduces Ukhaa Tolgod as an incredible fossil site from the Late Cretaceous and describes how the quality of the fossils was initially attributed to instant, deadly sandstorms that rapidly buried the animals. But new geological evidence challenges this idea.

P2: Geological Evidence: Types of Sandstone and Where Fossils Are Found

Here, we learn about three kinds of sandstone at the site. Only the third type—which isn’t windblown, has no layering, and contains large pebbles—holds the fossils. The other two types are related to wind activity but don’t have skeletons. This points away from the sandstorm theory.

P3: The Debris Flow (Sandslide) Hypothesis

This paragraph explains that the third sandstone type matches what you'd expect from a rain-soaked dune suddenly collapsing in a debris flow. Such an event could quickly bury animals just like an avalanche, explaining the fantastic fossil preservation.

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 were killed by sandslides?

Scientists have never directly observed
No Impact
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% picked this
The area of the Gobi
No Impact
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.
9% picked this
In areas of the Gobi
No Impact
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.
25% picked this
Geologists believe that no sandslides
No Impact
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.
6% picked this
Correct
There are several natural processes
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.
57% picked this

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