Reading Comprehension

PT156 · S1 · P4 · Q25 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.

25.

The information in the passage most helps to answer which one of the following questions?

  1. What kinds of mammals inhabited

    Unsupported

    We're only told that "dinosaur, lizard, and mammal fossils are extraordinarily well preserved". We don't know specifically which types of dinos, lizards, or mammals.

    18% picked this

  2. What kind of sandstone is

    Unsupported: currently

    We learn about the three types of sandstone that have accumulated in the past. We don't know what kind(s) of sandstone, if any, are currently being formed. And the fact that the final sentence of the passage makes us think that the condition at Ukhaa Tolgod 65 million years ago were different from those today makes speculating about today even harder.

    18% picked this

  3. Have fossilized plants ever been

    Unsupported: fossilized plants

    Plants are only mentioned in the final sentence of the passage, where we learn that the Gobi desert may have had rain and plant life 65 million years ago.

    6% picked this

  4. Have scientists found fossilized skeletons

    Unsupported: Other Sandstorms

    There isn't talk about sandstorms elsewhere or fossilized skeletons being found in other sandstorms. Presumably both exist, but the passage doesn't provide any acknowledgement of them.

    5% picked this

  5. Correct

    Are present-day sand dunes ever

    Why this is right

    This is definitely not concretely established, but it's somewhat supported by the text. The 3rd paragraph starts by saying: - the 3rd type has 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 a mudslide. This sentence has all the keywords from this answer choice. The intended meaning of the beginning of the 3rd paragraph is something like, "This pile of pebbly sand that we found these old fossils buried in look a lot like [this thing we observe nowadays]." The phenomenon described is talked about in the present tense: - a sand dune becomes drenched - the resulting avalanche can be If we were talking about something that only existed in the past, we wouldn't use that verb tense. For example, doctors no longer put leeches on you to perform a blood-letting, in the hope of ridding your body of the spirits causing your disease. Thus, if we were describing this now-obsolete procedure, we'd say "A doctor would put leeches on you", not "A doctor puts leeches on you". "Once you bled at least a pint of blood, the doctor ended the treatment", not "Once you bleed at least a pint, the doctor ends the treatment".

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

    53% picked this

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