`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.