Fossil preservation and rain-induced sandslides at Ukhaa Tolgod

How did these tiny bones survive so well? Ukhaa Tolgod contains dinosaur, lizard, and mammal fossils from roughly 65 million years ago, including skeletal structures less than two millimeters across. Exposed bodies could have been scavenged or damaged. The extraordinary preservation calls for an explanation.

The original answer was sudden sandstorms. Bury the animals quickly, and their remains are protected before anything can destroy them. But the first paragraph ends by telling us that new geological evidence challenges that account. We should expect a competing explanation for the same well-preserved fossils.

The sandstone supplies the first important distinction. The first type has tilted, neatly sorted layers typical of windblown sand. It contains dinosaur footprints, but no skeletons. The second type has less precise layering, yet enough similarity to suggest that wind created it too. The third type is different: no layers, and pebbles too large for wind to have carried. All the vertebrate skeletal fossils are in that third type.

That matters because the proposed cause was wind. The deposits that look wind-created aren't where these skeletons are found. The skeletons instead appear in a deposit whose structure needs a different explanation. This isn't merely a catalog of three rocks; it is evidence for changing our account of what buried the animals.

Now picture a stable sand dune—a large hill of sand—getting drenched by heavy rain. A layer can break loose and rush downhill, much like a mudslide or snow avalanche. That sudden flow could trap animals and cover them before their remains deteriorated. We still have rapid burial explaining preservation. What changes is how the sand reached the animals: a rain-triggered slide rather than a wind-driven storm.

The final paragraph asks how the slide might happen. Dust storms deposit clay, rain carries that clay into the dune, and the clay sticks to sand grains. As it accumulates, water has a harder time draining out. But the grains can still move. That combination is the key: water stays in the dune without the sand becoming fixed in place. Add unusually heavy rain, and a layer of wet sand can rush down the slope. The cause isn't completely understood, so we should keep this as an evidence-based explanation rather than a settled account of every detail.

The observations of modern dunes let the scientists work backward to the ancient landscape. Clay accumulates only where vegetation stabilizes a dune. In a bare desert, sand keeps blowing away and the dunes migrate, preventing that buildup. If clay-assisted sandslides killed these animals, the area must have had the stable dunes, plants, and rain that made those slides possible. We aren't seeing ancient plant fossils here; we are inferring the conditions needed for the proposed process.

That gives us a useful environmental conclusion, but keep the hierarchy straight. The central question was how the animals died and became so well preserved. The sandstone evidence supports sandslides over sandstorms; the reconstruction of the ancient climate follows from that explanation.