PrepTest 92+, Section 3, Question 24

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 across, are extraordinarily well preserved. The quality and completeness of their preservation was at first thought to have been due to immense, sudden sandstorms that buried the bodies of the animals before they could be scavenged or destroyed by exposure to the elements. However, new evidence gathered by 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. The second type of sandstone does not show the same fine-grained structure as the first, but similarities in tilt and layering indicate that it, too, was created by wind. The third type shows no layering whatsoever and contains pebbles that are too large to have been windblown. It is in 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 a debris flow at Ukhaa Tolgod in the Late Cretaceous period could have trapped whatever animals 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 rainwater from the dune but does not prevent the wet sand grains from shifting. Unusually heavy rains can therefore cause a layer of wet sand to rush down a dune's face. Observations of modern sand dunes indicate that clay accumulates only if the dunes are stabilized by vegetation; dunes in deserts lacking vegetation migrate across the desert as the sand is blown away, preventing clay buildup. If the animals of Ukhaa Tolgod were indeed killed by sandslides, this suggests that the Gobi area at that time was not a sterile desert, but a stable dune field with plant life and rain.

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 across, are extraordinarily well preserved. The quality and completeness of their preservation was at first thought to have been due to immense, sudden sandstorms that buried the bodies of the animals before they could be scavenged or destroyed by exposure to the elements. However, new evidence gathered by 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. The second type of sandstone does not show the same fine-grained structure as the first, but similarities in tilt and layering indicate that it, too, was created by wind. The third type shows no layering whatsoever and contains pebbles that are too large to have been windblown. It is in 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 a debris flow at Ukhaa Tolgod in the Late Cretaceous period could have trapped whatever animals 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 rainwater from the dune but does not prevent the wet sand grains from shifting. Unusually heavy rains can therefore cause a layer of wet sand to rush down a dune's face. Observations of modern sand dunes indicate that clay accumulates only if the dunes are stabilized by vegetation; dunes in deserts lacking vegetation migrate across the desert as the sand is blown away, preventing clay buildup. If the animals of Ukhaa Tolgod were indeed killed by sandslides, this suggests that the Gobi area at that time was not a sterile desert, but a stable dune field with plant life and rain.