Gobi dinosaurs killed by rainstorms, not sandstorms

Jeff Poling

Scientists announced in January, 1998, evidence of the life, environment and death of the animals found in one of the richest fossil sites on earth.

The team of scientists from the University of Nebraska, the American Museum of Natural History, the Berkeley Geochronology Center and the Mongolian Technical University studied the fossil rich site of Ukhaa Tolgod in the Gobi Desert of Mongolia. Discovered in 1993, Ukhaa Tolgod is one of the world's best sources of Late Cretaceous fossils. The fossils from this area are extraordinarily well preserved, including minuscule skeletal structures less than 1/16 inch in size.

The quality and completeness of preservation was long thought to have been due to swift, catastrophic events that buried the bodies of the hapless animals before they could be scavenged or destroyed by exposure to the elements. It had been presumed that immense sandstorms were the source of the catastrophes, but the exact nature of the occurrences were actually unknown. To find out what may have really happened, the scientists took a close look at the structure of the deposits at the site. The scientists discovered that there are actually three distinct types of sandstone at Ukhaa Tolgod.

The first type shows a well-defined bedding structure tiled at 25 degrees and sorted by particle size. Such arrangements are typical of deposits created from wind-blown sand. This sandstone may have been formed during violent sandstorms as had been presumed for the fossil rich area of the site, yet this deposit contains no skeletal remains. It does, however, contain something else equally interesting, numerous concave depressions measuring from several inches to 20 inches across. One of the scientists had seen depressions like these in deposits in Nebraska, and thus was able to identify what they are: the first dinosaur footprints ever discovered in the Gobi Desert.

The second type of sandstone does not show the same kind of fine-grained structure as the first, but similarities in tilt and layering indicates that it, too, was created by wind action. Insect burrows were found in the deposits, usually below a certain depth. The scientists speculate that this was due to dinosaurs walking on the sand and crushing the burrows, but due to the lack of fine-grained sands the tracks were not preserved.

The third type of sandstone shows no layering whatsoever and contains large pebbles and cobbles much too large to have been windblown. It is in this third type of deposit that all the fossils of Ukhaa Tolgod are found.

This sandstone shows a structure similar to that caused by a poorly known phenomenon in which otherwise stable sand dunes become drenched with water from heavy rains, trigging sudden debris flows. These avalanches of sand can be very powerful and violent, just as with snow avalanches and mudslides. Such a debris flow could have trapped the dinosaurs and other animals in its path, resulting in the exquisite quality of the remains.

What can trigger such avalanches is not well understood; however, scientists do think clay may play an important role in the process. Windblown clay is brought in by dust storms and deposited on the sand by water, which soaks into the sand carrying the clay with it. The clay coats the individual grains of sand, eventually inhibiting a dune's ability to absorb water. Unusually heavy rains can therefore cause a layer of wet sand to rush down a dune's face.

Clay accumulates in modern sand dunes that are stabilized by vegetation, preventing the dune's migration. Dunes in deserts lacking vegetation tend to migrate across the desert as the sand is blown away, preventing clay buildup. If the animals of Ukhaa Tolgod were indeed caused by sandslides, it suggests, then, that they lived not in sterile desert, but a stable dune field with abundant plant life and rain.