Jan. 7, 1998 A team of scientists from the University of Nebraska, the American Museum of Natural History, the Berkeley Geochronology Center, and the Mongolian Technical University, presents new evidence in the cover story of the January issue of Geology that the dinosaurs and other ancient creatures from the Gobi Desert's richest fossil site were killed by sudden avalanches of water-soaked sand flowing down the sides of dunes. The research also revealed the first dinosaur footprints ever discovered in the Gobi Desert. The scientists, part of a joint expedition organized by the American Museum of Natural History and the Mongolian Academy of Science, were working in the area known as Ukhaa Tolgod (Brown Hills), one of the world's richest Late Cretaceous fossil sites.
Discovered by the American Museum/Mongolian Academy team in 1993, Ukhaa Tolgod is virtually unparalleled in the extraordinary preservation of the specimens it yields. Minuscule skeletal structures -- some of them smaller than one of the letters in this sentence -- are perfectly preserved. This remarkable quality of preservation indicates that the animals at Ukhaa Tolgod were killed swiftly by catastrophic events that buried their bodies before they could be scavenged or destroyed by the elements. It has often been presumed that immense sandstorms were the culprit, their wind-blown clouds of grit burying the dinosaurs alive. However the true nature of these disasters remained a mystery.
The geological detective work conducted by the authors of the new paper reveals that cause of death was actually a little-known and only recently recognized phenomenon, a debris flow, or "sand slide," in which a massive quantity of wet sand rushes down the side of a dune, burying everything in its path in an avalanche of debris.
Unraveling the mystery began with a detailed examination of the geology of Ukhaa Tolgod. The team discovered that there were three distinct types of sandstones at the site, each revealing a different part of the puzzle. One type shows a well-defined bedding structure that is tilted at an angle of twenty-five degrees and is arranged by particle size; such structure is typical of wind-blown deposits. This sandstone was likely formed during violent storms like those long thought to be the dinosaur's killers, yet it contains no skeletal remains.
Even more surprisingly, the usually regular sandstone layers were pocked with numerous concave depressions, measuring from several inches to twenty inches across, and resembling a stack of bowls in cross-section. As luck would have it, first author of the Geology paper, David Loope, professor and chair in the Department of Geosciences, University of Nebraska, had studied depressions of the same type in Nebraska and was the first scientist to recognize them as the fossilized footprints of large animals. While it is not possible to identify the Gobi Desert trackmakers with absolute certainty, the only large fossils found in the area are dinosaurs, a strong indication that the footprints were made by such creatures as Protoceratops and ankylosaurs.
A second type of sandstone did not show the fine-scale structure of the first, but similarities in its texture and its large tilted and cemented sheets of sand made it clear that it too was created by the action of the wind. Burrow marks made by insects and other tiny creatures were present in the sandstone, but only below a certain depth. The researchers posit that these sandstones were trampled by dinosaurs, crushing the upper burrows and leaving the lower ones intact. The marks of the dinosaur trackways themselves were not preserved, due to the lack of fine-scale structure in the sandstone.
The third type of sandstone is the one in which all of Ukhaa Tolgod's hundreds of fossil have been found, and it drew particularly close attention from the geologic team. Unlike the other two types of sandstone, this showed no structured layering at all. Large pebbles and cobbles, which are much too big to have been carried by the wind, are sometimes present in these sandstones, indicating that the sandstones were not formed by wind action, and thus eliminating the possibility that windy sandstorms delivered the fatal blow to the dinosaurs of Ukhaa Tolgod. To corroborate this, the team reviewed research on the travel literature of Central Asia and Arabia to see if there were any modern-day accounts of animals buried alive in sandstorms. The research did not record any such mass smotherings.
Again, Dr. Loope drew on his experiences in the Nebraska Sandhills to provide the solution. Before leaving for Mongolia he had been studying a poorly known phenomenon in which otherwise stable sand dunes became drenched with water in heavy rains, triggering sudden debris flows. In talking to residents of the area he began to hear intriguing stories. In one instance, a pick-up truck parked at the base of a large sand dune was half buried by sand flows generated by a heavy July rainstorm. In another case a barn built on a dune slope was partly filled by a flow. Such "sand slides" in the Gobi Desert could have trapped the dinosaurs and other animals that were in the path of the debris, entombing them until they were uncovered by the paleontologists. This would account for the extraordinary quality of the Ukhaa Tolgod fossils and would explain why they are always found in the sandstones that lack the structured layering caused by wind action.
Precisely what triggers such debris flows is not well understood, but it appears as though clays that coat individual grains of sand play an important role. The clay is delivered by swirling dust storms and is deposited on the sand grains by rain water, which carries tiny clay particles, as it soaks into the dune. With time such clays inhibit the dune's ability to absorb water so that an unusually heavy rain can cause a slurry of wet sand to rush down its face.
The clay in the Nebraska dunes (and presumably in the ancient Gobi dunes) accumulates because the dunes are stabilized by vegetation and so do not actively migrate. Conversely, the dunes of most active dunefields have very small quantities of clay because such coatings are knocked off as the individual sand grains bounce over the desert floor. The team plans to conduct a series of experiments, including trying to trigger a "sand slide," in order to learn more about the nature of these debris flows.
The discovery indicates that the dinosaurs whose bones are found at Ukhaa Tolgod did not live in a howling, sterile desert, but rather in a stabilized dune field where plant life and rainfall were relatively abundant Authors of the paper are: David Loope, professor and chair in the Department of Geosciences, University of Nebraska; Lowell Dingus, research associate, American Museum of Natural History's Department of Vertebrate Paleontology; Carl Swisher, geochronologist, Berkeley Geochronology Center; and Chuluun Minjin, geologist, Mongolian Technical University.
In addition to support from the American Museum of Natural History, the Gobi Expedition is supported by Mercedes-Benz, which is the principal sponsor of the Museum's 1997, 1998, and 1999 expeditions to Mongolia, providing both financial support and vehicles for use by the expedition team. The Gobi project is also supported by the National Science Foundation, the Jaffe Foundation, and the Infoquest Foundation.
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