The Dinosaur Fossils in the Gobi Desert
The Gobi desert is home to real treasures for paleontologists. There are only a few places on earth where there are similarly well-preserved fossils from the Cretaceous period. A team of scientists has discovered with the help of detectives that the dinosaurs were buried not by sandstorms, but by sand avalanches that suddenly slid down during heavy rainfall. The researchers draw a not at all desert-like picture of the habitat of the "terrible lizards". A team of scientists from the University of Nebraska, the American Museum of Natural History, the Berkeley Geochronology Center, and Monogolian Technical University presented new evidence in the January issue of Geology that dinosaurs and other creatures in the Gobi desert of the time originated from a were killed by a sudden avalanche of water-soaked sand that slid off a dune side. The research also revealed the first dinosaur footprints ever discovered in the Gobi Desert. The scientists, part of an expedition organized jointly by the American Museum of Natural History and the Mongolian Academy of Science, worked in an area known as Ukhaa Tolgod (Brown Hills) and one of the world's richest Late Cretaceous fossil sites represents.
Unique quality of the fossils
Ukhaa Tolgod was only discovered by the team in 1993. There are practically no other finds in such good condition on earth. Tiny skeletal structures-some of them smaller than the letters on this page-are intact. This remarkable quality suggests that the animals of Ukhaa Tolgod were killed quickly by a cataclysmic event. Their bodies were buried before other animals could digest their remains or the natural processes of decomposition destroyed them. It was often assumed that strong sandstorms were the culprits that buried the dinosaurs alive under clouds of dust and grit. However, the true nature of this tragedy remained obscure.
Geological detective work performed by the authors of the new article unearthed the cause of death: it was the little-known and only recently recognized phenomenon of a debris slide or "sand slide". A large amount of wet sand slides down one side of the dune like an avalanche and buries everything under it.
The Hunt for the Clues
The unraveling of this mysterious mystery began with a detailed study of the geology of Ukhaa Tolgod. The researchers found three different types of sandstone in the area, each providing a crucial piece of the puzzle to the overall picture. The first type of sandstone shows a well-defined layered structure, tilted at an angle of 25 degrees, and ordered by particle size. Such a structure is typical of wind deposition. This sandstone was probably formed during strong storms. It was long believed that the storms killed the dinosaurs, but the sandstone in question contains no skeletal remains. Even more surprising in the usually regularly arranged sandstone were the "pockmarks", concave depressions from a few centimeters to about half a meter in diameter, which in cross section looked like bowls stacked on top of one another. The group had just that little bit of luck: David Loope, first author of the Geology article and associate professor in the Department of Geosciences at the University of Nebraska, had examined depressions of the same type in Nebraska and recognized them as fossilized footprints of large animals. While it is not possible to identify the tracks in the Gobi desert with any certainty, the only large fossils that have been found are of dinosaurs. This is considered strong evidence that creatures such as Protoceratops and Ankylosaurus left the footprints.
The second type of sandstone does not show the fine-pored, small-grained structure of the first. However, similarities in the large sloping layers of sand indicate that this type was also formed by wind deposition. Walkways and burrows for insects and other small animals could be seen in the sandstone, but only from a certain depth. The researchers posit that the dinosaurs trampled over the insects' upper canals and burrows, but left the lower ones intact. Because of the missing fine-pored structure in the sandstone, the actual footprints of the dinosaurs were not preserved.
Finally, the many hundreds of fossils of Ukhaa Tolgod were found in the third type of sandstone. Of course, this guy attracted the interest and attention of geologists. Unlike the first two types of sandstone, this one had no layered structure at all. Large cobbles and pebbles, too large to have been carried by the wind, occasionally occurred in these sandstones. Thus, the possibility that sandstorms would have buried the dinosaurs of Ukhaa Tolgod can be ruled out. To support this thesis, the group scoured the travel literature of Central Asia and Arabia for reports of sandstorms burying animals. There were no such reports.
The solution to the riddle
Dr. Loop his attention to the Nebraska Sandhills to find the solution. Before leaving for Mongolia, he had studied a little-known phenomenon whereby otherwise stable sand dunes become waterlogged during heavy rains, triggering a sudden debris avalanche. In conversations with residents of the affected areas, he heard interesting reports. In one instance, a pickup truck was parked at the base of one such large sand dune and was half buried by a sand slide triggered by a heavy rainstorm in July. In another case, a barn built on a dune slope was half filled by a slide. Such "sand slides" could have caught the dinosaurs and other animals that were in the direction of movement of the debris. The sand buried them until the remains of the animals were found by paleontologists. This might explain the extremely good quality of the Ukhaa Tolgod fossils and why they are always found in sandstones that lack the strata formed by wind deposition.
What triggers such avalanches of debris in detail is not yet well understood. It seems as if clays covering individual grains of sand play an important role. The clay is brought in by hurricanes full of dust. When it rains, the water seeps through the dune and is absorbed by the clay surrounding each grain of sand. Over time, the clay prevents the dune from absorbing the water, so heavy rain can cause the wet sand to slide away.
Life in the desert
The clay in the Nebraska Dunes (and presumably in the ancient Gobi desert dunes) accumulates because vegetation stabilizes the dunes and prevents them from moving. Conversely, the dunes of most active fields have very little clay, because these clay coatings break up as the individual grains of sand rub against the desert floor. The group plans to conduct a series of experiments, including trying to trigger a "sand slide," to learn more about the nature of these debris movements.
The discovery shows that the dinosaurs whose bones have been found at Ukhaa Tolgod did not live in an inanimate, sterile desert, but in a solid dune field where plant life and rainfall were relatively abundant and widespread.
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