Gobi dinosaurs article (long)

["Sharon A. Hill" <HILL.SHARON@a1.pader.gov>]

>From http://www.sciencedaily.com

Institution: American Museum Of Natural History 
Contact: Elizabeth Chapman , Department Of Communications 
E-mail: chapman@amnh.org, Phone: 212-769-5800 

Posted
1/7/98 
New Study Shows Dinosaurs From Gobi Desert Site Killed In Sudden "Sand 
Slides" Falling From Dunes - Study Also Reports First Dinosaur Tracks 
>From Gobi 

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.