Mongolian dinosaurs give up sandy secrets.

Ever since paleontologists first discovered dinosaur fossils in Mongolia in the 1920s, they have tried to make sense of how these animals lived and died in what appears to have been an ancient Sahara-like desert with epic sandstorms. When sedimentologist David B. Loope studied the Mongolian rocks recently, he came up with a different image: modern Nebraska.

Loope and his colleagues propose that the richest fossil site in Mongolia once resembled the Sand Hills region of northern Nebraska, a rolling landscape of giant dunes covered with meadows and wetlands that support a rich assortment of wildlife.

Rather than the traditional scene, says Loope, "the picture I would paint would be a much greener one." He presents the findings in the January GEOLOGY.

Loope reached this conclusion after analyzing the red sandstone at a site called Ukhaa Tolgod, the richest known fossil locale in Mongolia. Discovered in 1993, this southern Mongolian site has yielded nearly 1,000 lizards, more than 500 mammals, and over 200 dinosaurs, says Mark A. Norell, a paleontologist at the American Museum of Natural History in New York and a coleader of expeditions to Ukhaa Tolgod.

One of the most spectacular fossils found by Norell and his colleagues at the site was an Oviraptor dinosaur sitting on a clutch of eggs (SN: 1/6/96, p. 7). The paleontologists originally thought that this specimen and others had died when a sandstorm engulfed them more than 70 million years ago.

Loope noted, however, that the fossil-rich sandstone layers at Ukhaa Tolgod do not resemble eolian deposits, which are formed by wind. Such deposits have rounded sand grains of uniform size laid down in slanted sheets. Although such layers exist at Ukhaa Tolgod, the fossils come from a different type of deposit, one that lacks the sheeted structure and contains large cobbles that could not have been carried by wind, Loope reports.

The picture emerging from this site is one of a variable climate, occasionally dominated by dry, shifting dunes. During wetter times, a thriving community of Cretaceous animals inhabited the site. In these periods, some sort of vegetation may have covered the dunes, although geologists have not found a record of the plants. Modern grasses emerged much later, notes Loope.

The new explanation for Ukhaa Tolgod's sandstone makes sense, says David E. Fastovsky, a sedimentologist and paleontologist at the University of Rhode Island in Kingston. Still, the Nebraska analogy does not explain other fossil sites in Mongolia, where Fastovsky and his colleagues have found clear evidence of eolian deposition.

At a site called Tugrik, for instance, Polish and Mongolian paleontologists discovered a predatory Veiociraptor and an herbivorous Protoceratops locked in what appears to be a violent clash frozen in time. In this and other cases, says Fastovsky, the geologic evidence suggests that sandstorms suffocated the dinosaurs.