|A Layered Past|
Most of the fossils found in southern Mongolia represent animals that lived some 80 million years ago in the late Cretaceous Period. This was some 15 million years before the dinosaurs became extinct and when the Velociraptor and the shield-headed Protoceratops roamed the Earth. Paleontologists believe that the area now known as the Gobi Desert, though primarily arid, also contained marshy areas and ponds created by water run-off from the surrounding mountains. Enough vegetation grew then to support a wide variety of dinosaurs, lizards, and mammals (Loope et al., 1998).
These animals were trapped and buried in sediments in a number of different ways during the late Cretaceous Period. Some were taken
by flash floods and were buried in flood plains. Some drowned in lakes and were
covered by silt. Still others were swallowed by sandstorms. However, Novacek
explains, the best-preserved fossils from Ukhaa Tolgod came about as a result of
collapsing sand dunes. Stable, non-drifting sand dunes formed at and around the
marshlands and small ponds back then. "The dinosaurs would hunker down in
these dune fields and make their nests," said Novacek. Sometimes the dunes, which
were often many stories high, would become unstable. During a heavy rain, the top
layers of sand on the dune would slide down its sides like an avalanche, catching
dinosaurs unaware below and covering them instantly as they were nesting,
fighting, or collecting food (Loope et al., 1998).
Over time, layers upon layers of sediments built up on top of these dunes.
Under the intense pressure of these additional strata, the sand dunes turned to
red sandstone and the bones within fossilized. Millions of years later,
continental uplift and erosion from water and wind brought the fossils back to
the surface. The climate in this region became even more arid and sparsely vegetated, making the fossils easy to find.
Throughout the twentieth century, the red sandstone fossil beds yielded the finest specimens of both late Cretaceous dinosaur and mammal fossils in the world. In the 1920s while searching for human fossil remains, Roy Andrews, a paleontologist at the American Museum of Natural History, made the first big Gobi fossil discovery at a locality known as the Flaming Cliffs. Here he found a fossilized dinosaur nest as well as the first known skeletons of the infamous Velociraptor. For the 60 years that followed, during the time that Mongolia was under communist rule, researchers from Poland and Mongolia trekked further into the desert, turning up, among other things, a fossil of an Oviraptor in a fight to the death with a Protoceratops (Webster, 1996).
The museum resumed its campaigns shortly after the Soviet Union break up. Under the direction of Novacek, paleontologists uncovered the greatest Gobi fossil beds to date around an area known as Ukhaa Tolgod. "In this four-square-kilometer area we've found probably as many specimens as have been found in the rest of the Gobi combined," says Novacek. He explains this site has not only given them great specimens of fossils, but insights into the evolution of mammals and how dinosaurs raised their young. An Oviraptor skeleton they retrieved, for instance, shows that dinosaurs took care of their eggs in much the same way that birds do today (Norell et al., 1995). Well-preserved mammal skeletons dug up at the site have helped scientists fill in branches of the evolutionary tree that perhaps led to our existence.