Sand Dunes Swallow Dinosaurs

Despite the scale of the great Gobi sand dunes, it is still startling to learn that more than 65 million years ago those sand dunes were big enough to bury a dinosaur in an instant. At least, that is the conclusion of an international team of scientists who published a study in Geology.

The Gobi Desert has long been renowned as a fossil hunter's dream. Roy Andrews Chapman, a swashbuckling, publicity-grabbing scientist who became the model for the Indiana Jones movies, first discovered the surprisingly well-preserved dinosaur bones and eggs in the dry sands of the Gobi. Scientists have been mounting expeditions to that remote region ever since, trying to piece together a history of the dinosaurs.

A team of scientists writing in Geology has suggested a fascinating theory to explain the extraordinary preservation of dinosaur bones in the Gobi. Working in the Ukhaa Tolgod (brown hills), they sought an explanation for the rich variety of perfectly preserved fossils. They found they could study tiny skeletal structures in the buried bones, including features smaller than one of the letters in this sentence. Scientists have found the remains of fierce meat-eaters larger than tyrannosaurus rex, perfectly preserved with scarcely a bone missing. They sought to explain how an entire dinosaur was killed and buried so quickly that not even the quickest scavengers or bone-eating bacteria could get to the remains before the fossilization process started.

The scientists proposed a massive sand slide—an avalanche of watersaturated sand down the steeply curving side of a sand dune tall enough to dwarf even a 52-foot-long (14 m) monster like the tarbosaurus, a close relative of the tyrannosaurus. The geologists found three different types of sandstone at the site.

One had clearly visible layers tilted at a 25-degree angle and sorted by particle size, exactly the sort of sandstone that forms when a sand dune gets buried, compressed, and converted into rock. Some researchers had long ago suggested that the sandstorms creating such massive sand dunes might have suffocated and buried the dinosaurs, which would account for the well-preserved remains. Surprisingly, the researchers found very few fossils in these sand-dune-created sandstones.

The second type of sandstone had a different structure, but had clearly originally been laid down by the wind. The paleontologists found burrow marks made by insects and other small creatures that had dug into the surface of the dunes, but only below a certain depth. They concluded that this sandstone represented a flat, wind-blown layer and that dinosaurs had ambled over the surface and therefore crushed the insect and animal burrows in the upper levels, leaving the lower ones intact. This layer representing the windblown surface of a sand dune also contained no dinosaur fossils.

Almost all of the dinosaur fossils were found in the third type. This layer of sandstone proved much more surprising. The rock showed no structure or layering, as it would if laid down on top of a sand dune or in a bottomland deposit between dunes. Moreover, this type of sandstone contained rocks and pebbles much too large to be carried by the wind. Clearly, the dinosaurs buried and fossilized in this rock layer were not smothered by the sand-thick wind and then buried by the windblown sand. But scientists initially could not figure out how such a jumbled up layer of sand wound up with all of the fossils.

Fortunately, University of Nebraska geosciences professor David Loope had previously studied the Nebraska sandhills, much smaller dunes in a much wetter climate. He had collected strange stories of dunes that had suddenly collapsed and buried pickup trucks and other objects. In one case, a rain-drenched dune slid down and swallowed up most of a barn. Clearly, the wet, heavy sand dunes were prone to massive avalanches of sand. Loope concluded that in the Gobi's much wetter climate some 70 million years ago, the gigantic dunes became water-saturated in a storm and so slid down to bury an unsuspecting dinosaur at the base of the dune. That means that the dunes of the dinosaurs were probably wet, covered with vegetation, and not moving about constantly with the wind. They would have contained more clay, which would hold water until it got wet and slippery, at which point the clay layers would promote a catastrophic collapse. The overlying sand then protected the bones of the buried dinosaurs from scavengers, and the lack of oxygen deep beneath the sand prevented bacteria from breaking down the bones. As a result, the skeleton remained intact long enough for dissolved minerals in groundwater to crystallize and replace the organic material.

Various expeditions have uncovered remarkable dramas. One expedition located the nearly complete skeletons of two tarbosaurus, the largest known dinosaur predator. One had unlaid eggs still in her body. The sand preserved the details so perfectly that paleontologists could even study the barely formed bones of the embryos inside the eggs. Another expedition found the bones of a meat-eating dinosaur and his plant-eating victim entangled in the midst of their fatal struggle at the moment the avalanche of sand overwhelmed them both.

That remarkable preservation has yielded repeated surprises and inspired many scientific expeditions to the Gobi. For instance, one expedition found the oldest marsupial. Another expedition to the same area as the sand slide study discovered the well-preserved, 80-million-year-old bones of a tiny creature that lived in the dark shadow of the dinosaurs. The opossumlike delatheridium had sharp molars and long canines and a two-inch-long skull. It probably ate lizards and other early mammals. Scientists were surprised to find that creature in the Gobi Desert, since the marsupials today live mostly in South America and Australia. This finding suggests that the marsupial line originated in Asia and spread across the planet, but then died out in Asia in the face of competition from the placental mammals.