"Another skeleton with a perfect skull!” I shouted to the team, all of whom were face down on the quarry floor exposing other skeletons. In the years I had spent as a paleontologist, never had I seen anything like this. Our team of fossil hunters had been prospecting for only 15 days in the Gobi Desert of Inner Mongolia, but already we had uncovered a veritable graveyard of intact fossils.

Over the next few weeks we would apply chisel, pickax and bulldozer to the site, digging up more than a dozen examples of an ostrichlike dinosaur that was to become one of the most well known in the dinosaur world. But the story would soon grow far richer than a simple body count of fossil bones, as intact and well preserved as they might be. This group of individuals would reveal how these dinosaurs interacted with one another, how their society was built, as well as the circumstances surrounding their gruesome and untimely deaths. We were just beginning to uncover the first clues of this 90-million-year-old murder mystery. Little did I know that what we were about to learn would end up making this the richest site for a single dinosaur species I had ever encountered.

The Lure of the Gobi
Americans inevitably associate dinosaur discovery in the Gobi with Roy Chapman Andrews, the swashbuckling expedition leader from the American Museum of Natural History in New York City. In the 1920s Andrews ventured into the desert regions of Outer Mongolia and returned to great fanfare with the first known dinosaur eggs and the sickle-clawed wonder Velociraptor. Andrews was not the only explorer combing the desert, however. At around the same time, Swedish explorer Sven Hedin was recovering unprecedented fossils from the southern half of the Gobi in Inner Mongolia, a region that is now part of China.

In the intervening years, scientists searching near Hedin’s sites have uncovered dinosaur egg nests with brooding parents and sickle-clawed raptors that rival the best discoveries in Outer Mongolia. Yet scholars and public attention have favored Outer Mongolia; as a result, scores of international fossil expeditions have crisscrossed the area since it opened to the West more than a decade ago. In contrast, Inner Mongolia has remained relatively untouched.

I was a 27-year-old graduate student in geology in the middle of an around-the-world tour when I first visited Inner Mongolia in 1984—the first year that China allowed foreign tourists to travel in the country without an escort. After I arrived in the capital city of Hohhot by coal-powered steam locomotive, I visited the museum in the center of what was then a one-story town. Outside, dinosaur-age rock stretched for hundreds of kilometers west, flanking the fabled Silk Road linking the Mongolian steppe with the heart of Central Asia. When I returned to Beijing, I met with Zhao Xijin, a professor at the Institute of Vertebrate Paleontology and Paleoanthropology and one of China’s most accomplished fossil hunters, who had at the time already been responsible for discovering more than a dozen new species. We discussed exploring the area together at some time in the future. Some 16 years later the timing and circumstances finally aligned.

In 2000 I returned to Hohhot with Zhao to arrange the logistics of a major dig in the area. We stepped from the train onto the platform in Hohhot and were greeted by Tan Lin, a geologist and director of the Long Hao Institute for Stratigraphic Paleontology in Hohhot. Looking much younger that his 60 years, Tan energetically laid out the details of vehicles and supplies needed for a Gobi expedition the following spring. Fortunately, we would have no problem finding suitable expedition vehicles in Hohhot. The one-story town I knew had been replaced by a bustling metropolis with wide boulevards lined with flashing neon signs.

Tan suggested we revisit sites made famous by the fossil discoveries of Hedin and later expeditions. Certainly more fossils were there to be discovered. But I had other ideas. “Anywhere no one has been” was my refrain. Eventually the pull of the unknown won the day, and we decided to set out on the Silk Road in the spring, heading to the remote western reaches of the Gobi.

The First Clues
By mid-April 2001 our 16-person crew composed of American, French, Chinese and Mongolian fossil hunters had gathered in Hohhot. We divided ourselves among four field vehicles and a truck packed with tons of supplies for the 700-kilometer trip along the banks of the Huang He (Yellow River) and out into the desert.

We set up our first base camp not far from the tiny outpost of Suhongtu. The Gobi wind battered our iron-framed Chinese army tents, spraying a coat of silt and dust on everything inside. Hair soon stood erect as if gelled. Showers were out of the question, given the shortage of water and profound chill.

Every day we set out to hunt for fossils. Team members would walk for miles over the uneven terrain, searching for interesting finds that might be peeking out of the rock. With fossil hunting, it’s good to be lucky, but it’s better to be blessed with the “nose”—a natural talent for sniffing out fossils.

Montana State University paleontologist Dave Varricchio spotted the first major find—a three-toed footprint on the underside of a low ledge of rock. That footprint, notable for its short side toes, was small for a dinosaur, though bigger than his hand. We deduced that it was likely made by a large ornithomimid, or “bird mimic.” Soon we would have no doubt about what made the print.

The area surrounding the camp was Late Cretaceous in age—about 90 million years old—according to a Chinese geology map printed some 25 years earlier. Besides the footprint, our finds were limited to bones of small dinosaurs found before in the Gobi, and so we moved into a broad valley nearby where fossils were more plentiful. Soon team members were poking at several finds, including what was likely a primitive duckbill skull poking out of the surface. Another fossil appeared to belong to a small sauropod, the four-legged plant eaters that often grow to enormous size.

The most interesting site was a vertical wall of layered red and blue rock that was peppered with the leg bones of several relatively small dinosaurs. This was not a natural wall. Hewn by chisel and pick, it was the back wall of a fossil quarry. Someone had been there before us.

Tan explained that this site was originally found in 1978 during a mapping survey by a geologist and former classmate of his. “Look here,” he said, pointing to a small bone symbol on the geology map. Using this map, Tan had guided a joint Chinese-Japanese-Mongolian expedition to this site four years earlier in 1997. Running short of time and materials, they had collected multiple skeletons but stopped before the excavation was complete.

In my backpack was a paper from 1999 on a new ornithomimid from the Gobi by a young Japanese paleontologist, Yoshitsugu Kobayashi, who at the time was a graduate student at Southern Methodist University. I now realized that the paper was based on finds from the quarry before me. Kobayashi and his colleagues had noted the presence of many fossils as well as their exquisite preservation; some fossils even included stomach stones—pebbles that ancient sauropods (and modern-day alligators, seals and birds) ingest to help grind food and aid digestion. In 2003 Kobayashi would name this dinosaur Sinornithomimus dongi. Yet mysteries remained: Why were so many fossils preserved in one small area? Did they all die at the same time or over millennia? And if they did all perish at once, how?

The quarry was located at the base of a small, rocky hill in a desolate windswept region of the Gobi. The horizon had turned gray brown—an early warning of an approaching dust storm. In the desert, these warnings are best measured in minutes, not hours. We raced for the field vehicles, hoping to dash back to camp while we could still navigate by the tracks we left on the way there. Within minutes the wind began to sling sheets of sand, dulling the paint on the lower reaches of our trucks.

The pore-filling dust and bone-chilling cold was offset at day’s end by the camp cook, who regularly served seven-course meals for dinner—always a different seven from the night before. Fortunately, the camp was located not far from an army outpost, giving us access to fresh vegetables. Chinese cuisine, to my palate, is the best the world over. Equally renowned is Chinese beer, which we downed by the quart that season in celebration of our luck racking up noteworthy finds.

The Death Trap
We returned to that valley every day for the next few weeks, many of us devoted to unraveling the mystery of the ornithomimid quarry. One skeleton led to another, as our tools pushed the back wall of the quarry deeper into the hill. Others studied and sampled the cliff face, compiling a detailed log of the rocks entombing the graveyard.

When multiple individuals of a single species are preserved in one place, a paleontologist must ask whether that assemblage is natural—that is, was it a family group or herd congregating, as it might on a given day, only to be caught dead in its tracks? Most bone accumulations of a single species are not so interesting. Rather they are composed of unrelated individuals that, over some unknown length of time, died near a water hole or were washed in by a flood.

If we quickly bagged the remaining skeletons, the most interesting part of the story—how all these dinosaurs died—would be lost forever. Clues to the cause, circumstances and timing of death do not reside solely in the bones themselves but also in the position of skeletons, in the presence of tooth marks or splintered bone, and in the character of the sediment that is laid down before, during and after death. A crime scene, not a pa­leontologist’s trophy trove, is how we must view such a quarry.

We soon came to believe that these animals all met their fate at the same time. The skeletons were not randomly distributed—all the bones seemed to point in the same direction. That could have been the result of a flood or a river carrying multiple sets of bones to the same place, but we could not find any evidence that the bones were moved in this way. All the skeletons were intact.

In addition, the thin-layered red and blue rock of the cliff face implied that the area used to consist of fine-grained mud and silt. We found spots in the infilled mud cracks, suggesting that the area went through wet and dry periods. Tiny, flat shells of freshwater creatures called conchostracans blanketed some of the skeletons, the flotsam from an expanding lake. Near the skeletons the mud was nearly pure, lacking the worm burrows and roots of a soil that supported plant life. All in all, the rock surrounding the skeletons suggested the ebb and flow of an ancient lake—an oasis in an otherwise dry area.

A collection of fossils such as this was unheard of—it was (and remains to this day) the only known Pompeii-like sampling of a dinosaur species. As the quarrying operation continued, we spent many hours musing on death scenarios. Perhaps these dinosaurs perished from a nearby volcano or succumbed to a flash flood? “Perhaps they just got stuck in the mud?” team member Gabrielle Lyon suggested, while outlining with a jeweler’s needle the clenched digits of the foot of a fallen dinosaur. To me, the idea of a mud trap seemed a bit far-fetched. Though an experienced excavator, Lyon was an educator, not a paleontologist or geologist. Modern animals such as cows sometimes die near water holes—the large beasts get mired up to their kneecaps in mud and ultimately die of thirst, exposure and starvation. Yet it is extremely rare for entire herds to die this way (although sometimes it does happen to wild horses, noted Varricchio, the expedition’s expert on taphonomy—the science of death and dying).

As we dug, more clues began to accumulate. Dave spotted ­V-shaped patterns on the cliff face near the horizon preserving the skeletons. The layers of mud were deformed downward, as if pulled by the passage of a thin object such as the claw on a dinosaur’s toe. Was this evidence of a lethal dance in mud?

Unfortunately, we were not going to be able to dig for much longer. Our ornithomimid quarry was angling downward into the hill, getting more difficult to extract by the day. A complete excavation using the tools we had available would take months, if not years. Fortunately, we were soon to learn that in China, anything is possible.

On our day off we traveled out to the Chinese army outpost, where we were challenged to a game of basketball by the recruits. Professors Tan and Zhao watched from the sidelines, noticing the impressive heavy equipment parked nearby. Having befriended the soldiers during the basketball game, we took our case to officials at the base that evening, helped along with mind-numbing rounds of baijiu, a spirit that euphemistically translates to “white wine,” even though it is served by the shot. A few days later a giant bulldozer arrived at the site.

As the blade trimmed off the top of the hill a few centimeters at a time, we followed in its wake, searching for fossils above the graveyard. “Stop!” shouted Jeff Wilson, who was hunched over a block tipped up by the monstrous blade. A paleontologist at the University of Michigan at Ann Arbor, Wilson had spotted some jaws and teeth. We picked through the tailings to each side of the last pass until we found all the missing pieces. Locked in that concretion just one human body length above the graveyard was a 45-centimeter skull of an unknown predator.

By the fourth day at the quarry, the bulldozer had removed the entire hill, its blade just above the graveyard. We resumed excavation until we unearthed the last of 13 individuals. Skeletons usually collapse on a flat surface to be buried in mere centimeters of sediment, but as we chipped the mudstone from the main horizon containing the skeletons, the hind legs of several dinosaurs plunged deep into the mud. Some of the otherwise perfectly preserved skeletons were missing hipbones. These individuals looked as if they were trapped in mud, only to suffer the attention of ancient scavengers.

It was just what we would expect if a herd of dinosaurs on the move all became stuck on the same muddy shore. Lyon’s hypothesis, drawn from the panic she must have sensed in the bones, was emerging as the most likely death scenario.

In Life and Death
Back at the University of Chicago, members of my research team cleaned the skeletons one by one under the microscope, revealing a remarkable level of preservation. Not only were stomach stones preserved, but they seemed to retain the shape of the gizzard where they once pulverized plants. We also discovered a thin film of black carbon coating either side of the gizzard. The black material was the remains of the dinosaurs’ last meal.

Additional evidence helped to confirm one of our hunches from the excavation. In the desert we had noticed that all the skeletons in the quarry were immature. At a field site the best way to measure a dinosaur’s age is to examine the individual bones that constitute the backbone. Every vertebrae is made of a spool-shaped bone (the centrum) below and a curved structure (the neural arch) above. If these two parts are fully fused, the backbone is no longer growing, and the dinosaur is mature. All skeletons collected at Suhongtu had vertebrae preserved as two parts.

Yet this feature provided only a crude estimate of the dinosaurs’ ages. Back at Chicago, we sliced bones into thin sections to count their annual growth rings as you would a tree. We learned that the skeletons ranged from one to seven years of age, with most ranging between one and two years old. This pattern told us two things. First, it meant that Sinornithomimus must have required about 10 years to reach maturity. And second, we realized that the herd at Suhongtu was a band of adolescents—dinosaur teenagers cruising in a pack.

With this realization we could put together the full history of the dinosaurs—not just the way they died, but the way they lived as well. Paleontologists have speculated about the social habits of half-grown dinosaurs, but the herd at Suhongtu provides the best evidence to date. Because maturation took about a decade in Sinornithomimus, juveniles had plenty of opportunities to congregate. Adults were busy with a range of activities during the breeding season—courtship, nest building, nest defense, brooding, and nurturing of hatchlings. Juveniles seem to have wandered about, fending for themselves as they went.

This particular group had met an untimely end. To a passing herd, the mud trap would have looked like many other areas along the lake’s edge—mud that might record a footprint rather than swallow a foot. A central pair of skeletons most dramatically captures the 90-million-year-old tragedy. These two animals lay hopelessly trapped, their bodies collapsed sideways on the surface, one on top of the other, their feet deeply anchored in mud. Their skeletons were exceptionally complete except for their hipbones, which must have been pulled off by hungry scavengers. An isolated hipbone helped to confirm that scenario, the central portion of its blade crushed under the weight of an intruder’s toe.

Then the water level rose, at least briefly, gently sealing in mud the graveyard and its tale of woe.