In 2000 Clark joined Xu (pronounced shoe), a rising star at Beijing’s Institute of Vertebrate Paleontology and Paleoanthropology, for a scouting trip to the Junggar Basin. The following year the two scientists mounted a full expedition to the basin’s Shishugou formation, one of the few places on Earth where the exposed rock dates to the Middle Jurassic. Some 160 million years ago it was a marshy realm at the foot of a small mountain range riddled with volcanoes. Now it is a series of desiccated badlands and dunes splayed along the Gobi desert’s western edge.
“We intentionally chose an area where I had seen a lot of small fossils,” says Clark, noting that small prehistoric species tend to be rare, more difficult to find than large creatures. The excavation of one massive skeleton, like that of a multi-ton sauropod, can take an entire field season, Clark says. Instead, he and Xu focused on quantity, a strategy they hoped would yield plentiful clues about the missing segments of the dinosaur panorama.
As their team of Chinese and North American scientists and excavators began to dig, a menagerie of primitive creatures emerged from the rock, including new species of turtles, crocodilians, winged pterosaurs, and early mammals. Many exhibit the onset of traits that evolution would magnify in later species. A ceratopsian skull, for example, bears a bony halo, hinting at the massive horned frill that would crown its descendant Triceratops tens of millions of years later. A partial stegosaurian skeleton offers a look at one of the earliest known members of the famous armor-plated dinosaurs.
High on Clark and Xu’s wish list was to find a theropod, a type of two-legged, meat-eating dinosaur from the lineage that led to birds. So it was with giddy expectation that the team began to excavate a skeleton of an unknown bipedal hunter. As Clark and Xu dug down around the theropod, they kept encountering more bones. “We realized that another theropod was buried under the first,” says Clark. “And we suspected more were below that one. That’s when we got even more excited.” All told, they removed a column of rock containing five small dinosaurs.
Later, in Beijing, Xu and his technicians began extricating the fossils from the rock using micro-drills and dental picks. An analysis of the rock revealed large amounts of volcanic ash, suggesting an eruption had occurred not long before the animals died. David Eberth of the Royal Tyrrell Museum in Canada, the team’s geologist, believes that thick veils of ash fell into the marsh, creating a viscous mud. As massive sauropods lumbered across this soft ground, their footsteps could have made the pits that became dinosaur death traps.