Perhaps the shriek of a dying animal enticed the dinosaur into the trap. Or maybe it was the scent of rotting flesh. Whatever the bait, once the predator was lured into the mud pit, it quickly forgot its prey. It thrashed futilely in the mire for a long while, but its legs couldn’t reach the bottom. Doomed, the animal slowly accepted its fate and succumbed to exposure, but not before its struggle attracted another predator to the pit, continuing the cycle of the death trap. Eventually the mud turned to stone, entombing its victims, stacked one on top of another, for 160 million years.
This is the story contained in a column of rock unearthed in northwestern China’s Junggar Basin. But that column is just part of a startling collection of fossils excavated over the past seven years by paleontologists James Clark and Xu Xing with support from the National Geographic Society. Their discoveries are opening a new window onto an obscure period in Earth’s geologic history—a violent interval that lasted from about 165 to 155 million years ago and saw the continents breaking apart and dinosaurs undergoing a burst of evolution. As landmasses divided and animals became isolated from each other, a profusion of new branches sprouted on the dinosaur family tree. These new branches eventually yielded many famous dinosaur groups, including horned ceratopsians, armored stegosaurs, and tyrannosaurs. But the dearth of terrestrial fossils from this ten-million-year span had vexed scientists. “We could trace these groups back through time to this period, but then the trail went cold,” says Clark, a professor at George Washington University.
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.
So far, the team has located three of these death pits, each a diorama of a desperate struggle. One features a crocodilian huddled next to a small ceratosaur; another contains three headless ceratosaurs, possibly decapitated by predators big enough to avoid getting stuck in the trap. The most spectacular of the pits contains the remains of a 170-pound tyrannosauroid. Xu named it Guanlong, Chinese for “crowned dragon,” a reference to a crest that stretched from its snout to the back of its head. Its discovery sent rumblings through the paleontology community, because it represents the earliest and most primitive of the infamous “tyrant lizards,” a family of powerful predators that culminated more than 90 million years later with the most fearsome of them all, Tyrannosaurus rex.
“What’s striking about this find,” says Thomas Holtz, a tyrannosaur expert at the University of Maryland, “is the revelation that T. rex, the king of the tyrant lizards, comes from such humble origins. Most people think of tyrannosaurs as dominant predators, but that was toward the end of their line. For most of their history, they probably were the jackals of their day.”
To its surprise, the team found a second, smaller Guanlong beneath the first, leading to speculation that an adult had gone after a youngster trapped in the pit. It’s possible, says Holtz. “We have numerous tyrannosaur skulls that bear bite marks from other tyrannosaurs.”
Clark and Xu are eager to explore new parts of the Junggar Basin later this summer. “There’s still so much to find,” says Clark. “I can’t wait to see what turns up next.”