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.