The first of three dino-filled pits was unearthed nearly a decade ago in northwestern China’s remote Xinjiang region.
Inside the 3.5- to 6.5-foot-deep (1- to 2-meter-deep) depressions were the largely complete skeletons of several species of small theropods, bipedal raptors from the lineage that includes Tyrannosaurus rex.
The stacked fossils included Guanlong, or “crested dragon,” a T. tex ancestor with a Mohawk-like head adornment. Limusaurus, also found in the pits, was a probable herbivore with an intriguing hand that some experts believe links dinosaur limbs to bird wings.
Even as scientists celebrated these rare fossil finds, a mystery remained: What created the death traps in which the animals were entombed?