The mysterious "death pits" holding the fossil skeletons of nearly two dozen small dinosaur species may actually be the 160-million-year-old footprints of a giant dinosaur, a new study has suggested.
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 depressions were the largely complete skeletons of several species of small theropods, bipedal raptors from the lineage that includes Tyrannosaurus rex.
Though the fossil finds were quite rare, the mystery remained that what created the death traps in which the animals were entombed.
Analysis of the rocks surrounding the dinosaur fossils shows that the unfortunate animals were stacked up inside in a mixture of volcanic mudstone and sandstone, according to geologist David Eberth and colleagues.
"All of the geological data indicated to us that we're dealing with sediments that were originally very rich in fluids," said Eberth, of Albert"s Royal Tyrrell Museum. "These were never empty holes in the landscape," he added.
Instead, the death pits might have been created by the wanderings of the massive sauropod dinosaur Mamenchisaurus, Eberth's team suggests in the study.
According to the study authors, when Mamenchisaurus went for a walk across the strange landscape, the massive plant-eater's feet punched through the ashy surface, and the dinosaur's footprints became backfilled with thick mud.
Like beach walkers leaving their "vanishing" prints in wet sand, the huge dinosaur's filled-in prints became "invisible" pit traps.
At around 40 to 50 pounds (18 to 22.6 kilograms) each, comparatively tiny theropods and other small animals could have walked unhindered across most of the region's solid ash crust.
But, the dinosaurs would have easily gotten trapped if they had stumbled into Mamenchisaurus's muddy footprints.
"Theropods in particular would have had a tough time escaping, since the dinosaurs used only their hind legs for locomotion," Eberth said.
In addition, small theropods were most likely coated in feathers that, when covered with mud, would have weighed them down further.
When an animal died, it would become at least partially submerged. Other creatures would then fall into the muddy trap, creating layers of entombed bodies.
After a few months, it's possible some animals were able to escape death because they could stand on the piled-up corpses, according to the study authors.
Multiple individuals of the same species found together in the pits allow paleontologists to better understand the way dinosaurs grew and aged, as well as their roles in the larger ecosystem.