were stacked up inside in a mixture of volcanic mudstone and sandstone, say geologist David Eberth and colleagues, whose work was partially funded by the National Geographic Society.

"All of the geological data indicated to us that we're dealing with sediments that were originally very rich in fluids," said Eberth, of Alberta's Royal Tyrrell Museum. "These were never empty holes in the landscape."

Instead, the death pits might have been created by the wanderings of the massive sauropod dinosaur Mamenchisaurus, Eberth's team suggests in the study, which will appear in the February issue of the journal PALAIOS.

Although the pits lie in what is now the Gobi desert, 160 million years ago the region was a marshy wetland. (Related pictures: life in the Jurassic period.)



But at some point during the latefootprint idea is plausible, said Hans-Dieter Sues, curator of vertebrate paleontology at the Smithsonian's National Museum of Natural History.

"The small, possibly plant-eating theropod Limusaurus was common and conceivably could have traveled in small flocks or herds," said Sues, who was not involved with the study. "Many of these animals could become trapped on such treacherous ground."

"The larger predatory Guanlong would then have been attracted by seemingly easy prey and became itself ensnared in mud," he said.

Whatever the pits' origins, the importance of the skeletons found inside them is beyond doubt, he added.

Theropods of this era are considered closely related to birds and so are important pieces in reconstructing the evolution of flight. (Related: "New Feathered Dinosaur Found; Adds to Bird-Dino Theory.")

But even though the small raptors were common