January 19, 2010—Two predatory dinosaurs known as Guanlong wucaii struggle in a muddy pit in a painting of China's Xinjiang region during the late Jurassic period, about 160 million years ago. Guanlong, which means "crested dragon," was a small theropod, a group of bipedal raptors from the lineage that includes Tyrannosaurus rex.
The painting depicts one of China's mysterious dinosaur "death pits"—3.5- to 6.5-foot-deep (1- to 2-meter-deep) depressions filled with the largely complete skeletons of several small theropod species. Now a new study has pinpointed a possible origin of the traps: They may be the mud-filled footprints of the 20-ton sauropod dinosaur Mamenchisaurus.
—Reporting by Brian Handwerk
Illustration courtesy Michael Skrepnik
Excavated Fossil Pit in China
An excavated dinosaur "death pit" seen in a 2005 photograph created a column of sandstone and mudstone standing about 5.2 feet (1.6 meters) tall.
The column, the insides of one of three such pits found so far, is filled with the fossils of nearly two dozen small dinosaur species. Because of the nature of the surrounding rock, experts think the pits were actually depressions—possibly giant footprints—filled with watery mud.
"All of the geological data indicated to us that we're dealing with sediments that were originally very rich in fluids," said study co-author David Eberth, a geologist at Alberta's Royal Tyrrell Museum. "These were never empty holes in the landscape."
Theropods like Limusaurus were common during the late Jurassic in China's Xinjiang region. Now part of the Gobi desert, Xinjiang was a marshy wetland 160 million years ago. Experts think that at some point, heavy volcanic eruptions showered the area in ash, which then created a semisolid surface over pockets of quicksand-like volcanic mud.
When the huge sauropod Mamenchisaurus went for a walk across this strange landscape, the massive plant-eater's feet punched through the ashy surface, Eberth and colleagues suggest in a paper to be published in the February issue of the journal PALAIOS. The sauropod's footprints became backfilled with thick mud, setting the stage for the death pits.
Photograph courtesy David Eberth
Young Limusaurus in Stone
At around 40 to 50 pounds (18 to 22.6 kilograms), small theropods such as Limusaurus (above, the fossil bones of a juvenile found in 2001) would have easily become ensnared in the muddy pits created by Mamenchisaurus's footprints.
Theropods in particular would have had tough times escaping, since the dinosaurs used only their hind legs for locomotion, study co-author Eberth said.
"It's very likely that other kinds of animals would have entered these pits but were able to get out," Eberth said. "We picture quadrupeds being able to get out of these pits because they essentially had a natural four-wheel-drive to pull themselves out."
Photograph courtesy David Eberth
Limusaurus Fossil From China
Whatever the dinosaur "death pit" origins, the importance of the skeletons found inside them is beyond doubt, said Hans-Dieter Sues, curator of vertebrate paleontology at the Smithsonian's National Museum of Natural History, who was not involved in the new study.
Theropods of the late Jurassic are considered closely related to birds and so are important clues in reconstructing the evolution of flight. But theropod remains are quite rare, he said, most likely because their carcasses were typically ripped apart by predators, and their smaller bones were far less likely to survive to the present day.
Until the discovery of the death pits, Sues said, "we had known very little about dinosaurs and other land vertebrates from this particular time interval anywhere."