Paleontology, Science Education, Dinosaur Expeditions, Discoveries and Exhibits

Media Contacts:
Steve Koppes
(773) 702-8366
skoppes@uchicago.edu

Sarah Clark
(202) 828-5664
sclark@ngs.org

Release: March 16, 2009

Illustration © P. Sereno

Fig. 1—Map of Inner Mongolia in northern China showing the site of the discovery, a place near the outpost Suhongtu.

Illustration © Todd Marshall

Fig. 2—While approaching the edge of a lake in what is today the Gobi Desert of Inner Mongolia, a herd of young Sinornithomimus dinosaurs suddenly finds itself hopelessly trapped in mud some 90 million years ago.

Photo © Mike Hettwer

Fig. 3—Professor Zhao Xijin, legendary Chinese dinosaur hunter (left) and University of Chicago Professor Paul Sereno (right) compare fossil bones at the site of the buried dinosaur herd in the Gobi Desert of Inner Mongolia, China.

Photo © Mike Hettwer

Fig. 4—Professor Paul Sereno holds a plaque cast of two juvenile skeletons of the ostrich-mimic dinosaur Sinornithomimus.

Photo © Mike Hettwer

Fig. 5—Two juvenile skeletons of the ostrich-mimic dinosaur Sinornithomimus died when they were a little over one year in age. In their ribcages are stomach stones and the carbonized remains of their last plants they consumed. (cast)

Photo © Mike Hettwer

Fig. 6—Found under a ledge by Montana State University paleontologist David Varricchio, the uneven three-toed footprint cast matches closely the foot of an ornithomimid like those found at the herd site.

Photo © Mike Hettwer

Fig. 7—Entombed in red-stained mud, the forelimb of a juvenile of the ornithomimid dinosaur Sinornithomimus tells a story of a dinosaur death trap some 90 million years ago.

Photo © Mike Hettwer

Fig. 8—The skull of an ostrich, with a similar shape and no teeth, looks a lot like the skull of the 90 million-year old “bird mimic” dinosaur Sinornithomimus from the herd site.

A herd of young birdlike dinosaurs met their death on the muddy margins of a lake some 90 million years ago, according to a team of Chinese and American paleontologists that excavated the site in the Gobi Desert in western Inner Mongolia. The sudden death of the herd in a mud trap provides a rare snapshot of social behavior.

Composed entirely of juveniles of a single species of ornithomimid dinosaur (Sinornithomimus dongi), the herd suggests that immature individuals were left to fend for themselves when adults were preoccupied with nesting or brooding. “There were no adults or hatchlings,” said Paul Sereno, professor at the University of Chicago and National Geographic Explorer-in-Residence. “These youngsters were roaming around on their own,” remarked Tan Lin from the Department of Land and Resources of Inner Mongolia. Within an exquisite pair of the skeletons, prepared for display in Sereno’s lab and airlifted back to China in late February, stomach stones and the animals’ last meals are preserved.

Sereno, Tan, and Zhao Xijin, professor in the Chinese Academy of Sciences, led the 2001 expedition that found the fossils. Team members also included David Varricchio of Montana State University, Jeffrey Wilson of the University of Michigan and Gabrielle Lyon of Project Exploration. The findings are published in the December 2008 issue of Acta Palaeontologica Polonica, and the work was funded by the National Geographic Society and the David and Lucile Packard Foundation.

“Finding a mired herd is exceedingly rare among living animals,” said Varricchio, an assistant professor of paleontology at Montana State University. “The best examples are from hoofed mammals,” such as water buffalo in Australia or feral horses in the American West, he said. The first bones from the dinosaur herd were spotted by a Chinese geologist in 1978 at the base of a small hill in a desolate, windswept region of the Gobi Desert. Some 20 years later, a Sino-Japanese team excavated the first skeletons, naming the dinosaur Sinornithomimus (“Chinese bird mimic”).

Sereno and associates then opened an expansive quarry, following one skeleton after another deep into the base of the hill. In sum, more than 25 individuals were excavated from the site. They range in age from one to seven years, as determined by the annual growth rings in their bones.
The team meticulously recorded the position of all of the bones and the details of the rock layers to try to understand how so many animals of the same species perished in one place. The skeletons showed similar exquisite preservation and were mostly facing the same direction, suggesting that they died together and over a short interval.

The details provided key evidence of an ancient tragedy. Two of the skeletons fell one right over the other. Although most of their skeletons lay on a flat horizontal plane, their hind legs were stuck deeply in the mud below. Only their hip bones were missing, which was likely the handiwork of a scavenger working over the meatiest part of the bodies shortly after the animals died.
“These animals died a slow death in a mud trap, their flailing only serving to attract a nearby scavenger or predator,” Sereno said. Usually, weathering, scavenging or transport of bone have long erased all direct evidence of the cause of death. The site provides some of the best evidence to date of the cause of death of a dinosaur.

Plunging marks in mud surrounding the skeletons recorded their failed attempts to escape. Varricchio said he was both excited and saddened by what the excavation revealed. “I was saddened because I knew how the animals had perished. It was a strange sensation and the only time I had felt that way at a dig,” he said.

In addition to herd composition and behavior, the site also provides encyclopedic knowledge of even the tiniest bones in the skull and skeleton. “We even know the size of its eyeball,” Sereno said. “Sinornithomimus is destined to become one of the best understood dinosaurs in the world.”

High-resolution press photos are available from National Geographic for download.