Geoscientist Online

Ninety million years ago, in what is now the Gobi Desert, a herd of young birdlike dinosaurs became trapped in the muddy banks of a lake, writes Sarah Day

Geoscientist Online 17 March 2009

They died a slow death, flailing about in their attempts to escape, while scavengers waited for a chance to pick over their remains, according to researchers. These remarkable details of how more than 25 individual dinosaurs met their end have been preserved at a site in western inner Mongolia, first discovered by a Chinese geologist in 1978 at the base of a small hill. The first skeletons were excavated 20 years later by a Sino-Japanese team, who named the dinosaur Sinornithormimus (“Chinese bird mimic”).

Now, a team of Chinese and American palaeontologists, who led an expedition in 2001 to fully excavate the skeletons, have published their findings in Acta Palaeontologica Polonica (December 2008). Two exquisitely preserved skeletons have since been displayed at the University of Chicago, before being airlifted back to China in late February.

The sudden nature of the dinosaurs’ deaths provides a rare snapshot of their social behaviour. The herd was composed solely of juveniles of a single species, suggesting that they were left to fend for themselves while adults were nesting or brooding. Growth rings in their bones indicate that their ages were between one and seven years.

Turn your back for a minute and look what happens

‘These youngsters were roaming about on their own’, says Tan Lin of the Department of Land and Resources of Inner Mongolia, one of the 2001 expedition leaders.

Preservation at the site is so fine, plunging marks in the mud surrounding the skeletons can be seen, recording the dinosaur’s failed attempts at escape. The skeletons were similarly well preserved and most faced the same direction, suggesting they died together in a short space of time. Some hip bones were missing, an indication of scavengers at work on the meatiest parts of their bodies shortly after death.

‘These animals died a slow death in a mud trap, their flailing only serving to attract a nearby scavenger or predator’, remarks Paul Sereno, professor at the University of Chicago and National Geographic Explorer-in-Residence, another leader of the 2001 expedition.

Even the tiniest bones in the skeletons were perfectly preserved. ‘We even know the size of if its eyeball’, says Sereno. ‘Sinornithomimus is destined to become one of the best understood dinosaurs in the world’.

For David Varrichio, a member of the expedition from Montana State University, the unusual site made the dinosaurs’ plight seem far more immediate: ‘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’.