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)-(Mike Hettwer, Project Exploration) . Photo credit: The University of Chicago.

Picture it: 90-million years ago, a herd of young dinosaurs is left without a babysitter. The juveniles go roaming off on their own and soon find themselves in trouble.

Flash ahead to modern times, where paleontologists discover their mass grave and reveal what happened next.

The scene is in Mongolia’s Gobi Desert, where a team of Chinese and American scientists have been excavating the site for years.

They say the birdlike dinosaurs stumbled into a death trap on the muddy shores of a lake, sealing their fate, and preserving their bodies so well, even their stomach contents are still intact.

The find itself is rare enough, but even more unusual is what it reveals about the social behavior of these dinosaurs. Specifically that the herd made up of a single species of ornithomimid dinosaur (Sinornithomimus dongi) was left to fend for itself when adults were preoccupied with nesting or brooding.

The fossils were first found in 2001, but only last month were two of the spectacular skeletons airlifted from the site. More than 25 individuals were excavated in all, ranging in age from one to seven years, as annual growth rings in their bones revealed.

Along with herd composition and behavior, the site also turned up minute details. “We even know the size of its eyeball,” said University of Chicago paleontologist Paul Sereno.

Click here for the scientific paper on the herd site as published in Acta Palaeontologica Polonica.

Click here for more information about team members and the expedition.