Dino crew: While approaching the edge of a lake in what is today the Gobi Desert of Inner Mongolia, a herd of young Sinornithomimus found itself hopelessly trapped in mud some 90 million years ago. The discovery supports the idea that juvenile dinosaurs formed herds.
Credit: Todd Marshall
BRISBANE: Fossils have been discovered of an entire herd of young dinosaurs that died together when they became trapped in mud 90 million years ago.
“We found what you would call palaeontological gold,” said Paul Sereno, at the University of Chicago in the U.S. and a co-author of a study detailing the find. “It’s the first real population of dinosaurs caught in a snapshot – a death trap – that has ever been discovered.”
The remains of more than 25 dinosaurs, all juveniles of the same species, are incredibly well-preserved, even down to the gouges made in the mud as they tried to escape.
Lucky break
The unique find provides a rare insight into the social behaviour of the bird-like species Sinornithomimus dongi, according to the study in the journal Acta Palaeontologica Polonica.
There are several fossil examples of animals that died after becoming trapped in mud, including a flock of birds, but this is the first time a herd of dinosaurs has been found mired.
A joint Chinese and American team began excavating the fossilised herd in 2001 in the Gobi Desert in Inner Mongolia. What is now an arid hillside was once a lake, Sereno said, and the configuration of the skeletons tells the tale of a whole herd becoming hopelessly trapped in sticky mud along the shoreline.
Most of the skeletons’ leg and tail bones extend deeply down into layers of sediment, while the bones of the upper body lie in a horizontal plane, showing how the dinosaurs slumped over when they finally died of thirst or starvation.
Worked over by scavengers
Unusually, the bodies were not thoroughly worked over by scavengers, so bones and even traces of soft tissue are preserved in close to their original positions. A few of the fossils even contain the carbonised remains of their last meals.
Such anatomically accurate skeletons provide a wealth of information. “Sinornithomimus is destined to become one of the best-understood dinosaurs in the world,” Sereno said.
By counting annual growth rings in the dinosaurs’ bones, Sereno’s team discovered that the herd contained only juvenile dinosaurs between one and seven years old. This suggests that juvenile dinosaurs sometimes banded together to fend for themselves while the adults were preoccupied with nesting.
Palaeontologists had suspected that juveniles formed herds before, but this is the most compelling evidence yet found for the theory. “We think we’ve found a smoking gun, the site that preserves a juvenile or immature herd, out wandering on their own,” Sereno said.
Steve Salisbury, a palaeontologist at the University of Queensland in Brisbane, Australia, said that the find is exciting because of its potential to reveal dinosaur behaviour
“Most information about [dinosaur] social behaviour comes from trackways,” he said, referring to the fossilised footprints left by stampeding animals. “But without bones, it’s hard to tell about ages.”
