About 90 million years ago, in what is now Mongolia, the ground collapsed beneath a group of immature Sinornithomimus that had been walking on the edge of a drying lake bed. The ostrich-like dinosaurs struggled to free themselves, clawing at the thick mud and calling out in desperation, but to no avail. They soon perished of hunger and dehydration, and scavengers picked at the parts of the rotting carcasses that protruded from the mud. Still, much of the skeletons remained in the morass, and their death poses became preserved as minerals seeped into the bones over millions of years.
The fossils, recently discovered by paleontologists, are providing interesting clues to the social life of Sinornithomimus, which was first described in 2003. Typically, new dinosaurs are represented by a few scraps of material, but in the case of Sinornithomimus, scientists found at least 13 skeletons—all in the same geological bedding plane. There was little doubt that they had died together, and the fact that most of them were juveniles intrigued researchers.
According to a new study published in Acta Palaeontologica Polonica, the assemblage of skeletons represents a mass-death event. The bones did not accumulate over many years, but are the remains of a single tragedy. Paleontologists determined that the dinosaurs were young by looking at growth rings inside the leg bones. The majority of skeletons were from animals about 1 to 2 years old. Why was this age group so well-represented?
The authors of the new paper think that it has to do with breeding. Many dinosaurs, we now know from fossil evidence, constructed nests and looked after their offspring like living birds and crocodylians do. This takes a lot of time and energy, which required parent dinosaurs to focus on their nests and not on the last year’s brood. The juvenile Sinornithomimus were too old to stay in the nest but mature enough to wander around on their own. During this time, the immature individuals probably grouped together like juvenile ravens and ostriches do today. Although these groups may have formed due to social reasons, they also would have provided some protection from predators. The larger a group is, the lesser the chance a particular individual will be picked out and eaten, and all those eyes make it more likely that a predator will be seen before it has a chance to strike.
As illustrated by the death assemblage above, young dinosaurs probably suffered high rates of mortality. While parental care was focused on the new clutch of eggs or helpless hatchlings, the juvenile dinosaurs could get into all sorts of trouble. In the face of a predator, belonging to a group might raise the chances an individual might live to see another day, but this provided no defense against stumbling into the sucking muck that entombed them.