David Varricchio, an assistant professor and paleontologist in the Department of Earth Sciences, and colleagues from the University of Chicago and China wrote the paper after a 2001 expedition to the Gobi Desert. It was published in December in the journal Acta Palaeontologica Polonica.
The paper describes the team's work at a 90-million-year-old dry lake bed in western Mongolia. Over the past decade, paleontologists have recovered more than two dozen fossilized skeletons of the dinosaur Sinornithomimus.
All of the skeletons belonged to animals between one and seven years old and were well-preserved. Most of skeletons were facing the same direction, suggesting that they died together in a short period of time, Varricchio said.
"Normally there are a lot of post-mortem effects that transpire between when a dinosaur died and when it was buried," he said. "This site really provides, in my mind, better evidence than any other dinosaur locality of how the dinosaurs perished, and that's pretty rare for any fossil vertebrate."
Varricchio believes that the dinosaurs probably became mired in the mud around a partially dry lakebed during the Cretaceous Period. During times of drought, as were common in the region at the time, these oases likely attracted many animals, he said.
Many of those animals were probably weak from starvation and dehydration, which could explain why so many of them became trapped in the mud. It's a phenomenon that's still seen around dry desert lakes today, he said.
The fact that so many young dinosaurs of the same species died at roughly the same time and in the same place tells paleontologists something about the social behavior of the animals, Varricchio said. It may be that young dinosaurs -- too old for the nest but not yet old enough to fend for themselves -- roamed together in social herds, he said.
"We get a snapshot-like view of what a herd of these animals looked like back in the Cretaceous Period," Varricchio said. "That snapshot gives us a glimpse into their biology and their behavior."
Past studies have theorized that dinosaurs had strong and complicated parenting relationships with their young, Varricchio said. Female -- and even male -- dinosaurs were tied to a nesting spot for the breeding portions of the year while they took care of their eggs, he said.
The fact that the parent dinosaurs were busy with the eggs could explain why a group of adolescent dinosaurs was roaming together without adult supervision, Varricchio said. These and most dinosaurs would take several years, at least, to fully mature. Groups of juveniles would consist of those individuals too old to be cared for by parents, but too young to breed, he said.
"This site argues that this might be a general trend among dinosaurs," and is further evidence of the theory that dinosaurs were dedicated parents, he said.
Varricchio's collaborators include Paul Sereno from the University of Chicago, Tan Lin from he Department of Land and Resources of Inner Mongolia and Zhao Xijin from the Chinese Academy of Sciences. Also on the team were Jeffrey Wilson from the University of Michigan and Gabrielle Lyon from Project Exploration.
The work was funded by the National Geographic Society and the David and Lucile Packard Foundation.
Contact: David Varricchio at 406-994-6907 or firstname.lastname@example.org.