Dozens of fossils from a bird-like theropod dinosaur known as Avimimus have been unearthed in a single bonebed in Mongolia, providing strong evidence that they were gregarious, social beings.
Avimimus lived in the Upper Cretaceous in what is now Mongolia, approximately 70 million years ago. This artist’s rendering shows a pair of nesting bird-like dinosaurs. Image credit: Chuang Zhao.
“The common mythology of dinosaurs depicts solitary, vicious monsters running around eating everything,” said Gregory Funston, a paleontologist at the University of Alberta, Canada.
“Our discovery demonstrates that dinosaurs are more similar to modern animals than people appreciate.”
The discovery comes from a site at the Nemegt locality in Mongolia, first encountered by researchers in 2006.
Bonebeds provide good evidence that the animals were living together in colonies. Though rare in the Jurassic and Triassic, they dominate the Cretaceous period. However, this is the first discovery of a bonebed of bird-like dinosaurs.
“A monodominant bonebed of Avimimus from the Nemegt Formation of Mongolia is the first oviraptorosaur bonebed described and the only recorded maniraptoran bonebed from the Late Cretaceous,” Funston and co-authors said.
“Evidence from other bonebeds suggests that non-bird theropod dinosaurs had a tendency to form juvenile-dominated herds. Adult-dominated bonebeds of non-theropod species also typically contain a small percentage of juvenile individuals.”
“The Avimimus bonebed is therefore unusual in its near absence of juveniles,” they said.
Field locality of the Avimimus bonebed. Image credit: Gregory F. Funston et al.
The presence of more than two adults suggests that the bonebed is not an isolated family group, and the mix of subadults and adults in such a large aggregation argues against an assemblage of parents and their offspring.
“Although speculative, the paucity of juveniles in the bonebed may instead indicate that Avimimus formed age-segregated assemblages,” the scientists said.
“These groups may have enjoyed reproductive, antipredator, or foraging benefits, but the contribution of these factors to the formation of the assemblages is unclear.”
“Lekking behavior, where individuals group to display to potential mates, is known in multiple groups, especially birds. Aggregations may include as many as 100 individuals of varying age, size, and sexual fitness. The near absence of juveniles is consistent with, but not indicative of, a lekking assemblage.”
Alternatively, the Avimimus bonebed may be evidence of flocking or communal roosting behavior.
“With an assemblage like this, you can’t really understand why the dinosaurs died together unless you see the field site,” Funston said.
“We can tell that they were living together around the time of death, but the mystery still remains as to why.”
The bonebed also produced skull elements that were formerly unknown for Avimimus and provide important anatomical information.
The paleontologists report their findings in the journal Scientific Reports.
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Gregory F. Funston et al. 2016. The first oviraptorosaur (Dinosauria: Theropoda) bonebed: evidence of gregarious behaviour in a maniraptoran theropod. Scientific Reports 6, article number: 35782; doi: 10.1038/srep35782
