Fossils from a 190-million-year-old nesting ground have revealed that its dinosaurs likely lived in herds. The species – Mussaurus patagonicus – are now the oldest known herding dinosaurs, according to a paper in Scientific Reports.
“We’ve now observed and documented this earliest social behaviour in dinosaurs,” says Jahandar Ramezani, a researcher at the Massachusetts Institute of Technology, US, and co-author on the paper.
“This raises the question now of whether living in a herd may have had a major role in dinosaurs’ early evolutionary success. This gives us some clues to how dinosaurs evolved.”
The researchers gathered this evidence by examining Mussaurus eggs and skeletons from the fossil site, which is in the Laguna Colorada Formation, in southern Argentina.
“Such a preserved site was bound to provide us with a lot of information about how early dinosaurs lived,” says Diego Pol, a researcher at the National Scientific and Technical Research Council of Argentina (CONICET), who discovered the site and is lead author on the paper.
The site contains at least 80 Mussaurus skeletons and 100 eggs. The researchers scanned 30 of the eggs with a synchrotron, finding some had embryos inside them.
“We use high-energy X-rays to penetrate the sample without destroying it and get a full view inside it,” says co-author Vincent Fernandez, who did the scanning at the European Synchrotron in France.
“We spent four days scanning the eggs around the clock. It was tiring, but the exciting results were morale-boosting.”
All the embryos were the same species of Mussaurus, indicating that they came from a communal breeding site.
The researchers also examined the age of the dinosaur skeletons at the site.
“The bones of these dinosaurs grew in annual cycles, like tree rings, so by counting the growth cycles we could infer the age of the dinosaur,” says Pol.
Skeletons were also grouped by age, with one-year-old skeletons found in clusters together. Older adults and sub-adults were found in pairs or alone, but all within the same small area (a square kilometre in size).
“This may mean that the young were not following their parents in a small family structure,” says Ramezani. “There’s a larger community structure, where adults shared and took part in raising the whole community.”
The researchers calculated the site’s age at 193 million years using radiometric dating (specifically, uranium-lead dating). This is 40 million years prior to the current oldest evidence of “gregarious” (social) behaviour in dinosaurs.
“These are not the oldest dinosaurs, but they are the oldest dinosaurs for which a herd behaviour has been proposed,” says Pol.
The researchers suggest that their evidence throws the evolution of herding back even further. “Palaeontological understanding says if you find social behaviour in this type of dinosaur at this time, it must have originated earlier,” says Ramezani.
The researchers propose that herding may have started between 227 and 208 million years ago, at the same time as dinosaur bodies increased in size.
“Mussaurus belongs to the first successful family of herbivorous dinosaurs, so we postulate that being social and protecting their young together as a herd may have been part of the reason these long-necked dinosaurs were so common in all continents,” says Pol.