Patagonian fossils show Jurassic dinosaur had herd mentality

A vast trove of fossils unearthed in Argentina's southern Patagonia region is offering the oldestknown evidence that some dinosaurs thrived in a complex and well-organized herd structure, with adults caring for the young and sharing a communal nesting ground.

Scientists said the fossils include more than 100 dinosaur eggs and the bones of about 80 juveniles and adults of a Jurassic Period plant-eating species called Mussaurus patagonicus, including 20 remarkably complete skeletons. The animals experienced a mass-death event, probably caused by a drought, and their bodies were subsequently buried by wind-blown dust.

"It is a pretty dramatic scene from 193 million years ago that was frozen in time," said paleontologist Diego Pol of the

Egidio Feruglio Paleontological Museum in Trelew, Argentina, who led the research published in the journal Scientific Reports.

Mussaurus, which grew to about six metres long and about 1.5 tons, possessed a long neck and tail, with a small head. It was bipedal as an adult but newborns were quadrupedal. Mussaurus lived early in the Jurassic, the second of three periods comprising the age of dinosaurs. It was a relatively large beast for its time — much bigger than contemporaneous meat-eating dinosaurs. Dinosaurs became true giants later in the Jurassic.

"The site is one of a kind," Pol said. "It preserves a dinosaur nesting ground including delicate and tiny dinosaur skeletons as well as eggs with embryos inside. The specimens we have found showed that herd behaviour was present in long-necked dinosaurs since their early history. These were social animals, and we think this may be an important factor to explain their success."

The animals were found to have been grouped by age at the time of their deaths, with hatchlings and eggs in one area while skeletons of juveniles were clustered nearby. The eggs were arranged in layers within trenches. Adults were found alone or in pairs.

This phenomenon, called "age segregation," signals a complex social structure, researchers said, including adults that foraged for meals and cared for young. The researchers suspect members of the herd returned to the same spot during successive seasons to form breeding colonies.

"The young were staying with the adults at least until they reached adulthood. It could be that they stayed in the same herd after reaching adulthood, but we don't have information to corroborate that hypothesis," said paleontologist and study co-author Vincent Fernandez of the Natural History Museum in London.

Herd behaviour also can protect young and vulnerable individuals from attack by predators.

"It's a strategy for the survival of a species," Fernandez said.

The nesting ground was situated on the dry margins of a lake featuring ferns and conifers in a warm but seasonal climate. The eggs are about the size of a chicken's, and the skeleton of a hatchling fits in the palm of a human hand. The adults got as heavy as a hippo.

A scanning method called high-resolution X-ray computed tomography confirmed that the embryos inside the eggs were Mussaurus.