The fossilized remains of six young dinosaurs found together in a "nursery" at a site in China show these animals had started forming social groups much earlier than previously thought, scientists said on Thursday.
The find sheds light on the life of the beaked dinosaur Psittacosaurus and on the origins of social behavior in its descendents, including the horned Triceratops, said Paul Barrett, a paleontologist at Britain's Natural History Museum, who led the study.
"We don't know very much about the early behavior of dinosaurs in general," he said in a telephone interview. "This discovery shows the early relatives were already social and living in groups."
The international team, which published its findings in this month's Paleontology journal, found the remains in the Yixian Formation, an area in northeast China rich in fossils of primitive mammals, birds and feathered dinosaurs.
Psittacosaurus was a small herbivore that lived in China, Mongolia, Siberia and Thailand about 130 million to 100 million years ago. It was an early relative of Triceratops and Protoceratops.
The largest of the young dinosaurs, probably aged one-and-a-half to three-years old when they died, measured about 50 centimeters (1.6 feet) from the tip of the nose to its tail and weighed about a kilogram (2 lbs). Adults were about 2 meters long and weighed up to 30 kilograms.
The age range of the fossils suggested they came from different eggs, laid by different parents, he said. The remains formed a nursery with babies from at least two different parents, he added.
The baby dinosaurs were probably killed in a volcanic mudflow, but the way the researchers discovered them, lying side by side, indicates they lived in a herd, Barrett said.
"These animals had left the nest and were already hanging out with each other," he said.
The remains also help answer a question that has long puzzled paleontologists about whether features such as horns and frills found in Triceratops preceded the development of social behavior, Barrett said.
"It used to be thought that social behavior only occurred when these animals had their horns and frills," he said. "Now we know that they are incidental to it and that the social behavior comes before them."