A fossilised herd of baby Psittacosaurus dinosaurs has been discovered in China. This remarkable find sheds light on the life of these young dinosaurs, and reveals clues to how social behaviour evolved in their descendants.
Six complete skeletons of baby dinosaurs were found lying side by side in the exact positions in which they died - they were killed suddenly by a volcanic mudflow.
The herd was excavated from the Yixian Formation in the northeast of China. These are the same beds that have yielded the famous feathered dinosaurs.
The fossils are exceptionally-well preserved and Natural History Museum dinosaur expert, Dr Paul Barrett, led the research on them.
Paul's team took measurements of the dinosaur thigh bones and made estimates of the dinosaur body weight. From this, they were able to age the dinosaurs to between one and a half to three years old.
They could also tell that the six individuals came from more than one clutch of eggs and that they lived together in one social unit, like a crèche.
Psittacosaurus lived during the Early Cretaceous period, 130 to 100 million years ago. They lived in China, Mongolia, Siberia and Thailand and were early relatives of Triceratops and Protoceratops, which all belong to a group known as ceratopsians .
Unlike their later relatives, Psittacosaurus had no horns or frilled armour around their heads but they had a high powerful beak shaped like a parrot. The name Psittacosaurus means parrot lizard.
The reason for the evolution of horns has puzzled scientists for decades. Did horns evolve after dinosaurs began living in groups, maybe as visual displays for courtship? Or were they needed before group living began?
Finding young dinosaurs of different ages living together in this way, helps answer this question.
'This find proves that Psittacosaurus, which did not have fancy frills or horns, lived in herds,' says Paul. 'From this, we can now tell that the horns of their later relatives evolved after the advent of group-living in these dinosaurs.'
Paul Barrett worked with Zhao Qi of the Chinese Academy of Sciences and David A Eberth of the Royal Tyrell Museum of Palaeontology, Canada on this research. Their paper is published in this month's Palaeontology journal.