A dinosaur crèche has been found entombed in the volcanic debris that engulfed it on a hillside 123 million years ago. Six young Psittacosaurus, all less than three years old, died side by side.

It is the earliest known dinosaur nursery uncovered and casts new light on the evolution of social behaviour among dinosaurs, including the triple-horned Triceratops.

Palaeontologists have long been puzzled about whether horns and frills evolved because of, or resulted in, social behaviour and the herding instinct. The find reveals that social groups of the creatures had formed long before the elaborate adornments.

Descendants of Psittacosaurus, including Triceratops, which lived 65-68 million years ago, would have used their horns and frills to compete for mates, to mark out territory and to help them to recognise each other.

Herding has previously been established in earlier species of dinosaurs but this is the first time it has been found in a type in which horns and frills were absent.

Paul Barrett, of the Natural History Museum in London, one of the researchers, said that the fossilised juveniles appeared to have formed a crèche but it was impossible to be sure if they were part of a larger herd or if they grouped together for protection.

“This is the first time we’ve found a group of these dinosaurs together. For at least the first few years of life it looks like they stuck together. It answers the chicken-or-egg question of whether the social behaviour or the horns and frills came first,” he said.

The fossils were found near the village of Lui Tai near Yixian in Liaoning province, China. There are up to 12 species of Psittacosaurus– a type of ceratopsian dinosaur.

The research team, including scientists from the Chinese Academy of Sciences and the Royal Tyrrell Museum of Palaeontology in Alberta, Canada, suspect that they may be from a previously unknown species.

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