It's no Jurassic jest - meet the Velociraptor's vegetarian cousin.

While it may look every bit as bloodthirsty as the beasts from Jurassic Park, Argentine scientists say this newly discovered dinosaur liked to keep meat off the menu.

The prehistoric creature, named Chilesaurus diegosuarezi, was discovered in the Toqui Formation in southern Chile and is identified as a primitive kind of theropod that, unusually, was a herbivore.

Theropods were the dominant predators in most terrestrial ecosystems during the Mesozoic era between 66 and 252 million years ago, and their most famous members, the Tyrannosaurus Rex and Velociraptor, are often depicted ripping hunks of meat off their prey.

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Theropods were generally bipedal - bounding across the land on two hind limbs - and almost always carnivores.

Some theropods, however, became specialised as herbivores - something which tended to happen in highly derived groups.

In a newly published study in the journal Nature, the authors said a fully-grown Chilesaurus would have been about 3m long when it roamed the Earth about 150 million years ago.

Fossils revealed an unusual combination of features from various dinosaur groups, including relatively short arms, a long neck, small head and leaf-shaped teeth.

"Chilesaurus illustrates how much relevant data on the early diversification of major dinosaur clades [groups] remain unknown," the authors said.

Professor Ewan Fordyce, of Otago University's Department of Geology, said the find was a reminder that the history of life has had strange and unexpected twists and turns.

"What environmental changes would lead to some dinosaurs becoming herbivorous, abandoning the seemingly efficient carnivorous lifestyle, eating energy-rich meat?

"It will be interesting to see if the authors can do further research to indicate exactly what sort of plant material might have been eaten."

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The key specimen, a juvenile, had an incomplete skull and Professor Fordyce said it would be interesting to see whether other specimens had more complete skulls, "and in that case, whether the shape of the skull and inferred positions of feeding muscles are consistent with herbivorous diet".

"The authors suggest that the structure of the pelvis is consistent with having a gut that would digest plant material."

Their discovery comes soon after another dinosaur made headlines for strange reasons. After a century of debate, scientists this month formally declared the Brontosaurus did exist, justifying the giant's place in books and films.