Chilesaurus has a strange mix of skeleton features, some meat-eating and some vegetarian. © Gabriel Lío

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Weird Chilean dinosaur is plant-eating theropod

A new species of dinosaur that has surprised scientists with its mix of meat-eating and plant-eating features has been announced this week.

Chilesaurus diegosuarezi lived 145 million years ago and was the size of a small horse. It is named after Diego Suárez, who at age 7, discovered the first bones in the Toqui Formation in southern Chile.

'Chilesaurus shows an exceptionally weird mix of features in its skeleton,' said Natural History Museum dinosaur expert Prof Paul Barrett who saw some of the Chilesaurus fossils while on a research trip to Argentina in 2014.

'Some bones resemble those of different carnivore groups and others are more similar to those of long-necked plant-eating dinosaurs.'

'The balance of evidence suggests that Chilesauris is a theropod dinosaur – a group largely composed of meat-eaters like T. rex – but its teeth and skull bones show it had given up meat for a vegetarian diet, something else that is highly unusual for the group.'

The team, led by Fernando Novas of the Bernardino Rivadavia Natural Sciences Museum in Buenos Aires, have published their findings this week in the journal Nature.

They uncovered extremely well-preserved fossil bones from the Upper Jurassic Toqui Formation site. Importantly, they were also articulated ie found in the earth as three-dimensional near-intact skeletons.

'If these bones were found individually, instead of part of a skeleton, it would be easy to assume that they came from more than one type of dinosaur, not just one unusual one,' said Barrett.

The site revealed five Chilesaurus individuals, along with remains from other species such as sauropod dinosaurs. The team says Chilesaurus was the most common dinosaur in its ecosystem.

Barrett explains, 'This is unusual as theropods are usually rare in comparison with other herbivorous dinosaur groups.

'In most dinosaur ecosystems it is either the long-necked sauropods, such as Diplodocus, or the bird-hipped dinosaurs like Triceratops or Iguanodon that are the most conspicuous and abundant.

'However, Chilesaurus probably owes its abundance to its vegetarian diet and it is likely that it was occupying the same ecological role that other more standard plant-eating dinosaurs occupied elsewhere in the world.'

So was Chilesaurus unique? No. Barrett says that several other theropod dinosaurs became vegetarian during the evolutionary history of the group, but most of these lived later in time than Chilesaurus.

Other weird plant-eating theropods include Caudipteryx and Erlikosaurus.