A missing link between plant-eating dinosaurs and meat-eaters such as the Tyrannosaurus rex may have been found, suggests a new study from the University of Cambridge and the Natural History Museum.

After analyzing more than 450 anatomical characteristics of early dinosaurs, the researchers found that Chilesaurus — first described in 2015 — fills the gap between the two groups.

Chilesaurus, which lived 150 million years ago in the Late Jurassic Period, was a puzzling dinosaur that looked like a meat-eating raptor, yet was a herbivore.

"It was so mysterious," Matt Baron, lead author and a PhD student from Cambridge University, told CBC News. "When it was first published in 2015 it really caused a stir, because it was so unusual, a Frankenstein dinosaur. It was made up of such a weird combination of features."

The head of the Chilesaurus resembles that of a carnivore. However, its hip structure was more like Ornithischia (which means "bird-hipped" in Greek). It also had flat teeth for grinding plant material. Making things more puzzling, it had short forelimbs like the allosaurus, a carnivore. However, it lacked claws and instead had stump-like fingers.

'It was so unusual, a Frankenstein dinosaur. It was made up of such a weird combination of features.' - Matthew Baron, Cambridge University

"Most of the time with fossils and particular dinosaurs, you can look at bones from the animal and there are certain features of the bone that you can look at and say, 'This belongs to this group because it has this and this,'" Baron said. "With Chilesaurus, if you were to pick up any bone from its body and look at it individually, you'd be forgiven for thinking the bones belonged to a whole range of animals.

"It looked like it had been stitched together from lots of different animals. It was baffling. It was really weird," Baron said.

Evolution in a changing world

Baron was part of a team that sought to reclassify all dinosaurs earlier this year in an effort to account for the advances and discoveries made since the first dinosaur family tree was laid out in 1888. It was this project that led to the findings on Chilesaurus.

Chilesaurus possessed both carnivorous and herbivorous dinosaur traits, Baron said, because it was adapting to a changing planet, one that was developing lush green landscapes with plenty of plants to consume.

A body that digests plant matter is vastly different from one that needs to digest meat, Baron said. One key difference is a larger gut, a reason for wider hips.

Baron said that the findings aren't just a window on the past, but also help us to better understand how animals adapt to a changing world.

Allosaurus dinosaur

While Chilesaurus had some qualities similar to the carnivorous Allosaurus (above), it also had features found in herbivores. (Noiel/Shutterstock)

"We can learn a lot from the way in which they adapted and changed to a changing environment," Baron said. "Given the Earth is changing so much right now, it's good to have things in the fossil record that we can look at and can say, 'Oh, well look: this is one of the ways in which life responded to this change."

Baron acknowledged that there's likely to be pushback from the researchers' findings. Fernando Novas, who first described Chilesaurus in 2015, has expressed some concerns.

In an email statement about the findings, Novas said that Baron and his co-author Paul Barrett overlook strong similarities the dinosaur shares with meat-eating Theropods, including various bone structures.

"I guess the discussion on Chilesaurus relationships [has] just begun, and the years to come will testify fluctuating interpretations on the early dinosaur diversification." he said. "However, I welcome the novel interpretation by Baron and Barrett, because it promotes a necessary debate on poorly known aspects of dinosaur evolution as a whole."