Raptor-Like Vegetarian Dinosaur Links Carnivores and Herbivores

Chilesaurus diegosuarezi is an odd mix of features and could be the common ancestor of herbivorous and carnivorous dinosaurs, uniting these two seemingly disparate groups.

When Chilesaurus diegosuarezi was first described in 2015, the 150-million-year-old dinosaur baffled nearly everyone. Its head resembled that of a carnivore, while its flat teeth were well-suited to grinding up plant materials. Paleontologists even nicknamed C. diegosuarezi  “the Platypus,” because of its unusual combination of features.

“Chilesaurus is a truly bizarre animal and ranks as one of the most interesting new dinosaur discoveries of the past 20–30 years," Paul Barrett of the UK's Natural History Museum said. “It has an exceptionally odd mixture of features, so much so that if you found these bones separately, rather than together, you would probably end up placing each bone in different dinosaur groups.”

New research by Barrett and lead author Matthew Baron of the University of Cambridge's department of earth sciences suggests that C. diegosuarezi could be, or at least could be very closely related to, the common ancestor of herbivorous “bird-hipped” dinosaurs and carnivorous “lizard-hipped” dinosaurs.

Hips turn out to be important to diet, according to the paper, which is published in the journal Biology Letters. Baron explained that many plant-eating dinosaurs evolved inverted hips, wherein the pubis — one of the three bones that makes a dinosaur hip — is pointed backward.

“This appears to have evolved to increase the amount of free space between the legs of the animals that had them, for guts to be held in,” Baron said.

“Many dinosaurs switched to eating plant matter, which is harder to process than meat and thus requires longer and larger digestive organs,” he continued. “This added gut matter needed to be kept somewhere, but without the added weight being put too far ahead of the legs as this would have caused the animals to become unbalanced, so the bird-hipped dinosaurs rearranged the hips to avoid this.”

C. diegosuarezi indeed had both inverted hips and the larger gut suitable for processing plant matter.

What the dinosaur lacked, however, was a prominent lower beak seen in all other members of the herbivorous bird-hipped group, technically referred to as ornithischia. This suggests that C. diegosuarezi was one of the first vegetarian members of this dinosaur group that includes Stegosaurus, Triceratops, and Iguanodon.

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Earlier this year, Baron, Barrett, and their colleague David Norman shook up the world of paleontology by proposing a new hypothesis of dinosaur relationships and early dinosaur evolution. Writing in the journal Nature, they found that the bird-hipped plant-eating dinosaurs and the meat-eating lizard-hipped dinosaurs, such as Tyrannosaurus, were closely related, an idea that no one else had ever suggested.

“Chilesaurus is a perfect half and half mix of the two groups, so plugs that gap that was created by our rearrange,” Baron said. “It is good evidence for the argument that ornithiscian dinosaurs have much more in common — and share a common ancestor with — the big meat-eating dinosaurs, rather than the long-necked tree browser kinds, like Diplodocus.”

The researchers’ new analysis of C. diegosuarezi involved a comprehensive dataset that looked at more than 450 anatomical characteristics of early dinosaurs. It enabled Baron and Barrett to correctly place the species in the dinosaur family tree.

This lineage shows that the ancestors of C. diegosuarezi were likely carnivores. As for why some of them switched to an all-plant diet, Baron explained that during the Late Triassic–Early Jurassic period, Earth’s single continent Pangea was starting to break up.

“When this break up happened, new shallow seas spread between the newly created continents, and so more moisture reached what used to be inland areas,” Baron said. “As a result, more plant life started to appear and spread around the world. I think that this increase in plant life drove the evolution of, and increase in, herbivory in the dinosaur groups.”

“Many predators no doubt preyed upon this increased number of herbivores too," he added, "and we also see an increase in the body size and diversity of the theropod (meat eating) dinosaurs.”

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By the late Cretaceous, the “king of dinosaurs” — Tyrannosaurus rex — had evolved, to feast on many of the herbivores.

Paleontologists still debate what the first ever diet was for all dinosaurs. In other words, they still are not sure whether or not the first dinosaur was an herbivore or a carnivore. It could be that, even though the ancestors of C. diegosuarezi were carnivores, their even earlier ancestors might have been plant eaters.

Today’s dinosaurs, birds, perhaps reflect this mixed-up diet. Many modern birds, like humans, have evolved the ability to process both certain meat, and/or fish, and plant materials.

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