Chilesaurus diegosuarezi has a strange mix of skeleton features

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

Dinosaur Q&A: What was Chilesaurus diegosuarezi ?

New research co-authored by Museum scientists explains why the dinosaur Chilesaurus diegosuarezi has a bizarre mix of anatomical features.

It is a missing link between plant-eating dinosaurs like Stegosaurus, and carnivorous dinosaurs like T. rex.

The research builds on work published by the same research team earlier in 2017, which proposed a reorganisation of the traditional dinosaur family tree.

We caught up with co-author Prof Paul Barrett, a dinosaur expert at the Museum, to find out what's interesting about Chilesaurus and how the animal fits into the wider proposed changes to the family tree.

When was Chilesaurus discovered?

Chilesaurus was named in 2015 by a team of Argentinian and Chilean palaeontologists, but the first fossils were found in 2004 in Chile by seven-year-old Diego Suarez. The species name, diegosuarezi, is named after him.

Chilesaurus is from the Late Jurassic period and is about 150 million years old. Researchers have found several skeletons of the dinosaur so we have a fairly good idea about what the animal looked like.

What's interesting about Chilesaurus?

Chilesaurus has a bizarre mix of anatomical features that we normally associate with quite different sets of dinosaurs.

It has bird-like hips that look like those of an ornithischian dinosaur, similar to Stegosaurus.

It has a body outline and body proportions much more similar to the meat-eating dinosaurs, such as Tyrannosaurus.

And it has some other features, such as the structure of its ankle, that even hint at a relationship with some of the long-necked dinosaurs, like Plateosaurus.

So it has this strange combination of body parts that make it difficult to classify.

Cast of Chilesaurus

A cast of a fossil Chilesaurus skeleton © Evelyn D'Esposito licensed under CC BY-SA 2.0 via Wikimedia Commons
 

What do we know about how Chilesaurus lived?

Chilesaurus was a fairly small dinosaur, two to three metres in length. It was a biped that walked on its hind legs, with a long tail to balance the rest of its body.

It had grasping hinds, which again makes it look like a meat-eating creature, but when we look at the skull of Chilesaurus, its jaws and teeth were those of a plant-eater.

Its teeth were adapted for slicing through foliage. They are not the knife-like teeth of a meat eater.

Why is this new research so interesting?

It turns out that Chilesaurus is not a meat-eating dinosaur as originally thought, but actually a primitive member of a big group of plant-eating dinosaurs that we call the bird-hipped dinosaurs, or ornithischians. That's the group that includes animals like Stegosaurus, Triceratops and Iguanodon.

I don't use the term lightly, but this does look like a genuine missing link between more standard-looking meat-eating dinosaurs and early plant-eaters.

It has a combination of features that we still find in meat-eaters, but also advanced features that we only otherwise see in those bird-hipped dinosaurs.

These suggest that it has been 'caught in the act' of turning into a plant-eating dinosaur from a meat-eater. It helps to bridge those two groups.

Stegosaurus and Stegosaurus battle

An artist's illustration of a fight between a Stegosaurus (right), an ornithischian dinsoaur, and Ceratosaurus, a theropod. Chilesaurus shared physical characteristics of both groups of dinosaur © The Trustees of the Natural History Museum, London/Nicholls (paleocreations.com)

What does this mean for your other research?

This fits nicely with work we published earlier in the year looking at the structure of the dinosaur family tree in general. We suggested the new idea that the ornithischian dinosaurs were very closely related to the meat-eaters like Tyrannosaurs, and that both of those groups together were more distantly related to the long-necked dinosaurs like Diplodocus.

This overturned the prevailing idea, which suggested that the meat-eaters and the long-necked dinosaurs formed a group to the exclusion of the other plant-eaters.

Chilesaurus has a combination of features that we see in the meat eaters and in the ornithischians. This helps make our proposed new grouping stronger.

What was the response from the community when you published your proposed new dinosaur family tree earlier in the year?

So far the response has been sceptical, but supportive. Most of my colleagues think that it is an interesting idea, but they are still burrowing through our data, to see if they agree with what we said.

We know that a group of our colleagues are working on a response to our paper. That's going to be coming out fairly soon, and our reply to their response will come out at the same time.

Most people are now thinking that the traditional tree has got some things wrong with it. So even if we are not right, our work has highlighted that these issues need looking at.

It's going to be at least four or five years before we know if our idea is widely accepted. Researchers need time to digest it, and see if it gives them a better explanation for what they are seeing, before they start preferring it.

Nyasasaurus parringtoni

Nyasasaurus parringtoni may be the oldest known dinosaur. Although only fossil fragments have been found, they show features unique to dinosaurs. © Mark Witton/The Trustees of the Natural History Museum, London
 

Are there other dinosaurs that might benefit from being looked at using the new tree?

There are a number of other dinosaurs that are quite hard to fit in the traditional family tree.

Having our new tree means we can test some of those dinosaurs out and see if they can move around it.

One of those dinosaurs is Nyasasaurus, and it is here in the Museum's collection. Some of us think it is the earliest-known dinosaur, but that idea has been controversial. We are trying to work out where Nyasasaurus might fit in this new family tree.

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More information

Read the scientific paper on Biology Letters.

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