Old artwork of two herbivorous dinosaurs Hypsilophodon

The ornithischians were a large and diverse group of herbivorous dinosaurs whose later members evolved a wide range of eating styles. Image © The Trustees of the Natural History Museum, London

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Early dinosaur skulls show how meat-eaters became vegetarian

The skulls of early herbivores are helping scientists to understand how a large and diverse group of dinosaurs may have evolved different ways of eating plants.

Reconstructing the jaw muscles and measuring the bite force of these dinosaurs shows a surprising variation in eating styles early on in the evolution of this group.

Dinosaurs evolved to have a wide variety of diets and feeding behaviours.

While much is already known about how different dinosaurs consumed their food, but relatively little is currently understood about how they evolved their preferred eating style.

A new paper published in Current Biology analysed the skulls of some of the earliest members of the ornithischians – a large and diverse group of plant-eating dinosaurs that would go on to include animals such as Triceratops, Stegosaurus and Ankylosaurus.

The study looked at how vegetarianism may have evolved within this group, whose early members lacked a lot of the specialisations of their later relatives.

Professor Paul Barrett, a dinosaur expert at the Museum and a senior author of the study, says, 'What we're interested in here was seeing how those early plant-eaters started to adopt different ways of being a herbivore.'

'Because they're all descended from a carnivorous ancestor, we want to know how they start experimenting with different ways of eating plants in their transition to becoming vegetarians.'

The Ornithischian Psittacosaurus dinosaur skull on whote background

The skull of the ornithischian dinosaur Psittacosaurus, an early member of the horned dinosaurs. Image © The Trustees of the Natural History Museum, London

What are ornithischians?

Ornithischians were one of the two major groups of dinosaurs that consisted mainly of herbivores. Their name means 'bird-hipped' because their hip bones are arranged similarly to birds, with the pubic bone pointed downwards and towards the tails instead of forwards like in other reptiles.

Early ornithischians were small bipedal dinosaurs that evolved over 200 million years ago during the Late Triassic. Over millions of years, they evolved into a diverse group of dinosaurs, including well-known members such as Triceratops and Stegosaurus.

This study looked at the skulls of five early representatives from the major groups of ornithischians: Heterodontosaurus, Lesothosaurus, Scelidosaurus, Hypsilophodon and Psittacosaurus.

'These dinosaurs show an extensive range of adaptations to eating plants in the later members of the group, but the early members are much less well-studied,' explains Paul. 'There's been a lot of general examination of them, but no real analysis of how the skulls worked as feeding machines.'

ornithischian skull reconstructions

The CT reconstructions of the skulls and jaws, together with diagrams of where the jaw muscles attach in each of the five Ornithischian dinosaurs. Image © David Button. 

How do we know how dinosaurs would have eaten?

The five dinosaur skulls were CT scanned to make three-dimensional models, which were then corrected to restore missing bits of the skulls.

The jaw muscles were then reconstructed using living birds and crocodiles to help figure out where muscles would have been attached in the dinosaurs. The force those muscles might have generated could then be worked out based on their size.

The models then went through a finite element analysis. This involved dividing the skulls into thousands of individual parts, called elements, that were labelled depending on what they were made of, such as bone, enamel or dentine.

The skulls were then simulated to bite an imaginary object at different points along their tooth row to see how different elements respond to the applied force. These models generated heat maps showing the areas of the skulls that were strongly stressed and those that were weakly stressed.

Comparing the models for each of the dinosaur skulls revealed that although all of these animals were eating plants, they each had different ways of doing it.

Dr David Button, a researcher at the University of Bristol and lead author of the study, says, 'When we compared the functional performance of the skull and teeth of these plant-eating dinosaurs, we found significant differences in the relative sizes of the jaw muscles, bite forces, and jaw strength between them.'

'This showed that these dinosaurs, although looking somewhat similar, had evolved very different ways to tackle a diet of plants.'

'This research helps us understand how animals evolve to occupy new ecological niches. In particular, it shows that even similar animals adopting similar diets won't always evolve the same characteristics. Consequently, this highlights how innovative and unpredictable evolution can be.'

Models of ornithischians bite performance

Models comparing bite performance across the five ornithischian dinosaurs. Hot colours (red and pink) represent areas that are highly stressed. Image © David Button. 

How did early herbivores eat differently?

Although lacking the specialisations of later ornithischians, the skulls of these five early dinosaurs showed a surprising variation in how they ate plant material.

For example, Heterodontosaurus appears to have had large jaw muscles relative to its skull size. Therefore, it could have produced a high bite force ideal for consuming tough vegetation.

Scelidosaurus, on the other hand, may have had a similar bite force but relatively smaller jaw muscles compared to its skull. It is possible that as these animals had a greater overall body size and could achieve a stronger bite. In contrast, the Hypsilophodon skull didn't have big muscles. Instead, it reoriented its muscles to bite more efficiently but with less muscle force.

'This is setting the scene for why dinosaurs became so diverse,' says Paul. 'Most dinosaurs were plant-eaters, so getting an idea about how they could feed on a wide variety of vegetation is one of the key ways in which we can understand how they then built on that to literally take over the world later on.'