Herbivorous dinosaurs were incredibly diverse, ranging from animals just a few kilograms in weight to those that weighed 70 tonnes. But despite this, the same patterns in evolution kept cropping up again and again  © Gerhard Boeggemann/Wikimedia Commons

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The way dinosaurs evolved mirrors life on Earth today

From huge titanosaurs to the diminutive Bagaceratops, herbivorous dinosaurs evolved into a massive variety of species. Yet despite this impressive diversity, there were particular features that kept cropping up time and again. 

A new paper has shown how unrelated plant-eating dinosaurs repeatedly evolved similar characteristics to help them chomp down on vegetation. 

Over the span of 180 million years dinosaurs evolved into a great variety of forms to fill the ecological niches in the environment. While for most of this evolutionary history the dominant plants which formed the basis of these food webs were not the same ones that we see today, the dinosaurs still filled similar roles as that of large mammals. 

Today, Earth supports vast sweeping grassy plains being grazed by gazelles and rhinos. In the time of the dinosaurs, there would have been savannas of ferns that supported herds of ornithopod dinosaurs such as Hysilophodon and Parasaurolophus

Just as giraffes have evolved to eat the tender leaves found on the tops of trees, there were towering sauropods like Brachiosaurus which exploited the crowns of huge conifers. Smaller creatures such as Psittacosaurus developed beak-like mouths to slice through tougher vegetation and possibly even to crack seeds and nuts like modern-day parrots. 

Large mammalian herbivores today usually grind their food before swallowing it, but this is not the only way to eat plants © Wilson Christopher/Wikimedia Commons

Dr David Button, a researcher at the Museum, co-led new research which shows that within this huge diversity of herbivorous dinosaurs, unrelated groups evolved the same solutions to eating plants. 

'People often think of dinosaurs as a swansong for extinction or that they were a failed species,' says David. 'But they were actually extremely successful in terms of how different species' anatomies evolved, particularly in herbivores.'

How to eat plants

Large-bodied herbivores have been a central feature of land-based ecosystems for some 300 million years. As one group of large herbivores went extinct, another would evolve into similar forms to take their place. 

Yet generally speaking, plants are difficult to eat. This has resulted in two main ways to be a herbivore. The first involves crushing and grinding the tough plant material with teeth before it is passed to the gut. This is the primary method employed by most large herbivorous mammals, such as wildebeest and tapirs. 

The second are those animals which use their skulls as a way of simply acquiring the plant material, and instead have a muscular gut to help break it down. This is seen in many reptiles and birds, like geese and tortoises.  

There were two main ways that dinosaurs evolved herbivory: convergent regime one involved 'snipping' plant material and eating it whole, while convergent regime two meant that dinosaurs chewed the vegetation first © Button et al. 2019

When it comes to dinosaurs, both of these methods of herbivory have been observed. The large sauropods, such as diplodocus, had tiny heads that are not well adapted to chewing but large bodies perfect for fermenting plant material. The hadrosaurs, on the other hand, were more modest in size but had large flat teeth adapted to crushing plants. 

How these different strategies related to each other has, not been well understood. David and his colleagues have now shown that both types of herbivory, and the associated adaptations to their skulls, evolved independently multiple times across the dinosaur family tree. 

Converging bodies

When two unrelated organisms - be they animals, plants or fungi - evolve similar physical characteristics, it is known as convergent evolution. 

This has happened over and over throughout the history of Earth. Dolphins have evolved a similar body plan to the ichthyosaurs that swam the oceans millions of years before, while bees and flower beetles have both evolved similar looking mouthparts perfect for sucking up nectar. 

It is thought that as different animals move into similar environments, evolution favours similar characteristics. This can be difficult to test, however, as organisms take millions of years to evolve.

By looking at the skulls of 160 different dinosaurs that lived throughout the Triassic, Jurassic and Cretaceous Periods, the team were able to see how dinosaur teeth and bite force differed and how they converged. This gave a deep-time perspective of more than 100 million years to look at convergent evolution and to see if any trends held.    

Diplodocus skull

Diplodocus had a skull suited to pulling leaves off and swallowing them whole © The Trustees of the Natural History Museum, London

While they could see multiple different feeding strategies emerge among the herbivorous dinosaurs, it became apparent that two main modes kept cropping up again and again.

'We have what we're calling Convergent Regime One, but they could be the 'snippers',' explains David. 'In these groups we see the skull and the jaw get less robust, they have fewer numbers of teeth - or no teeth at all - with quite small biting surfaces.

'So we think these had quite low biting forces, and were using their mouths to snip off vegetation to swallow down whole.'

Those species that fell into this group included the ostrich-like ornithomimosaurs, the large-clawed therizionsaurs, and the diplodicoid and titanosaur sauropods. Despite having dramatically different looking skulls on the surface, the research showed that they independently elongated their skulls, reduced the biting surfaces, and evolved weaker bite forces. 

'On the other hand, in the in the ornithischian dinosaurs we see multiple groups such as the tusked heterodontosaurids, the armoured ankylosaurs, the ornithopods like the duck-bills and the iguanodontids, and the horned ceroptopsians,' says David.

'In all of these we see densely packed complicated teeth and an increased leverage of the jaw muscles. This is indicative of greater biting efficiency, particularly towards the back of the mouth where chewing would be performed.'

Parasaurolophus skull

Parasaurolophus, on the other hand, had a skull full of complex teeth suited to crushing and chewing plants © The Trustees of the Natural History Museum, London

Understanding ecosystems

The research helps us to understand that convergent evolution needs not only similar environmental conditions, but also similarities in their ancestors.

Effectively, the ancestors to both titanosaurs and Gallimimus, for example, were both predisposed to snipping vegetation. So despite them appearing so different on the surface they were more likely to head down the same evolutionary path when placed under similar environmental conditions.

Delving into the evolution of big herbivores also helps us to figure out how large-scale ecosystems work in general.

'One of the reasons that palaeontology is so useful is that we don't really know from modern ecosystems alone how they really work, because they have been so catastrophically messed with,' says David. 'If we look at modern ecosystems we just have a single highly biased time slice and we won't really understand how things properly work unless we have a long time series, such as what we see in the dinosaur fossil record.'