1. Evolutionary Biology

Fossil sheds new light on early evolution of plant-eating dinosaurs

Analysis of a new Heterodontosaurus specimen reveals how ornithischian dinosaurs evolved a unique body structure and sheds light on what this meant for their breathing.
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Scientists have revealed how a species of plant-eating dinosaurs called ornithischians evolved a bodily structure that was substantially different to that of other dinosaurs.

Illustration of Heterodontosaurus tucki. Image credit: Viktor Radermacher (CC BY-NC-ND 4.0)

Their findings, published today in eLife, provide new insight on the early evolution of these dinosaurs and how their unique features may have impacted their breathing.

Dinosaurs fall into three broad families: theropods, including two-legged carnivores such as T. rex; sauropods, giant long-necked dinosaurs such as Diplodocus; and ornithischians, including those with body armour such as Triceratops, Stegosaurus and Heterodontosaurus. Ornithischian dinosaurs were prominent herbivores of the Mesozoic Era that lived on every continent on Earth.

“Comparing the skeletons of dinosaurs to their living relatives – birds and crocodiles – shows researchers how these species grew and moved, what they ate, and how they structured their social groups,” says first author Viktor Radermacher, paleontologist and paleoartist at the Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa. “While other aspects of ‘paleobiology’ have been well studied in ornithischians, the respiratory mechanisms of these species are still poorly understood and controversial.”

The evolution of dinosaur lungs and how the animals breathed is important for better understanding their biology and how they coped with changes in ecological conditions, including shifts in climate.

To explore this further in ornithischians, Radermacher and the team examined a new, fully articulated skeleton of the early evolving species Heterodontosaurus tucki – a small and swift two-legged dinosaur. The specimen was discovered in a part of the upper Elliot Formation, a geological formation in South Africa and Lesotho, dating to roughly 200 million years ago. It has well-preserved bones that were not previously known to exist in ornithischians. These include gastralia – toothpick-shaped bones embedded in the stomach muscles – two sternal plates, and unusual sternal ribs shaped like tennis rackets.

“Historically, the loss of gastralia was used to define what an ornithischian is,” Radermacher explains. “But this new specimen shows that they were present in the ‘beginning days’ of this group and were lost in the early stages of their evolution.”

Additionally, the team identified some of these bones as similar to those that form the framework of breathing mechanics in species alive today, such as lizards, crocodiles and birds.

Their analysis also suggests that the chest skeleton of Heterodontosaurus and other ornithischian dinosaurs was complex and could move significantly to allow the body to expand during breathing cycles. But the team also found evidence for the early evolution of a different method of breathing, termed the ‘pelvic bellows’. This type of breathing would have been most similar to how modern crocodiles breathe, using a muscle that connected the hips to the lungs. The authors suggest Heterodontosaurus was caught at the crossroads – that is, preserving some of its breathing mechanics from its ancestors, but showing the early evolution of a new mode of aerating its lungs.

“It’s fascinating to me how different vertebrate breathing mechanisms are,” concludes senior author Jonah Choiniere, Professor at the Evolutionary Studies Institute, University of the Witwatersrand. “This study shows us a moment frozen in time when ornithischians were in between breathing strategies. It helps explain why the ornithischian skeleton is so different to other dinosaurs and perhaps even why they became some of the most successful vegetarians of all time.”

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An accompanying press pack including images and videos from the study is available to access here.

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