Why the plant-eating ancient reptile Rhynchosaur starved to death in old age

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Before they were replaced by dinosaurs, the ancient reptile Rhynchosaur walked the planet some 250-225 million years ago. Rhynchosaurs were a group of herbivorous reptiles that ranged all over the world in the Triassic Period, which is also known as the ‘age of reptiles’.

These ancient reptiles were small, just about a meter long, and had wedge-shaped skulls and small but blunt teeth for eating plants. They were probably the most commonly found plant eaters on land at the time, before the rise of plant-eating dinosaurs at the end of the Triassic.

There were major changes in the anatomy of the rhynchosaur through their evolutionary history, until their extinction. A team of researchers from the University of Bristol studied the rhynchosaur specimens found in southwestern England’s Devon and found an interesting fact about the reptile’s life.

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The researchers studied the specimens using CT scanning to see how their teeth were worn down as they fed, and how new teeth were added at the back in rows as the animal grew in size. The findings reveal that these early herbivores likely struggled to get enough nutrition in a day as the vegetation took a toll on their teeth. They eventually starved to death in old age.

Co-author of the study Thitiwoot Sethapanichsakul studied the jaws as part of his MSc in Palaeobiology. He said, “They were clearly eating really tough food such as ferns, that wore the teeth down to the bone of the jaw, meaning that they were basically chopping their meals by a mix of teeth and bone.”

After a certain age, their growth slowed down and the area of wear and tear just got deeper and deeper. Dr Rob Coram, co-author of the paper and also the one who discovered the Devon fossils, said, “It’s like elephants today – they have a fixed number of teeth that come into use from the back, and after the age of seventy or so they’re on their last tooth, and then that’s that.”

Professor Mike Benton from Bristol’s School of Earth Sciences said, “I first studied the rhynchosaurs years ago and I was amazed to find that in many cases they dominated their ecosystems. If you found one fossil, you found hundreds.”

The team also compared examples of Devon rhynchosaurs with later examples from Scotland and Argentina. They were able to show how their dentitions evolved through time, and how their unique teeth enabled them to diversify twice, in the Middle and then in the Late Triassic, said the press release.

Coram said, “The fossils are rare, but occasionally individuals were entombed during river floods. This has made it possible to put together a series of jaw bones of rhynchosaurs that ranged in age from quite young, maybe even babies, through adults, and including one particularly old animal, a Triassic old-timer whose teeth had worn right down and probably struggled to get enough nutrition each day.”

The study was published in the journal Palaeontology.

Study abstract:

Rhynchosaurs were key herbivores over much of the world in the Middle and Late Triassic, often dominating their faunas ecologically, and much of their success may relate to their dentition. They show the unique ankylothecodont mode of tooth implantation, with deep roots embedded in the bone of the jaw and low crowns that were rapidly worn down in use. During growth, the main area of oral food processing, located in the middle and posterior portions of the occlusal surfaces of the jaws, moved posteriorly relative to the anterior tips of the jaws, which curved up. As the maxilla and dentary grew by addition of new bone posteriorly, the dental lamina fed in new teeth at the back of the tooth rows. CT scanning of the holotype skull of Bentonyx sidensis from the Middle Triassic of England reveals previously concealed details of the dentition. Together with new dentary material from the same location, this has enabled us to examine the tooth replacement process and elucidate ontogenetic changes in dentition and jaw morphology as the animals aged. There were major changes in rhynchosaur anatomy and function through their evolutionary history, with the early forms of the Middle Triassic dying out before or during the Carnian Pluvial Episode (233–232 Ma), and the subclade Hyperodapedontinae, with broad skulls and adaptations to chop tough vegetation, subsequently diversifying worldwide in a successful ecological expansion until their global extinction 227–225 Ma.