Rhynchosaurs: Ancient herbivores that starved to death in old age

Researchers from the University of Bristol have provided new insights into the life of Rhynchosaurs, ancient reptiles that existed between 250-225 million years ago. Rhynchosaurs were a relatively unknown group of reptiles that thrived during the Triassic Period, characterized by warm climates and tough vegetation.

Using CT scanning techniques, the team analyzed specimens found in Devon to examine how the teeth of Rhynchosaurs wore down during feeding and how new teeth were added as the animals grew in size.

The study, recently published in Palaeontology, suggests that these early herbivores likely experienced starvation in old age due to the impact of vegetation on their teeth.

Professor Mike Benton, the leader of the research team from Bristol’s School of Earth Sciences, commented, “When I first studied the rhynchosaurs years ago, I was fascinated to discover that they were often the dominant species in their ecosystems. If you found one fossil, you would find hundreds. They were the equivalent of sheep or antelopes during their time, yet they possessed specialized dental structures that appeared to be adapted for consuming large amounts of tough plant material.”

Dr. Rob Coram, who made the discovery of the Devon fossils, explained, “These fossils are rare, but occasionally, some individuals were preserved during river floods. This has allowed us to gather a collection of rhynchosaur jaw bones representing various ages, ranging from very young individuals, possibly even babies, to adults. Among them, we found one exceptionally old animal, a Triassic veteran whose teeth were completely worn down, suggesting it likely struggled to obtain sufficient nutrition each day.”

3D model of the skull of Bentonyx from CT scans, showing the teeth of the upper (blue) and lower (pink) jaws rooted deeply in the bone. Credit: Thitiwoot Sethapanichsakul

Thitiwoot Sethapanichsakul, who conducted the jaw analysis as part of his MSc in Palaeobiology, explained, “By examining the sequence of fossils over their lifespan, we observed that as the rhynchosaurs aged, the worn area of their jaws moved towards the back of the skull, exposing new teeth and bone to wear. This indicates that they consumed exceptionally tough plant material, such as ferns, which caused their teeth to be worn down to the jawbone. Essentially, they relied on a combination of teeth and bone to break down their meals.”

Dr. Coram added, “However, at a certain age—although we are unsure of the exact timeframe—the growth rate of their teeth slowed down, and the worn area became fixed and progressively deeper. This phenomenon is akin to modern-day elephants, which have a limited number of teeth that come into use from the back. Once they reach around seventy years of age, they use their last tooth, and that’s the end of it. While we don’t believe rhynchosaurs lived that long, their jaws were subjected to such challenging plant material that they simply wore out, likely leading to eventual starvation.”

During the Triassic Period, rhynchosaurs played a crucial role in terrestrial ecosystems as life rebounded from the devastating mass extinction that occurred at the end of the Permian Period. These reptiles contributed to the formation of new ecological systems, setting the stage for the dominance of dinosaurs and later mammals as the modern world slowly took shape.

Teeth of upper and lower jaws, X-ray section along the length of the jaws showing teeth in wear. Unerupted teeth at left are made from dentine (de) with an enamel (en) cap. Many teeth show the pulp cavity (pc) at the base. Tooth wear is so extreme that teeth are worn flat to the bone, and in many cases bone works against tooth in chomping. Credit: Thitiwoot Sethapanichsakul

Through the comparison of earlier rhynchosaur specimens found in Devon with later specimens from Scotland and Argentina, the research team also traced the evolutionary changes in their dentition over time. This analysis revealed that the unique teeth of rhynchosaurs played a significant role in their diversification, occurring twice during the Middle and Late Triassic periods.

However, as the study indicates, climate change, particularly alterations in available plant species, played a crucial role in the decline of rhynchosaurs. The changing environmental conditions likely provided favorable conditions for the rise of dinosaurs, ultimately leading to the extinction of rhynchosaurs and the subsequent dominance of dinosaurs in the ecosystem.

Source: University of Bristol

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