242-million-year-old mini predator changes lizard evolution

Today, lizards and their close relatives, including snakes and the distinctive tuatara from New Zealand, form the most diverse group of land vertebrates. With more than 12,000 species, they outnumber birds and mammals. Their collective group, known as Lepidosauria, has achieved remarkable success, raising long-standing questions about which early traits gave them such an advantage.

Reconsidering Early Lepidosaur Features

Scientists had anticipated that the earliest lepidosaurs would show traits also present in many modern lizards and snakes. These expected features included a partly hinged skull, an open lower temporal bar, and numerous teeth on the roof of the mouth (palate). In living species, these adaptations allow the animals to open their jaws extremely wide (skull hinge) and use palate teeth to secure struggling prey.

The lower temporal bar functions much like a cheek bone. It connects the cheek region to the jaw hinge and is missing in modern lizards and snakes. Although many living species share flexible skulls and other advanced traits, only the tuatara still retains a complete lower temporal bar, giving it a more ancestral appearance. The tuatara also maintains large palatal teeth, which further highlight its links to early reptiles.

Unexpected Anatomy Revealed

"The new fossil shows almost none of what we expected," said Dan Marke, who led the study as part of his MSc in Palaeobiology at Bristol. "It has no teeth on the palate, and no sign of any hinging. It does though have the open temporal bar, so one out of three. Not only this but it possesses some spectacularly large teeth compared to its closest relatives."

Advanced Scanning Exposes Hidden Details

"In modern paleontological studies we often X-ray scan the fossils," noted Dr. David Whiteside, a co-supervisor of the project. "But the exceptional resolution and quality of scans from synchrotron X-ray sources show us all the fine details and save any risk of damage.

"An earlier MSc student, Thitiwoot Sethapanichsakul, had worked on the regular scans and found fantastic detail, but it's so tiny -- the skull is only 1.5 cm long, and we could barely see the teeth. So, we were so grateful to be able to make synchrotron CT scans to get even finer resolution, using two powerful beamlines at the European Synchrotron Radiation Facility (France) and the Diamond Light Source (UK)."

A Tiny Skeleton With Remarkable Preservation

"When you look at the fossil, the whole skeleton sits in the palm of your hand," explained Michael Benton, another co-supervisor and Professor of Vertebrate Paleontology in the School of Earth Sciences at the University of Bristol.

"But after the scans and the hard work of our students cleaning up the scan data, we can see the most amazing detail. The new beast has relatively large triangular-shaped teeth and probably used these to pierce and shear the hard cuticles of its insect prey, pretty much as the tuatara does today."

A New Species With Unique Traits

"The new animal is unlike anything yet discovered and has made us all think again about the evolution of the lizard, snakes and the tuatara," said Dan Marke. "We had to give it a name to distinguish it from everything else, and we chose Agriodontosaurus helsbypetrae, quite a mouthful, meaning 'fierce toothed lizard from the Helsby rock" after the Helsby Sandstone Formation in which it was discovered.

"This specimen not only provides important information about the ancestral skull of all lepidosaurs but also builds on the growing knowledge that the tuatara, while often called a "living fossil"; belongs to a once-diverse order of ancient reptiles with a rich evolutionary history."

Tracing Early Reptile Evolution

The fossil dates to 242 million years ago during the Middle Triassic, just before the emergence of dinosaurs. From that point onward, lepidosaurs underwent several waves of diversification. Early forms likely slipped through the undergrowth beneath the feet of dinosaurs and succeeded by evolving a variety of strategies for capturing prey. These included flexible jaws and, in some lizards and snakes, venom.

A Chance Find on a Devon Beach

"When I found the specimen back in 2015 on the beach in Devon, I had no idea what it was because there was so little of it exposed," added Dr. Rob Coram. "It's been great to see such an amazing fossil coming from a site that has been providing fossils for 150 years."