By James Ashworth
The oldest known relative of lizards has been uncovered in the UK.
A new species of ancient reptile, known as Agriodontosaurus helsbypetrae, reveals that the evolution of these animals was quite different than scientists had imagined.
An extinct species has revealed new twists in the tale of lizards and their relatives.
Discovered near the town of Sidmouth in Devon, Agriodontosaurus helsbypetrae was an insect-eating reptile that lived more than 241 million years ago during the Middle Triassic. It’s part of a reptile group known as the lepidosaurs, which contains snakes, lizards and an animal known as the tuatara.
Today, there are over 12,000 species of lepidosaurs, making them the most species-rich groups of land vertebrates. But it’s hard to know where they came from as the fragile bones of their early ancestors are vulnerable to being damaged or destroyed.
Agriodontosaurus is currently the oldest known member of the group, being as up to seven million years older than any other known lepidosaur. But the fossil has surprised scientists because it it’s missing some of the key features early lepidosaurs were expected to have, such as teeth on the roof of its mouth.
Dan Marke, who led the research into the reptile as part of his Masters degree at the University of Bristol, says that Agriodontosaurus “is unlike anything yet discovered and has made us all think again about the evolution of the lizard, snakes and the tuatara.”
“This specimen not only provides important information about the ancestral skull of all lepidosaurs but also builds on growing knowledge of the tuatara. While often called a ‘living fossil’, this animal belongs to a once-diverse order of ancient reptiles with a rich evolutionary history.”
The findings of the study were published in the journal Nature.
How did Agriodontosaurus live?
As well as being the oldest known lepidosaur, Agriodontosaurus is also the earliest known rhyncocephalian. This is a specific group of lizard-like animals of which the tuatara is the only surviving member.
The tuatara uses its teeth to help it pierce the exoskeletons of insects and shear them apart, and it’s likely that Agriodontosaurus did something similar. The fossilised teeth of the ancient animal are much larger than those of its relatives and gave the species its name, as Agriodontosaurus means ‘fierce toothed lizard’ in ancient Greek.
The team believe that it probably fed on large insects such as cockroaches, crickets and grasshoppers. This is a lifestyle that hasn’t been found in any other Triassic lepidosaur before. Agriodontosaurus’s large eye sockets suggest it would have been good at finding prey, while it probably had good hearing to aid in hunting.
Once it caught the invertebrates, Agriodontosaurus would have held them tight in its mouth thanks to its strong jaw muscles and tongue, and then used it teeth to cut the insects up.
During the Middle Triassic ecosystems were changing, and the researchers think that Agriodontosaurus part of an evolutionary burst of reptiles that was taking over from mammal-like animals known as the synapsids. By the end of the Triassic, the ancestors of modern lizards and snakes would be taking their first steps towards global dominance alongside the dinosaurs.
Lepidosaur evolution has surprises in store
The evolutionary journey taken by the lepidosaurs, however, wasn’t quite as expected. Dr David Whiteside, one of our scientific associates who co-authored the research, said that detailed scans of the fossil revealed Agriodontosaurus’ unusual characteristics.
“In modern palaeontological studies we often X-ray scan the fossils,” David says. “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 Masters student, Thitiwoot Sethapanichsakul, had worked on the regular scans and found fantastic detail, but Agriodontosaurus is 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.”
It had been thought that the first lepidosaurs would have had some of the key characteristics of modern snakes and lizards which these animals had inherited. These include a partially hinged skull, teeth on the roof of the mouth and a gap between skull bones known as an open temporal bar.
This allows them to move different parts of the skull independently to stretch out their jaws and allows them to swallow much larger prey. However, Agriodontosaurus defied expectations, having just one of these three features.
“The new fossil shows almost none of the characters we expected,” said Dan. “It has no teeth on its palate and no sign of any hinging, just an open temporal bar.”
It suggests that these characteristics may have appeared multiple times in the history of these animals as they evolved, rather than being inherited from a common ancestor. This could have helped give lepidosaurs an edge as they became such a recognisable part of life on Earth.