A tiny skull found on a beach in Devon has rewritten the early history of lizards, snakes and the tuatara, the last survivor of an ancient reptile lineage found only in New Zealand today. The new species dates back 242 million years, making it the earliest known lepidosaur.
Bristol University scientists made a surprise discovery when investigating the fossilized remains of what looked like some sort of small ancient lizard. The animal turned out to be a lepidosaur – a member of the reptile group that today includes lizards, snakes and New Zealand's native tuatara (Sphenodon punctatus), and which has since diversified into more than 12,000 living species.
In trying to identify the unknown specimen, the researchers assumed the first lepidosaurs would look like a rough draft of a lizard – with flexible skull joints, teeth on the roof of the mouth, and an open lower temporal bar. Instead, the Devon fossil had only the last of these traits, lacking both palate teeth and a skull hinge. And its skull housed unusually large, triangular teeth not seen in its closest relatives. Rather than a simple "proto-lizard," the specimen showed that the earliest lepidosaurs were already experimenting with different skull shapes and feeding strategies.
“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.”
The rhynchocephalian A. helsbypetrae dates back to the Middle Triassic, around 242 million years ago – making it the earliest known lepidosaur, some three to seven million years older than anything we've so far identified. Today, the order of Rhynchocephalia has just one living species, the tuatara.
The tuatara has long been described as a “living fossil” because it looks so archaic compared to modern reptiles, retaining traits lost in lizards and snakes (which largely slot into the order Squamata). But this discovery shows the tuatara is more than a relic – it’s the sole survivor of a once-diverse branch, whose history stretches back to the dawn of the lepidosaurs.
Synchrotron X-ray scans of the specimen's 1.5-cm (0.6-in) skull revealed none of the anticipated palate teeth and no sign of a skull hinge. Instead, it showed a fully open temporal bar and those large teeth – ideal for piercing and slicing through the tough exoskeletons of insects.
“When you look at the fossil, the whole skeleton sits in the palm of your hand,” said Michael Benton, 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 pray, pretty much as the tuatara does today.”
An open lower temporal bar – a gap in the cheek region of the skull – makes reptile jaws more flexible. Modern lizards and snakes share this feature, while the tuatara has retained the older condition of a complete bar, making its skull stronger but less mobile. Finding this open bar in such an ancient fossil shows that lepidosaurs were already experimenting with jaw flexibility very early in their evolution – a shift that would later help make the group so successful.
This alone tells an interesting story of divergent evolution driven by environmental pressures. Many lizards and snakes were pushed by competition and opportunity to evolve more flexible skulls, in order to tackle larger or more varied pray. Losing this temporal bar gave their jaws extra mobility – they could open wider, twist and handle their food in different ways. The tuatara, on the other hand, didn’t lose the bar because it didn’t need to; it kept its ancestral bony bar because its function served its purpose for its ecological niche, helping the reptile snap up insects, worms and small vertebrates with a powerful, slicing bite. That conservative strategy may explain why the tuatara survived in isolation, even as most of its rhynchocephalian relatives vanished.
So while this newly identified species was small enough to fit in the palm of a hand, it's an important discovery that adds to the ancient family tree of reptiles. And its distinct features show why the suborder Lepidosauria – which today includes lizards, snakes and the tuatara – went on to become so successful. From the very beginning, they were already experimenting with different skull structures and feeding strategies that gave them such evolutionary flexibility – which continues to be a hallmark of the strongest survivors on Earth.
And while the tuatara may be the last of its kind, it carries the legacy of a dynasty that once stretched across the globe.
The research was published in the journal Nature.
Source: University of Bristol
