World’s oldest lizard fossil forces rethink of reptile family tree

The fossilised remains of a small lizard discovered in rock from the Italian Alps has shaken up the evolutionary family tree of reptiles and shed new light on the survivors of the most devastating mass extinction the world ever faced, researchers say.

Thought to have lived in the triassic period, about 240 million years ago, the creature, known as Megachirella wachtleri, has been unveiled as the oldest known member of a group of reptiles known as squamates – which includes lizards, snakes and peculiar legless creatures known as worm lizards.

Researchers say the finding not only sheds light on what the last common ancestor of such creatures would likely have looked like, but reveals that squamates probably appeared far earlier that previously thought, and survived one of the planet’s greatest catastrophes.

“All lizards and snakes are descendants from Megachirella or a Megachirella-like lizard,” said Dr Massimo Bernardi, co-author of the study from the University of Bristol, adding that Megachirella would probably have measured about 25-30cm from its nose to the tip of its tail.

Writing in the journal Nature, an international team of researchers describe how they reanalysed the 240m year old fossil of the creature which was first discovered in rock from the Dolomites in the early 2000s by an amateur collector.

Using an x-ray technique known as CT-scanning, the team were able to examine in 3D previously hidden features of the fossil. In addition, they spent about 400 days visiting and examining some 150 specimens of ancient lizard-like creatures held in collections around the world, and analysed both skeletal and molecular data – including DNA – from living squamates.

The results reveal that Megachirella is a squamate, pushing back the earliest known member of the group by 75m years and backing up some previous molecular studies that had proposed squamates existed in the triassic. A further result of the new family tree, the team say, is that it settles a long-standing debate, revealing that geckoes evolved earlier than iguanas.

And there’s more. Combining both the molecular data and observations of skeletal features, the team were able to estimate when the earliest squamates appeared, revealing they probably originated just before the “Great Dying” – a catastrophic mass extinction event 252m years ago when more than 90% of marine creature and 70% of land vertebrates died. That, Bernardi said, overturns current theories that they arose after the disaster, and reveals that many different species appeared in the wake of the catastrophe due to factors such as a lack of competitors. “It is like the other side of extinctions,” he said. “Squamates, for example, were actually there before the extinction, they went through [it] in some way, and they took the opportunities that opened up just after the extinction,” he said.

The international study of the Megachirella wachtleri fossil allowed the authors to re-write the history of all fossil and living lizards and snakes. Photo credit: MUSE - Science Museum, Trento, Italy
The international study of the Megachirella wachtleri fossil allowed the authors to re-write the history of all fossil and living lizards and snakes. Photo credit: MUSE - Science Museum, Trento, Italy

Bernardi says Megachirella probably lived along shorelines, and that the specimen found in the Dolomites met with a watery end amid a thunderstorm. “At that time geological reconstructions show us very clearly that the Dolomites were a series of islands with rich vegetation and fine sand beaches, and probably Megachirella was walking along one of those beaches,” he said. “[We think] it was taken by the thunderstorm because in the very same [rock] layers you see a lot of plants and debris and things that were coming from land into the sea.

David Martill, professor of palaeobiology at the University of Portsmouth who was not involved in the study, said the revelation that squamates arose before the Great Dying casts the creatures in a new light. “This means squamates are real survivors,” he said. “The Permo-Triassic extinction event was a dangerous time to be alive. Not much escaped its deathly touch.”

Nicola Davis