The first lizards were a hardy bunch, it seems.

Based on a small fossil found in the Italian Alps, these ancient reptiles appear to have survived a mass extinction that wiped out more than 90 per cent of all species.

Key points: There are around 10,000 species of snake and lizard alive today, but their earliest evolutionary history is unclear

There are around 10,000 species of snake and lizard alive today, but their earliest evolutionary history is unclear 3-D imaging of a fossil reptile found in Italy revealed previously unseen anatomical details

3-D imaging of a fossil reptile found in Italy revealed previously unseen anatomical details Palaeontologists calculate its common ancestor with modern snakes and lizards lived 252 million years ago, before a mass extinction that killed most species on Earth

The fossilised creature — called Megachirella wachtleri — isn't a direct relative of modern snakes and lizards, although they shared a common ancestor that scampered around 260 million years ago.

But its existence means lizard-like critters first appeared before the Great Dying mass extinction around 252 million years ago, not afterwards, said Alessandro Palci, a Flinders University palaeontologist and co-author of the research reported in Nature today.

"Many people thought lizards evolved after the mass extinction," he said.

"It seems now that lizards were already around before the mass extinction and survived, then took advantage of the fact that there weren't many competitors around and diversified."

Lizard family tree

Snakes and lizards belong to a group of animals called "squamates". Today, they're found almost everywhere: living in trees and underground; in barren deserts and on mountaintops.

But tracing their evolutionary lineage has proved problematic.

This is where Megachirella comes in. The only known specimen was discovered in 1999, embedded in a fossil-rich region of the northern Dolomites in Italy.

The partial skeleton comprised part of its skull, plus ribs, spine and front limbs. All up, it measured less than 6 centimetres long and was around 240 million years old.

The only preserved specimen of Megachirella wachtleri was discovered in the Dolomites in 1999. ( Supplied: MUSE - Science Museum, Trento, Italy )

It was unveiled in 2003 by a pair of Italian palaeontologists and examined over the next decade.

But Tiago Simoes, an evolutionary biologist at the University of Alberta in Canada, thought the little fossil's hidden side deserved a close look too.

The problem was doing so without damaging the precious remains, so he and his colleagues scanned Megachirella using a non-invasive 3-D imaging technique called micro-CT.

Micro-CT is becoming more widespread in fossil analyses, said Christy Hipsley, a palaeontologist at the Melbourne Museum, who wasn't involved in the work.

"A lot of interesting features [on fossils] are on the inside, but you can't see them because they're embedded in the rock," she said.

"With micro-CT, suddenly you can 'flip' the fossil over out of its rock."

When the team examined the skeleton, they discovered features found only in squamates, such as a "kneecap" on its elbow and specific curves in its collarbone.

Armed with these new details and a technique that uses DNA mutation rates to back-calculate when Megachirella and modern squamates diverged, Dr Simoes and his colleagues found the first lizards appeared before The Great Dying — not after it.

"It pushes the origin of lizards back 75 million years," Dr Palci said.

So does Megachirella have any living relatives today? So far, it appears not, he added.

"People tend to say [Megachirella was] the 'mother of all lizards' because it sounds very cool, but it went extinct."

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Strolling and slithering

Tracing the history of squamate reptiles can help palaeontologists address what Dr Hipsley calls the "limbless dilemma".

All snakes and lizards descended from a four-legged creature. Somewhere along the line, snakes stopped strolling and started to slither.

But where it seems snakes lost their limbs once along the evolutionary line, lizards did it over and over again, Dr Hipsley said.

"Some have long legs, some have flippers, others have no legs at all," she said.

These varying degrees of limb-loss makes tracing squamate evolution through anatomy alone particularly tricky.

There's also the question of the earliest evolving squamate group.

Anatomical studies suggested the iguana group takes the title, but DNA points to geckoes and blind skinks.

Dr Simoes and his team found their anatomical and DNA data indicated geckoes were among the earliest-evolving squamates.

As more fossils are uncovered and old specimens re-examined with new technology, palaeontologists will fill in more gaps in the squamate family tree, Dr Hipsley said.