The ‘World’s Oldest Reptile’ Wasn’t a Reptile After All

A famous “early reptile” turns out to have had gills and a far stranger evolutionary story.

Meet Westlothiana lizziae, also known as “Lizzie,” a tiny Scottish fossil that spent decades holding the crown as one of the oldest known reptiles.

At roughly 345 million years old, Lizzie seemed to have the right credentials. It had five-toed feet, limbs that looked capable of supporting its weight on land and pointed fingertips that resembled claws. Those features placed Lizzie close to the evolutionary roots of reptiles, birds, and mammals.

Then scientists peered inside its skull.

Using high-energy X-rays, researchers uncovered something that did not fit the reptile story at all: an internal gill apparatus and a mouth lined with thousands of tiny tooth-like structures. Lizzie, they conclude in a new study, was not an early reptile relative. It belonged much deeper on the family tree of four-limbed vertebrates and probably lived an aquatic or amphibious life.

The finding suggests that some features associated with life on land evolved surprisingly early, in animals that still retained distinctly aquatic anatomy. And that, in turn, complicates efforts to identify the earliest reptiles from fossil footprints alone.

More Like “Fishie”

Westlothiana was discovered at East Kirkton in Scotland in 1984. When Timothy Smithson and W.D. Ian Rolfe named it in 1990, they called it the “earliest known reptile.” Later studies softened that claim, often placing it near the stem of the amniotes, the group that includes reptiles, birds, and mammals.

The fossil survives flattened between two slabs of rock, with much of its crucial anatomy hidden from view. Researchers took the holotype, the specimen that defines the species, to the European Synchrotron Radiation Facility in Grenoble, France. There, intense X-rays allowed them to digitally reconstruct bones buried in the rock. The team analyzed only this specimen because preliminary observations suggested another fossil previously assigned to Westlothiana might actually belong to a different species.

“Lizzie has been an icon for the early evolution of amniotes for decades,” Xavier Jenkins, a co-lead author, said in a statement. “But when we were finally able to see inside the fossil, we found an animal that looked very different from what we expected.”

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The scans exposed a much more fish-like animal. Bony supports for internal gills carried grooves for blood vessels that would have supplied them, strong evidence that the gills functioned in life. Its palate and jaws were also covered with fields of minute teeth suited to feeding underwater.

Yet Lizzie had five toes and a forelimb less specialized for swimming and potentially better suited to bearing weight. Its terminal finger and toe bones curved into claw-like points. However, they lacked features expected of true claws covered by a keratin sheath.

Lizzie had gills and fed underwater, yet parts of its skeleton were already adapted for life on land.

The road onto land was not a straight line

When the researchers added the newly visible anatomy to their evolutionary analysis, Westlothiana moved well away from amniotes and onto the stem of the broader tetrapod lineage, an earlier branch leading toward living four-limbed vertebrates.

The revision fits a wider trend. High-resolution scans have repeatedly exposed hidden anatomy that rearranges the early tetrapod family tree. A 2017 study on hidden diversity among early tetrapods helped move several supposedly more advanced groups onto deeper branches of the tree.

There is an especially timely consequence. In 2025, researchers described clawed fossil tracks from Australia and argued that they pushed the origin of amniotes back by roughly 35 to 40 million years. That interpretation depended partly on claw marks identifying an amniote or one of its closest relatives.

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But if a more primitive, gill-bearing tetrapod could possess claw-like fingertips, a claw mark in some fossil does not by itself identify its maker as an amniote. The new paper argues that those Australian tracks can no longer securely establish such an early origin for the group.

“This is a big surprise,” Roger Benson, a study co-author, said in the museum statement. “It shows that, although we think of amphibians as being more ‘primitive’ than reptiles today, some of the traits of reptiles were already there in the ancestor of both amphibians and reptiles.”

There is also an apt historical twist. Smithson, who helped introduce Westlothiana as the earliest known reptile in 1990, is also an author of the new study that removes it from that perch.

Lizzie did not become any less important. Lizzie shows that vertebrates did not simply trade gills for legs and march onto land. Evolution assembled the machinery for terrestrial life slowly, sometimes in animals still very much at home in the water.

The study was published in the journal Nature.