The scans revealed:

• A surprisingly primitive skull
• Ossified internal gills — gill structures preserved in bone
• A fish-like mouth lined with thousands of tiny teeth

These features rule out the possibility that Westlothiana was an ancestor of reptiles. They suggest instead that the animal lived an aquatic or amphibious lifestyle, not a terrestrial one.

"The surprisingly primitive features of the skull show us that Lizzie was not only not a reptile but belonged to a much more ancient lineage," said Ben Igielman, co-lead author of the study, who began the research during his graduate work at the University of Oxford. "It turns out looks can be deceiving, and Lizzie lived a completely different lifestyle than we previously thought."

Why do the claws matter?

The findings reach beyond one misidentified fossil. Researchers now conclude that anatomical traits once assumed to be exclusive to land-adapted amniotes — including claws and a five-digit hand and foot structure, a feature shared by mammals and reptiles today — had already evolved among earlier, more aquatic members of the stem lineage of tetrapods. Tetrapods are the larger group of four-limbed animals that includes amniotes plus amphibians.

"This is a big surprise," said study co-author Roger Benson, the museum's Macaulay curator of paleontology. "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."

In other words, a reptile-like foot no longer proves an animal was a reptile relative.

What does this mean for fossil trackways?

The study also affects how scientists interpret ancient footprints. Some researchers have recently used fossil trackways — preserved impressions of an animal walking on four limbs with claw-like digits — to argue that amniotes originated during the Devonian Period, more than 359 million years ago.

If early tetrapod relatives such as Westlothiana also had claws, then clawed tracks alone can no longer establish that the animal which made them was an amniote. The research opens the possibility that some ancient trackways previously attributed to early amniotes were instead produced by more primitive, amphibious members of the tetrapod lineage.

"The broader lesson is that terrestrial-looking traits, such as a reptile-like foot, a weight-bearing forelimb and claw-like phalanges, were not unique features of tetrapods, but rather accumulated piecemeal in their close relatives, in animals that were still living in and out of the water," Jenkins said.

What are the study's limits?

The findings come from a single, deformed specimen, and interpreting crushed fossils from the Carboniferous Period always involves uncertainty. Still, the synchrotron scans provided direct anatomical evidence — gills, skull structure and teeth — that earlier surface-based descriptions could not access. The same technique could now be applied to other disputed early tetrapod fossils.

The study's co-authors include Jason Head of the University of Cambridge, Vincent Fernandez of the European Synchrotron Radiation Facility, Lucy Roberts of the Natural History Museum in London, and Timothy Smithson of the University Museum of Zoology Cambridge. The paper, led by Ben Igielman, appears in Nature under the title "Evolution of terrestrial anatomy revealed by a derived stem tetrapod" (DOI: 10.1038/s41586-026-11090-6).

Lizzie keeps her nickname. She just loses her place in the reptile family tree — and takes a chunk of the amniote origin story with her.