The story of Westlothiana began in 1984, when the fossil was discovered in Scotland embedded in two slabs of rock. When it was formally described in 1990, researchers could rely only on what was visible at the surface of the crushed specimen. Even so, the skeleton appeared remarkable: a nearly complete body with four limbs, ending in fingers and toes tipped with superficially claw-like structures. At a time when many of its contemporaries retained fin-like limbs better suited to swimming than walking, these features seemed to mark Westlothiana as a pioneer of terrestrial life. Its lizard-like appearance earned it the nickname Lizzie, and its five-toed hands and feet—a configuration familiar from modern mammals and reptiles—reinforced the impression that this was an animal adapted for life on land.

Because of those surface features, Westlothiana was widely treated as the earliest land-adapted amniote, and for nearly 30 years it served as a calibration point in debates about the origin and timing of the amniote lineage. Textbooks, phylogenetic analyses, and popular accounts of early land vertebrates repeatedly leaned on the Scottish fossil as evidence that reptile-like anatomy had emerged by the Early Carboniferous. The fossil, however, was deformed and notoriously difficult to interpret, and its status rested almost entirely on external anatomy that generations of researchers had never been able to examine from the inside.

That changed when a research team led by Xavier Jenkins, a postdoctoral fellow in the American Museum of Natural History’s Division of Paleontology, and Ben Igielman, who began the research during his graduate work at the University of Oxford, turned to high-resolution x-ray imaging. At the European Synchrotron Radiation Facility in Grenoble, France, the team used x-ray scanning to peer through the rock surrounding the specimen and reconstruct anatomical structures that had been invisible from the surface for four decades. The technique allowed the researchers to build a three-dimensional picture of the animal’s internal anatomy without damaging the fragile fossil, exposing features that would ultimately overturn its scientific identity.

What the scans revealed came as a profound shock. Inside the rock, the team found a surprisingly primitive skull, ossified internal gills, and a fish-like mouth lined with thousands of tiny teeth. Each of these features is incompatible with the interpretation of Westlothiana as a reptile or as any kind of amniote ancestor. Internal gills in particular point to an aquatic or amphibious lifestyle, while the primitive skull indicates that the animal belonged to a lineage far more ancient than the amniote stem. As Jenkins put it, Lizzie has been an icon for the early evolution of amniotes for decades, but once the researchers were finally able to see inside the fossil, they found an animal that looked very different from what anyone had expected.

Igielman emphasized just how deceptive the fossil’s outward appearance had been. The surprisingly primitive features of the skull, he noted, show that Lizzie was not only not a reptile but belonged to a much more ancient lineage altogether. In other words, the lizard-like silhouette that captivated paleontologists in 1990 masked an anatomy rooted deep in the tetrapod family tree, and the animal lived a completely different lifestyle than previously believed—one spent in and around water rather than striding across dry ground.

The implications extend well beyond the reclassification of a single specimen. The study concludes that several anatomical traits once assumed to be exclusive to land-adapted amniotes had, in fact, already evolved among earlier, more aquatic members of the stem lineage of tetrapods—the broader group of four-limbed animals that includes amniotes as well as amphibians. Claws and the five-toed foot structure of Westlothiana, long treated as hallmarks of terrestrial adaptation, now appear to have arisen piecemeal in animals that were still amphibious. Roger Benson, the Museum’s Macaulay Curator of Paleontology and a co-author of the study, described the result as a big surprise, noting that although amphibians are commonly thought of as more primitive than reptiles today, some of the traits associated with reptiles were already present in the ancestor of both groups.

The findings also carry weight for one of the most contentious questions in early vertebrate paleontology: the timing of the amniote origin. In recent years, fossil trackways—preserved impressions of animals walking on four limbs with claw-like digits—have been used to argue that amniotes originated during the Devonian Period, more than 359 million years ago. The logic seemed straightforward: clawed, weight-bearing tracks implied a clawed, land-adapted animal, and clawed, land-adapted animals were presumed to be amniotes. The new work breaks that chain of inference. If early tetrapod relatives such as Westlothiana also possessed claws, then the presence of clawed tracks alone can no longer establish that the track-maker was an amniote.

As a result, some ancient trackways previously attributed to early amniotes may instead have been produced by more primitive, amphibious members of the tetrapod lineage—animals that could walk on land but were not yet part of the reptile-mammal branch of the tree. Jenkins summarized the broader lesson: 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 gradually in their close relatives, in animals that were still living in and out of the water. The evolutionary transition from fish to land vertebrate, in this view, was not a single dramatic leap marked by the sudden appearance of reptile-grade anatomy, but a long, mosaic process in which land-ready features appeared one by one in creatures still tied to aquatic environments.

The study, published in Nature, was a collaborative effort that included 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. Support came from the National Science Foundation, the Natural Environment Research Council, and the Biotechnology and Biological Sciences Research Council. For paleontologists, the dethroning of Westlothiana is a reminder of how much remains hidden inside even the most famous fossils—and of how modern imaging technology can rewrite chapters of evolutionary history that seemed settled for generations. Lizzie may no longer be the world’s oldest reptile, but in revealing that claws and five-toed feet evolved before the first amniotes ever walked the Earth, the Scottish fossil may ultimately prove even more valuable to science than it was before.

Subject of Research: Reclassification of the 345-million-year-old Westlothiana lizziae fossil from Scotland as an aquatic stem tetrapod rather than the earliest known reptile, based on synchrotron x-ray imaging

Article Title: Fossil long known as world’s oldest reptile dethroned

Article References: Fossil long known as world’s oldest reptile dethroned. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Westlothiana, paleontology, amniotes, tetrapods, synchrotron imaging, Carboniferous, early reptiles, fossil trackways, evolution, Scotland, Nature study, aquatic adaptation

The new species, whose name translates to Jean’s crawler from Mazon Creek, is described in the Journal of Vertebrate Paleontology. Jeanerpeton mazonensis belongs to a group of early tetrapods that sit close to the ancestry of modern amphibians. To picture the animal, imagine a salamander in general body plan, but compressed and rotund, more like a frog in proportions. It was a small predator of the coal swamps, part of the sprawling evolutionary experiment in terrestrial and semi-aquatic life that followed the first vertebrates onto land. Its formal description, authored by co-lead scientists Payton Kohlberg and Allison Sefcovic along with postdoctoral researcher Cam So, formally establishes the animal as anatomically distinct from the species to which it had long been assigned.

The exceptional quality of the specimen owes everything to the geology of Mazon Creek. During the Carboniferous, the delta environment buried dead animals and plants rapidly under layers of fine sediment, a perfect recipe for fossilization. Rapid burial in anoxic conditions limited scavenging and decay, allowing delicate structures to survive the hundreds of millions of years between death and discovery. Today, Mazon Creek is recognized as one of the world’s premier fossil sites, a Konservat-Lagerstätte of the kind that preserves records of soft tissues and fine anatomical details normally lost to the fossil record. Where most fossil sites yield bones and shells alone, Mazon Creek can preserve body outlines, skin surfaces, and internal soft anatomy, giving scientists an unusually complete window into a 300-million-year-old ecosystem.

That preservation is precisely what makes the new species so scientifically valuable. What’s really cool about Mazon Creek is that it preserves all of these unusual details, things like toe pads and body outlines, says Allison Sefcovic, co-lead author of the paper and a 2025 Women in Science intern at the Field Museum. In the fossil of Jeanerpeton, researchers can see some of the scales on its back and tiny belly ribs called gastralia. Even the impressions of the eyes are visible. These features, preserved in remarkable fidelity, point to a deep origin for classic traits seen in today’s frogs and salamanders, anchoring the anatomy of modern amphibian lineages to a specific moment in the late Carboniferous.

The story of the fossil itself is a familiar one in paleontology. The specimen was found many years ago by an amateur collector and donated to the Field Museum, where at a glance it was classified as Amphibamus grandiceps, a species with a well-established presence in the region. For decades, the fossil sat in the collection under that label, its true distinctiveness masked by an old taxonomic habit. In paleontology, poorly understood specimens are sometimes lumped together into what scientists call a waste bin taxon, a metaphorical dumping ground for fossils that seem related but have not been carefully studied. Such taxa are convenient shorthand, but they can hide genuine diversity, quietly absorbing distinct species into a single catch-all name until someone takes a closer look.

That closer look is exactly what the new study provides. What we’re finding from the Field Museum’s collection are a bunch of things that historically were grouped together into this waste bin taxon known as Amphibamus grandiceps, says Cam So, co-author on the study and a postdoctoral researcher at the Field Museum. Jeanerpeton is one of those species. And now, we’ve found enough evidence to support it being an anatomically distinct animal. With this publication, we officially pull Jeanerpeton out of this waste bin taxon. The formal designation of a new genus and species is more than a naming exercise; it is a correction to the fossil record’s accounting, restoring a lineage that had been hiding in plain sight within museum drawers.

The research also carries a human story that has resonated widely. The study’s co-lead authors, Payton Kohlberg and Allison Sefcovic, were part of last year’s cohort of Women in Science Interns at the Field Museum, a program that champions young women pursuing careers in science and museums. Both worked under the mentorship of Arjan Mann, the Field’s assistant curator of fossil fishes and early tetrapods in the Negaunee Integrative Research Center, alongside So, who is also part of the Mann lab group. That a new species was described, named, and published by student scientists early in their careers is a testament to the model of hands-on research training, in which interns take genuine ownership of scientific questions rather than serving only as assistants.

For Kohlberg, the project was transformative as much as it was scientific. Before working in the Mann Lab, I didn’t know much about paleontology, she says. My experience at the Field and in the Women in Science program has made me more confident in my abilities to try something new. It is scary to take on a project when you feel like you don’t know as much as you should, but I learned with my practical experience that it is possible to overcome that fear with hard work. The choice of the name Jeanerpeton, honoring two mothers named Jean, reflects the personal significance of that journey, and it gives the ancient animal a warm and memorable identity in the scientific literature.

Scientifically, the description of Jeanerpeton mazonensis opens a door for researchers as they continue to chronicle the biodiversity that once lived at Mazon Creek. Amphibamiforms, the group to which the new species belongs, are central to debates about the origins of modern amphibians, the lissamphibians comprising frogs, salamanders, and caecilians. Every anatomically distinct species added to the group refines the character data that phylogenetic analyses use to reconstruct those relationships. The exceptionally preserved features of Jeanerpeton, from its dorsal scales to its gastralia to its eye impressions, supply exactly the kind of fine-grained anatomical evidence that such analyses require, and its removal from the Amphibamus grandiceps waste bin taxon cleans up a source of noise in previous comparisons.

Each new species described from Mazon Creek adds another piece to a detailed mosaic of what life looked like in Illinois more than 300 million years ago, a time when the foundations of modern terrestrial ecosystems were being laid. The Carboniferous world of Jeanerpeton was one of rising forests, oxygen-rich air, and evolutionary experimentation among the first four-limbed vertebrates to make a living on land and in fresh water. That a pair of student interns could pull a new animal out of a century-old museum collection and give it a name rooted in family gratitude is a reminder that paleontology’s greatest discoveries are often not made in remote badlands, but in the careful re-examination of fossils already in our care, waiting for the right eyes.

Subject of Research: A new exceptionally preserved Carboniferous proto-amphibian species from Mazon Creek and its taxonomic reclassification

Article Title: Mother of amphibians: New species of proto-amphibian described by two student scientists, named in honor of their moms

Article References: Mother of amphibians: New species of proto-amphibian described by two student scientists, named in honor of their moms. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Jeanerpeton mazonensis, Mazon Creek, Carboniferous, paleontology, amphibians, Field Museum, tetrapods, fossil preservation, waste bin taxon, Women in Science, Amphibamus grandiceps, early tetrapod evolution