At first glance, the animal hardly seems like a revolutionary. Measuring just fifty centimeters from nose to tail tip and weighing roughly two kilograms, Megacauda sungei would look modest next to any of the mammals alive now, from rhinoceroses to whales and elephants. Yet in the Mesozoic Era, spanning roughly 230 to 66 million years ago, size meant something entirely different. This was the age of the dinosaurs, and the early ancestors of mammals lived in the shadows of these giants, often weighing only a few grams. Against that backdrop, a half-meter aquatic predator was a genuine heavyweight, which is why the researchers describe it with a certain irony as a top predator of its day. Its very name, combining the Latin for large tail with an homage to Professor Sun Ge, Honorary Director of the Paleontological Museum of Liaoning, signals the animal’s most striking external feature.

That feature is also a window into its lifestyle. With its short coat and flattened tail, Megacauda sungei resembles an otter, a beaver, or a platypus, and like those modern animals it was clearly at home in the water. Three-dimensional reconstructions built from computed tomography images of the fossil suggest a swimming style that was both coordinated and efficient: the animal paddled with its front legs, propelled itself with wave-like undulations of its tail, and steered with its hind legs. According to paleontologist Professor Thomas Martin of the University of Bonn, this is the earliest evidence of an aquatic lifestyle in mammals, pushing back the timeline for when mammal relatives first committed to life in rivers and lakes. The finding adds a swimming specialist to a docodont family tree that already included tree-dwellers and burrowers comparable to moles, underscoring how experimentally diverse these early mammaliaforms were.

The teeth tell their own story of adaptation. The specialized canines of Megacauda sungei are distinctive in having three roots, an unusual anatomical arrangement, and the cusps of its teeth curved backwards. That backward curvature is a classic adaptation for gripping slippery, struggling prey, and it fits neatly with the picture of an aquatic hunter chasing fish or other slick animals through the water. Combined with the streamlined tail and paddling limbs, the dentition paints a coherent portrait of a predator fully integrated into its aquatic ecosystem, not merely an occasional visitor to the water’s edge. For an animal living in the shadow of dinosaurs, such specialization likely meant the difference between ecological success and obscurity.

But the true evolutionary revolution revealed by this fossil lies deeper than teeth or tails, hidden in the anatomy of the throat. Many animal groups lack a soft palate entirely and possess only a simple hyoid bone apparatus without any true swallowing mechanism. Crocodiles illustrate the alternative strategy vividly: to get unchewed prey down their throats, they raise their heads and use jerky movements, relying on gravity to do the work. As Professor Martin explains, this is achieved with the aid of gravity alone. Mammals, by contrast, evolved a complex hyoid structure in the pharynx that allows them to swallow against gravity. It is the reason a human being can drink upside down through a straw while continuing to breathe, an ability that seems trivial until one appreciates how few animals on Earth can manage it.

The soft palate is the keystone of this system. By separating the oral cavity from the pharynx, it permits swallowing and breathing to proceed without interference, and it keeps food and liquid from entering the nasal passages. This anatomical arrangement is a critical prerequisite for suckling, allowing newborn mammals to feed on nutrient-rich milk safely and efficiently. In the words of Professor Martin, it was this adaptation that made brain development, eventually leading to humans, possible. The soft palate and the shaping of the oral cavity and pharynx are directly linked to the evolution of mammals as a lineage. What the new fossil demonstrates is that this transformation was already underway, and apparently well advanced, in Megacauda sungei tens of millions of years before the emergence of modern mammals.

Classifying the animal required extensive comparisons with other fossils and with living species. The team, led by Professor Chang-Fu Zhou of Shandong University, Professor Zhe-Xi Luo of the University of Chicago, and Professor Thomas Martin of the University of Bonn, determined that the specimen, discovered at Xiayingzi in Inner Mongolia, belongs to the docodonts, an extinct group of early mammaliaforms that flourished primarily during the Jurassic period. Docodonts are not on the direct line to today’s placental mammals, marsupials, and monotremes, but they are close relatives whose anatomy offers a rare glimpse of the evolutionary experiments that accompanied the origin of the mammalian body plan. Within this group, Megacauda sungei stands out as the largest Jurassic mammaliaform yet known, a fact that reshapes assumptions about how big these animals could grow while dinosaurs dominated the land.

The path from discovery to publication was long and, as is so often the case in paleontology, began with chance. The Paleontological Museum of Liaoning in Shenyang acquired the specimen years ago together with other finds, and its significance only became apparent during a joint examination by the three senior researchers. Once the unique nature of the fossil was recognized, it took years of painstaking work before all examinations were complete and the new species could be securely classified, described, and documented. Martin describes the outcome as the result of a long period of teamwork, and the effort paid off handsomely: the description of the new species has unlocked countless secrets relating to the origin of mammals, from the mechanics of aquatic locomotion to the deep history of the throat anatomy that makes suckling, and ultimately large mammalian brains, possible.

Beyond its individual significance, Megacauda sungei changes the broader picture of early mammaliaform evolution. Together with other finds, it shows that docodonts occupied an astonishing range of ecological niches across the Northern Hemisphere during the Jurassic: some lived in trees, others underground like moles, and others, like the new species, in water. As Martin summarizes, this demonstrates how widespread docodonts were and that this diverse group of animals developed very different lifestyles at an incredibly early stage. A creature barely half a meter long, once dismissed as just another small fossil, thus carries an outsized message: the fundamental innovations that define mammals, from swimming bodies to swallowing throats, were assembled piecemeal and remarkably early, in the shadows of the dinosaurs, by animals small enough to hold in two hands.

Subject of Research: The anatomy and evolutionary significance of the Jurassic aquatic mammaliaform Megacauda sungei

Article Title: “Top predator” from the Jurassic measures just half a meter

Article References: “Top predator” from the Jurassic measures just half a meter. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Megacauda sungei, docodont, Jurassic, mammaliaforms, soft palate, paleontology, Inner Mongolia, aquatic adaptation, hyoid apparatus, mammalian evolution, Nature, fossil

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 research team, led by Ben Igielman of the University of Oxford together with Xavier Jenkins and Roger Benson of the American Museum of Natural History, 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 in Cambridge, subjected the holotype specimen, catalogued as NMS G.1990.72.1 and held at National Museums Scotland, to high-resolution synchrotron X-ray computed micro-tomography. The scans were performed on beamline ID19 at the European Synchrotron Radiation Facility in Grenoble, France, under proposal ls2687, and allowed the researchers to digitally segment and reconstruct individual bones of the skull roof, palate, braincase, mandible, vertebrae and limbs in three dimensions without risking damage to the fragile fossil. The resulting digital models, now openly available on MorphoSource, reveal anatomical details that could never have been extracted from the flattened, part-and-counterpart slabs in which the animal was preserved.

What the team found inside the skull came as a genuine surprise. Far from showing the suite of derived features expected of an early amniote relative, Westlothiana preserves unexpected plesiomorphies, that is, ancestral characteristics, in the skull roof, palate, braincase and lower jaw. More striking still are the traces of a thoroughly aquatic lifestyle: the scans revealed an ossified internal gill skeleton, including ceratobranchial elements of the hyobranchial apparatus, as well as extensive fields of denticles, tiny tooth-like structures, covering the palate and the mandible. An internal gill skeleton is a feature associated with breathing water rather than air, and shagreen-like denticle fields on the roof of the mouth are characteristic of suction-feeding aquatic predators among early tetrapods and their fishy relatives. Together, these traits paint a picture of an animal that was far more comfortable in water than its reputation as a pioneering land-dweller had suggested.

When the researchers fed their new anatomical observations into an expanded phylogenetic dataset and ran Bayesian analyses under a time-calibrated fossilized birth-death model, the tree returned Westlothiana not as a stem amniote but as a stem tetrapod, occupying a much deeper position on the lineage leading to all limbed vertebrates. This is a dramatic demotion in phylogenetic terms, moving the animal from the very threshold of the reptile lineage back to the broader stem of Tetrapoda itself, the branch that lies outside the crown group containing modern lissamphibians, the frogs, salamanders and caecilians, and amniotes. The result aligns with recent suggestions that the tetrapod stem lineage was taxonomically far more inclusive, and far more anatomically diverse, than traditional classifications allowed, echoing earlier work that highlighted hidden morphological diversity among early limbed vertebrates.

Yet the paradox that makes the new study so compelling is that Westlothiana simultaneously retains a set of unmistakably terrestrial adaptations. Its foot shows an amniote-like pedal formula, the pattern of bones in the digits that is associated with weight-bearing locomotion on land. Its forelimb has lost the fin-like functional anatomy seen in many earlier stem tetrapods, whose limbs were better suited to paddling than propping. And its terminal finger and toe phalanges are superficially claw-like, a feature that in living animals is often associated with traction, digging or climbing. The coexistence of these land-oriented traits with gill arches and palatal denticles in a single small-bodied animal is the clearest demonstration yet that the transition from water to land did not proceed as a neat, coordinated package of changes.

Instead, the authors argue, terrestrial anatomy evolved in a mosaic fashion, with individual traits appearing at different times, in different lineages and in different ecological contexts. Ancestral character reconstruction performed in the study, using equal-rates Mk models in the R statistical environment, suggests that traits associated with aquatic ecologies, such as sensory lateral line canals, branchial arches with grooves for the afferent gill artery, and gill arch articulation with the braincase, were present deep within the tetrapod stem, while postcranial traits such as L-shaped humeri and patterns of limb reduction and body elongation also map onto transitional, amphibious ancestors. In other words, the amniote-like foot and the claw-like phalanges of Westlothiana evolved in an animal that still breathed and fed like an aquatic predator, long before the origin of the tetrapod crown group. Terrestrial competence, on this view, was assembled piecemeal in creatures that were neither fully aquatic nor fully terrestrial, blurring the boundary that palaeontologists have long tried to draw between the two worlds.

The timing matters as much as the anatomy. Limbs first appeared in stem tetrapods by the Late Devonian, roughly 365 million years ago, and trackway evidence from Poland and Ireland shows that four-limbed animals were walking on land even earlier in the Middle Devonian. But the fossil record of the ensuing interval, particularly the poorly sampled stretch known as Romer’s gap in the earliest Carboniferous, has obscured how the first limbs were transformed into the limbs of modern-style land vertebrates. Many early taxa either retained limbs with fin-like functional anatomy or secondarily lost their limbs altogether, complicating any simple narrative of progressive adaptation to land. Westlothiana comes from the Viséan stage of the Early Carboniferous, and recent uranium-lead dating work at East Kirkton Quarry has helped pin down the age of the fauna it belongs to, a community that also included the stem amniote relative Silvanerpeton, the enigmatic mosaic form Eucritta melanolimnetes and a range of temnospondyl amphibians.

The new phylogenetic placement also feeds into a broader, ongoing recalibration of early tetrapod evolution. Recent studies of the earliest amniote tracks have pushed back the timeline of tetrapod evolution, and debates over whether recumbirostran ‘microsaurs’ sit on the amniote stem, and over the deep divergences that eventually produced lissamphibians and amniotes, continue to hinge on the anatomy of small-bodied Carboniferous and Permian fossils. The authors of the new study emphasize that their findings illuminate substantial gaps in anatomical knowledge, especially for small-bodied taxa, that have obstructed understanding of early tetrapod evolution. Many of the most evolutionarily informative early tetrapods are, like Westlothiana, tiny animals preserved as crushed slabs, and it is only with the advent of synchrotron-grade micro-tomography, phase-contrast reconstruction techniques and open digital repositories that their anatomy has become accessible in the detail phylogenetic analysis demands.

For the public imagination, the rehabilitation of ‘Lizzie’ is a reminder that even icons of palaeontology are provisional interpretations rather than settled facts. The specimen was named in 1990 by Timothy Smithson and W. D. Ian Rolfe, who at the time proclaimed it the earliest known reptile, and subsequent work in 1993 by Smithson, Robert Carroll, A. L. Panchen and S. M. Andrews cemented its status as a key to the amniote stem. Thirty-six years later, the same specimen, scanned rather than simply looked at, has told a different story: that of a small, gill-breathing, denticle-mouthed stem tetrapod that nonetheless walked on amniote-like feet with claw-like tips. The mosaic it displays suggests that the assembly of terrestrial anatomy was a long, experiment-rich process unfolding in amphibious animals across the tetrapod stem, and that the road to life on land was walked, at least partly, by creatures that never entirely left the water behind.

The study also carries a practical message for the field. The phylogenetic dataset, Nexus scripts for the Bayesian analyses and R code for ancestral state reconstruction are all stored openly on the Open Science Framework, and the computed tomography data and three-dimensional models of the specimen are freely downloadable, ensuring that future researchers can re-analyse the evidence as new fossils and methods emerge. As scanning technology spreads through museum collections worldwide, more small-bodied Carboniferous fossils are likely to receive the same treatment, and more long-held positions in the tetrapod tree may shift. What is already clear is that the origin of terrestrial vertebrates was not a single leap onto dry land but a mosaic of evolutionary experiments, and that some of the most important experiments were hiding in plain sight, inside a small Scottish fossil that has been studied for more than three decades.

Subject of Research: The mosaic evolution of terrestrial anatomy in early stem tetrapods revealed by computed tomography of Westlothiana lizziae

Article Title: Evolution of terrestrial anatomy revealed by a derived stem tetrapod

Article References: Igielman, B., Jenkins, X., Head, J., Fernandez, V., Roberts, L., Smithson, T., & Benson, R. (2026). Evolution of terrestrial anatomy revealed by a derived stem tetrapod. Nature. https://doi.org/10.1038/s41586-026-11090-6

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11090-6

Keywords: Westlothiana lizziae, stem tetrapod, tetrapod evolution, Early Carboniferous, East Kirkton, amniote origins, synchrotron tomography, phylogenetics, aquatic adaptation, terrestrial anatomy, palaeontology, Romer's gap