A skull you could balance on a fingertip just rewrote the lizard family tree
A fossil skull about 0.59 inches (1.5 cm) long, pulled from rock along the shoreline near Sidmouth, Devon, belongs to a species newly named Agriodontosaurus helsbypetrae. It is now the oldest confirmed lepidosaur on record, the group that contains every living lizard, snake, and the tuatara. The specimen comes from the Middle Triassic Helsby Sandstone Formation and dates to more than 241 million years ago, making it 3 to 7 million years older than the previous record holder, a species called Wirtembergia. The animal itself was small even by the standards of its relatives, an estimated 3.9 inches (10 cm) long nose to tail, the kind of skeleton that fits entirely in one hand. There’s no body with this fossil, just the skull. Paleontologist Robert Coram excavated it in 2015, and researchers think the animal was caught in a flash flood and buried fast enough to preserve detail that normally doesn’t survive.
What the scans actually showed
A team led by Dan Marke, working on a master’s degree at the University of Bristol, with Dr. David Whiteside of the Natural History Museum as a co-author, examined the skull using synchrotron X-ray CT, a scanning method that fires an intensely bright, tightly focused X-ray beam through a specimen to build a 3D model without cutting into it. The scans turned up oversized teeth, large eye sockets, and an open lower temporal bar, a gap in the rear architecture of the skull behind the eye. They also turned up what wasn’t there: no teeth on the roof of the mouth, and no hinge joint anywhere in the skull. The study, published in Nature on September 10, 2025, describes this exact combination of features as one never documented together in any lepidosaur, living or extinct.
Built to eat like a tuatara, a quarter billion years early
The teeth are what stopped the team cold. They’re so large relative to the rest of the skull that researchers first wondered if they’d found a juvenile that hadn’t grown into its own mouth yet. The skull belongs to an adult. Professor Michael Benton of the University of Bristol concluded the triangular teeth were likely used to pierce and shear the hard outer casing of insect prey, the same basic job the modern tuatara of New Zealand still does to cockroaches, crickets, and grasshoppers. Large eye sockets and anatomy suggesting sharp hearing point to an animal hunting by sight and sound. Nobody had evidence for that feeding strategy this early in the group’s history before. It means an insect-shearing skull was already a working solution for reptiles close to 241 million years ago, long before most living reptile lineages existed. The tuatara’s reputation as an unchanging “living fossil” doesn’t hold up either: Whiteside points out it belongs to a once highly diverse order with a long, complicated evolutionary history, not a species that simply stopped changing.
The myth this skull kills
Paleontologists had long assumed that a hinged skull, teeth on the palate, and an open lower temporal bar arrived as a package, inherited together from one common ancestor at the root of the lepidosaur tree. Agriodontosaurus has exactly one of the three traits, the open temporal bar. No teeth on the palate. No hinge anywhere in the skull. That’s not a gap in the fossil record; it’s the fossil overturning the assumption. If those three traits really were a single inherited set, the oldest known lepidosaur ought to carry all of them. It carries only one. The simpler explanation is that these features evolved separately, more than once, in different lineages arriving at similar skull solutions independently rather than inheriting a shared starting point from one ancestor.
This matters for anyone who has ever assumed a family tree works like a filing cabinet, one trait filed neatly under one ancestor, passed down in a clean line to every descendant. Real evolution is messier than that. The same solution can get invented more than once, in branches that never compared notes. A half-inch skull from a Devon beach just proved it.
What nobody knows yet
The study doesn’t say how many separate times these traits reappeared across the lepidosaur family tree, or what kept pushing unrelated lineages toward the same architecture. It also doesn’t explain why one half-inch skull is enough to overturn an assumption that had stood for decades, only that it is. The researchers place the find inside a larger, still-developing story: the Middle Triassic “Triassic Revolution,” when reptiles began replacing mammal-like synapsids and modern-style land ecosystems started taking shape, setting up the rise of lizards and snakes alongside the first dinosaurs by the end of the period. What actually drove that shift is still under study.
The scan that almost didn’t happen
Before the synchrotron came into the picture, an earlier master’s student, Thitiwoot Sethapanichsakul, worked on the fossil with conventional scanning equipment and could barely make out the teeth. The skull was simply too small for the resolution available. It took synchrotron X-ray CT, with its far finer detail, to bring the teeth and the rest of the skull into focus sharp enough to describe and name the species: Agriodontosaurus, fierce-toothed lizard. Benton has described holding the entire skeleton in one hand and then watching the scan data resolve into detail his team hadn’t expected, once students had finished cleaning it up. The animal waited 241 million years. The name only happened once the imaging finally caught up with the fossil.