The “smile” in this headline is a fossil jaw, not evidence of fish behavior. In a 2021 study, scientists reconstructed teeth and jaws from Canadian acanthodians—extinct spiny fishes that lived more than 400 million years ago—and found two different dental designs. Some teeth formed successive rows attached to the jaw; others formed a semicircular whorl at the front. Evolutionary analysis indicates that these elaborate arrangements arose independently, rather than representing one standard design inherited by all early jawed vertebrates.
The result changes how living sharks should be used as a guide to early tooth evolution. Teeth probably existed in the common ancestor of living jawed vertebrates, but conveyor-belt replacement, tooth whorls, dental laminae and other complex systems evolved, disappeared and re-evolved multiple times. The study is about fossil anatomy and evolutionary history, not a test of a treatment for people or pets.
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What fossil fish was studied?
The central fossils belonged to acanthodians, an extinct group of early jawed fishes often called “spiny sharks” because of their fin spines. They are not modern sharks, and the specimens in this study came from Canada. They are more than 400 million years old, placing them deep in the early history of jawed vertebrates.
The headline’s “fish smile” is editorial shorthand for the preserved tooth-bearing jaws. A fossil cannot show a fish smiling; it can preserve the position, shape and growth sequence of teeth well enough for researchers to reconstruct how the jaws worked.
The formal paper is Acanthodian dental development and the origin of gnathostome dentitions, published in Nature Ecology & Evolution in 2021 by Martin Rücklin, Benedict King, John A. Cunningham, Zerina Johanson, Federica Marone and Philip C. J. Donoghue.
How the hidden teeth were reconstructed
The jaws were preserved inside limestone. The researchers dissolved the surrounding rock with acid, exposing the delicate fossil material without having to cut through the jaws. They then used high-energy synchrotron X-rays at the TOMCAT beamline of the Swiss Light Source at the Paul Scherrer Institute in Switzerland.
The reported resolution was 0.74 micrometers (0.00074 millimeters). At that scale, the scans could distinguish individual teeth, their attachment to the jaw and the geometry of developing tooth rows. Computer reconstructions turned the X-ray data into three-dimensional models, allowing the team to examine growth history that would be difficult to see from a flattened fossil.
- Expose the fossil: acid removed the limestone around the Canadian jaws.
- Scan without sectioning: synchrotron X-rays recorded internal structure at 0.74-micrometer resolution.
- Build a 3D model: the scan slices were assembled into a digital jaw and tooth reconstruction.
- Read developmental clues: tooth position and size were used to infer which teeth formed first and where later teeth were added.
- Compare arrangements: the acanthodian patterns were examined alongside dentitions known from placoderms and early bony fishes.
- Test evolutionary histories: a phylogenetic tree and probabilistic ancestral-state analyses, including thousands of simulations, estimated which features were likely present in earlier ancestors.
That final step is an inference, not a direct photograph of an ancestral animal. The fossil jaws directly show particular acanthodian arrangements; the tree-based analysis estimates how those arrangements relate to one another through time.
Two different tooth designs in the acanthodian jaws
Successive rows attached to the jaw
In one arrangement, teeth were attached along the jaw in successive rows. New teeth were added as the animal grew, producing an organized sequence rather than a random scatter of isolated teeth. The position of each tooth relative to the jawbone and neighboring teeth provides evidence about the order in which the dental series developed.
A semicircular tooth whorl at the front
Other studied jaws had a semicircular whorl toward the front of the mouth. New teeth were added over time to this curved structure, creating a tightly arranged series unlike the marginal rows. A whorl is an arrangement of teeth; it is not a separate kind of tooth and does not imply that every tooth in the mouth was organized this way.
| Feature | Jaw-attached rows | Anterior tooth whorl |
|---|---|---|
| Where teeth occur | Along the jaw margin in successive rows | In a semicircle toward the front of the jaw |
| What the fossil directly shows | Teeth fixed to jawbone in an ordered series | A curved series with teeth added through time |
| Replacement or addition | Successive teeth appear as the series develops | New teeth are added to the whorl over time |
| Evolutionary status | An arrangement documented in the studied acanthodians | A separate arrangement documented in the studied acanthodians |
| What the ancestral analysis says | Not established as the single ancestral design | Not established as the single ancestral design |
These are two solutions to the mechanical problem of maintaining functional teeth while a jaw grows. Their coexistence in acanthodians is important because it shows that early jawed fishes were experimenting with dental organization rather than following one universal blueprint.
What the evolutionary analysis infers about the first jawed-vertebrate teeth
The researchers mapped dental traits onto an evolutionary tree that included acanthodians and other early jawed vertebrates. Probabilistic ancestral-state methods were then used to estimate the most likely condition at older branch points. Because the result depends on the fossils included, the relationships in the tree and the model used, it should be read as a reconstruction with uncertainty—not as direct observation of an animal that no longer exists.
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The study’s interpretation is deliberately narrower than “teeth evolved many times.” It supports the presence of teeth in the crown ancestor of jawed vertebrates, the lineage containing living jawed vertebrates and their closest common ancestor. What evolved repeatedly were more elaborate systems: organized dentitions, tooth whorls, dental laminae (the tissue that generates tooth germs) and coordinated replacement.
“Using state of the art probabilistic ancestral state estimation methods, we build on this discovery to show that teeth existed in the crown-ancestor of gnathostomes, whereas complex dentitions, tooth whorls, a dental lamina and coordinated replacement, have all evolved independently and been lost several times in the early evolution of jawed vertebrates.”
—Benedict King, Naturalis Biodiversity Center
In other words, the basic presence of teeth may have a deep common origin, while the way those teeth were arranged and replaced was evolutionarily flexible. Similar-looking structures in separate lineages can therefore be examples of parallel evolution rather than inherited features from one immediate ancestor.
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Why modern shark teeth are not a simple ancestral model
Many living sharks replace teeth continuously. A new tooth develops behind a functioning tooth and moves forward as the older one is lost, creating a conveyor-belt-like supply. Because this system is familiar and visually striking, it has often been treated as a reasonable model for how the earliest jawed vertebrate teeth developed.
The acanthodian evidence weakens that assumption. Early jawed fishes had multiple dental arrangements, and the evolutionary analysis does not recover the modern shark pattern as a universal starting point. A shark’s replacement conveyor is one successful evolutionary solution, not a time capsule preserving the first vertebrate dentition.
“We used high energy x-rays at the TOMCAT beamline of the Swiss Light Source at the Paul Scherrer Institut in Switzerland, to study tooth and jaw structure and development among shark ancestors.”
—Martin Rücklin, Naturalis Biodiversity Center
The distinction matters for biology and medicine. A living shark can reveal how one lineage grows and replaces teeth, but that does not make it a reliable stand-in for the earliest jawed vertebrate or a ready-made blueprint for regenerating human teeth. The study did not test a dental therapy, grow replacement teeth in a patient or evaluate an intervention for pets.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →“Dentitions of vertebrates are characterized by an organised arrangement to enable occlusion and efficient feeding over the lifetime of an animal.”
—Professor Philip Donoghue, University of Bristol School of Earth Sciences
Occlusion—the way upper and lower teeth meet—places mechanical demands on a growing jaw. Different animals can meet those demands with rows, whorls or other arrangements. Similar function does not prove common ancestry, and common ancestry does not require every descendant to retain the same replacement system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is directly observed, and what is reconstructed?
| Question | Evidence level | Careful interpretation |
|---|---|---|
| Were teeth present in the studied jaws? | Direct fossil observation through 3D scans | The acanthodian specimens contained preserved teeth and jaw structures. |
| Did the specimens have rows and whorls? | Direct reconstruction of fossil anatomy | The studied jaws showed jaw-attached successive rows and a separate anterior semicircular whorl. |
| What did the common ancestor of jawed vertebrates have? | Probabilistic ancestral-state estimate | The analysis places teeth in the crown ancestor, while the exact complex arrangement remains a reconstruction. |
| Did one arrangement evolve into all the others? | Phylogenetic comparison and simulations | The study favors independent evolution, repeated loss and modification of complex dentitions. |
| Can the findings produce a human or pet dental treatment? | Not tested by this study | The fossils inform evolutionary analogy; they do not demonstrate a clinical treatment. |
Accessible accounts of the study do not establish specimen species names or collection identifiers in enough detail to use them confidently here. Those labels are not needed for the central result: the fossils preserve more than one dental architecture, and their evolutionary history is not a single straight line from an acanthodian jaw to a modern shark mouth.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsDo not merge these fossils with the Australian placoderm stories
Separate reports from the Australian National University discuss Buchanosteus, an armoured placoderm from the Lake Burrinjuck area of Australia. A 2016 story addressed tooth-like denticles and whether they represented a transitional stage; a 2017 report concerned placoderm jaw structure and anatomy.
Those are different animals, localities and studies from the Canadian acanthodian material analyzed in 2021. They cannot be used as additional specimens of the acanthodian tooth rows or whorl, and their dates should not be substituted for the age of the Canadian fossils.
How this changes the story of tooth evolution
- Teeth are older than any one modern replacement system: the analysis supports teeth in the crown ancestor of jawed vertebrates.
- Complex dentitions are evolutionary experiments: whorls, dental laminae and coordinated replacement appeared independently and were lost repeatedly.
- Parallel shapes can mislead: two tooth arrangements may look alike or perform similar jobs without descending from the same immediate ancestor.
- Fossils can reveal development: the order and attachment of teeth in a 3D scan provide clues about growth that a surface view alone would miss.
- Sharks are informative but not ancestral stand-ins: their conveyor-belt replacement explains shark biology, not necessarily the first jawed vertebrates’ dentition.
The lasting contribution of the study is therefore a more careful separation of origin from elaboration. The presence of teeth can have a common deep history even when the rows, whorls and replacement schedules seen in later animals were invented more than once.
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