It's easy to assume that teeth always grow inside mouths, yet a new study has revealed that some animals – such as the spotted ratfish (Hydrolagus colliei) – grow them elsewhere.
Sometimes referred to as 'ghost sharks', adult male spotted ratfish have strange-looking hooked protrusions that appear on a head structure called the tenaculum, which the fish use to grasp mates during reproduction. The new research suggests that these protrusions are real teeth, and not merely lookalikes as previously assumed.
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” says Karly Cohen, a researcher at the University of Washington’s Friday Harbor Labs.
“The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”
The study, published in Proceedings of the National Academy of Sciences, says that these teeth develop from the same tissue responsible for oral teeth. The discovery suggests tooth-forming cells may have migrated outside the mouth early in evolution, adding to our understanding of how dental structures evolved.
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Spotted ratfish: the animal with teeth growing out of its forehead
Spotted ratfish belong to an ancient group of cartilaginous fish called chimaeras, which split from sharks on the evolutionary tree millions of years ago. They measure about two feet in length and are named for the long slender tails that make up half of their body length.
Only adult males develop the tenaculum, a white, peanut-shaped appendage between their eyes that becomes hooked and barbed during courtship. “Sharks don’t have arms, but they need to mate underwater,” says Cohen. “So, a lot of them have developed grasping structures to connect themselves to a mate during reproduction.”
To uncover the tenaculum’s origins, researchers caught and analysed hundreds of ratfish from the waters around San Juan Island in Puget Sound, a large ocean inlet off the west coast of Washington, USA.
They used micro-CT scans and tissue samples to track the structure’s development and compared their findings with fossils of related species. The scans revealed that both male and female ratfish begin growing a tenaculum early in life, but only males develop it fully.
“When we saw the dental lamina for the first time, our eyes popped,” Cohen says. “It was so exciting to see this crucial structure outside the jaw.” The dental lamina, a tissue layer responsible for producing new teeth, is typically found only in the mouth. Its presence in the tenaculum, alongside genetic evidence, confirmed these were true teeth, not the hardened skin denticles that cover many sharks and rays.
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“We have a combination of experimental data with paleontological evidence to show how these fishes coopted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction,” says Michael Coates, professor at the University of Chicago and co-author of the study.
Gareth Fraser, senior author and professor at the University of Florida, says the discovery highlights the diversity of dental structures through history. “If these strange chimaeras are sticking teeth on the front of their head, it makes you think about the dynamism of tooth development more generally."
Cohen adds, “Chimaeras offer a rare glimpse into the past. I think the more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw.”
Top image: spotted ratfish. Credit: Seattle Aquarium
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