Life is strange in the deep sea. For the male spotted ratfish — a ghostly relative of sharks, also known as the chimaera — it gets stranger. When he finds a mate, he doesn’t just sidle up. He grabs her with a retractable club sticking out of his forehead. And that club isn’t smooth. It’s bristling with rows of sharp, living teeth.

This is not a campfire monster story. It’s just… biology? The kind of biology that reminds us that teeth don’t necessarily belong in the mouth.

Teeth Outside the Jaw

For a long time, since the ratfish was first discovered, biologists wondered what to make of its odd forehead organ, called a tenaculum. At first glance, it looks like a little white peanut lodged between the eyes. In action, it unfurls into a barbed hook that males use to clasp females during sex.

Many fish have pelvic claspers to hang onto a mate, but ratfish take it a step further — forehead first. The mystery was whether those barbs were just skin denticles, the sandpapery armor on sharks, or something more.

The answer came from a team led by Karly Cohen at the University of Washington. They raised spotted ratfish, scanned them with micro-CT, and studied the tissue under microscopes. What they found floored them: the tenaculum teeth form from a dental lamina, the very same stem-cell tissue that builds jaws full of teeth.

“When we saw the dental lamina for the first time, our eyes popped,” Cohen said in a press release. “This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures.”

These aren’t structures that merely look like teeth. They have pulp cavities, dentine layers, mineralized tips — the full tooth toolkit. And they don’t just sit there. Like a shark’s jaw, they grow in conveyor-belt rows, constantly replaced.

Ancient Origins

This discovery doesn’t just solve a fishy puzzle. It pushes back against how scientists think teeth evolved in the first place.

For over a century, paleontologists have argued whether teeth came “outside-in” from armored skin denticles migrating into mouths, or “inside-out” from pharyngeal dentitions moving forward. Ratfish suggest a third option: vertebrates may have once had multiple zones capable of sprouting teeth, not just the jaw.

Michael Coates, a co-author from the University of Chicago, framed it this way: “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.”

The fossil record backs him up. Ratfish relatives from 315 million years ago, like Helodus simplex, had tenacula covered in whorls of teeth almost identical to their oral ones. It seems evolution simply shifted the tooth-making machinery forward — then left it there.

Not everyone is convinced it was always about sex. Dominique Didier, who studies chimaeras, points out that some females retain remnants of a tenaculum. She suspects it may have begun as a defensive weapon or warning signal. “I think it’s utterly amazing that ghost sharks have teeth growing out of their forehead,” she told the New York Times.

Future Implications

So, why should we care about ghost sharks with dental headgear? Because they reveal how flexible our biology really is. Teeth don’t just belong in mouths. They can show up in surprising places if the right genetic program is nudged.

As Gareth Fraser, an evolutionary biologist at the University of Florida, noted, “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.”

That dynamism could matter for humans. We lose our dental lamina after adult teeth come in, which is why we don’t regrow molars like sharks. But understanding how ratfish keep that tissue alive — in an entirely new body part — could eventually help researchers unlock new ways to repair or maybe even regenerate human teeth.

The findings were reported in the journal Proceedings of the National Academy of Sciences.