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Male "ghost sharks"—eerie deep-sea fish known as chimaeras that are related to sharks and rays—have a strange rod jutting from their foreheads, studded with sharp, retractable teeth. New research in Proceedings of the National Academy of Sciences reveals these are not merely lookalikes, but real rows of teeth that grow outside the mouth.

What's more, the toothy appendage is likely used for mating. Found only in males, the forehead rod—called a tenaculum—is the ghost sharks' only source of distinct teeth, and it seems to be used to grasp females in much the same way sharks use their toothy mouths in mating.

"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," said Gareth Fraser, Ph.D., a professor of biology at the University of Florida and senior author of the study. "If chimaeras can make a set of teeth outside the mouth, where else might we find teeth?"

The team, including scientists from the University of Washington and the University of Chicago, studied both fossils and living specimens to solve the mystery. A 315-million-year-old fossil showed the tenaculum attached to the upper jaw, bearing teeth incredibly similar to those in the mouth. Modern chimaeras collected from Puget Sound revealed the same tooth-growing process on the head, seen in modern-day shark jaws. Genetic testing confirmed they expressed the same tooth-specific genes as oral teeth.

"What we found is that the teeth on this strange appendage look very much like rows of shark teeth. The ability to make teeth transferred onto that appendage, likely from the mouth," Fraser said. "Over time, the tenaculum shortened but retained the ability to make oral teeth on this forehead appendage."

Fraser collaborated with Washington's Karly Cohen, Ph.D., and Michael Coates, Ph.D., from Chicago on the study.

As experts in shark evolution and anatomy, the scientists were intrigued by these tooth-filled rods sprouting from the ghost shark foreheads. The central mystery: Is the tenaculum covered in true teeth related to oral teeth, or more similar to the tooth-like scales plastering the skin of sharks and some ghost sharks?

CT scans of the fossils and modern chimaeras gave the scientists unprecedented, detailed insights into the development of the tenaculum teeth, which looked remarkably similar to the teeth of today's sharks. The nail in the coffin came from genetic evidence. The tenaculum teeth express genes found only in true teeth, never in shark skin denticles.

"What I think is very neat about this project is that it provides a beautiful example of evolutionary tinkering or 'bricolage,'" said Coates, a professor of biology at the University of Chicago. "We have a combination of experimental data with paleontological evidence to show how these fishes co-opted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction."

Cohen, a postdoctoral researcher at the University of Washington's Friday Harbor Labs and first author of the paper, said scientists had never spotted teeth outside the mouth in this way before.

"The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw," she said.

The bizarre path from a mouth full of teeth to forehead teeth used for mating demonstrates the impressive flexibility of evolution, the researchers say, always ready to repurpose structures for strange and unexpected new uses.

"There are still plenty of surprises down in the ocean depths that we have yet to uncover," Fraser said.