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."