This ‘ghost shark’ has teeth on its forehead

Male “ghost sharks” exhibit an extraordinary mating behavior by using teeth that sprout from their foreheads.

These fascinating creatures, also known as spotted ratfish, have developed a unique mechanism for grasping mates: they utilize teeth that extend from a fleshy appendage located between their eyes. This unusual area, termed the tenaculum, caught researchers off guard, says Karly Cohen, a biologist from the University of Washington Friday Harbor Laboratories.

Unlike the predominant dental structures found in most animals, which are situated within the jaw, the teeth of ratfish curve outward like feline claws from this peculiar forehead structure, as reported by Cohen’s team in the September 4 issue of the Proceedings of the National Academy of Sciences. “It’s quite remarkable to uncover actual teeth positioned outside of the mouth,” Cohen explains. Their finding marks a groundbreaking discovery, overturning the long-held belief that vertebrate teeth were only found in oral cavities.

The spotted ratfish (Hydrolagus colliei) has intrigued scientists due to its distinct features. Classified under chimaeriformes, these creatures glide through the ocean using their expansive wing-like fins, giving them an ethereal appearance. Unlike their shark relatives, ratfish do not possess rows of conventional jagged teeth; instead, their skin is smooth and shiny, contrasting sharply with the rough texture typically associated with sharks. The curious spike-like projections from their forehead have been conventionally viewed as evolutionary leftovers; however, Cohen’s research indicates that these structures are indeed teeth. 3D imaging techniques applied to both juvenile and adult ratfish reveal that these spikes emerge from a tissue known as the dental lamina, consistent with the development of true teeth, further reinforced by the expression of tooth-related genes.

Interestingly, fossil records provide evidence that such anatomically peculiar teeth were present in ancient relatives of the ratfish, supporting the findings of Cohen and her team.

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Meghan Rosen is a senior writer who covers life sciences for Science News. With a Ph.D. in biochemistry and molecular biology focusing on biotechnology from the University of California, Davis, she later completed the science communication program at UC Santa Cruz.