Researchers have found genuine teeth developing on the forehead of the spotted ratfish, challenging the idea that vertebrate teeth evolve only in jaws. The discovery, detailed in a new study, reveals that these head teeth share genetic origins with oral teeth and aid males in mating. This finding reshapes understanding of dental evolution in cartilaginous fish.
The spotted ratfish, a shark relative common in the northeastern Pacific Ocean and Puget Sound, possesses a unique structure called the tenaculum on the forehead of adult males. This cartilage-based appendage, which resembles a hooked organ when raised, is lined with rows of teeth that help males grip females by the pectoral fin during underwater mating. Spotted ratfish, part of the chimaera group that diverged from sharks millions of years ago, grow to about 2 feet long, with tails comprising half their length.
For years, biologists assumed teeth in vertebrates were confined to jaws, essential for feeding. However, studying the tenaculum has overturned this view. "This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures," said Karly Cohen, a University of Washington postdoctoral researcher at Friday Harbor Labs. The team analyzed hundreds of ratfish using micro-CT scans and tissue samples, comparing them to fossils of related species.
Development begins early: both sexes form a tenaculum precursor, but in males, it elongates, erupts through the skin, and sprouts seven or eight rows of hooked teeth rooted in dental lamina—a band of tissue typically found only in jaws. Genetic analysis confirmed these are true teeth, expressing the same genes as oral teeth across vertebrates, unlike tooth-like denticles on the body. "When we saw the dental lamina for the first time, our eyes popped," Cohen noted.
The tenaculum also serves for display, warding off rivals, and aligns developmentally with pelvic claspers used in mating. "We have a combination of experimental data with paleontological evidence to show how these fishes coopted a preexisting program for manufacturing teeth," said co-author Michael Coates of the University of Chicago. This research, funded by the National Science Foundation and others, suggests teeth may have evolved more dynamically outside jaws, offering insights into vertebrate history.
The study appears in Proceedings of the National Academy of Sciences (2025; 122(37), DOI: 10.1073/pnas.2508054122).