A Head Full of Teeth

Teeth might come in different shapes and sizes, but across vertebrates, they share common features: genetic origins, physical characteristics, and location in the jaw.

Now, the latter feature is being called into question after scientists observed a row of teeth outside the mouth of a male spotted ratfish (Hydrolagus colliei).

“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” Karly Cohen, author of a new study, said in a statement. “The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”

Adult male spotted ratfish display rows of teeth on a cartilaginous appendage called the tenaculum, which protrudes from their forehead like an antenna when it is erect and resembles a small white peanut between their eyes when at rest. The modern adult male spotted ratfish is capable of growing seven or eight rows of hooked teeth on its tenaculum.

Researchers said males flare their tenaculum to intimidate competitors and use it to hold females by the pectoral fin to avoid drifting apart while mating.

Spotted ratfish, shark-like fish originating from the northeastern Pacific Ocean, are a type of cartilaginous fish known as chimeras, which diverged from sharks millions of years ago.

All male chimeras have this peculiar facial tenaculum, but researchers studied its development in spotted ratfish from nearby Puget Sound – an inlet in Washington State – using micro-computed tomography (CT) scans and tissue samples to analyze tenaculum development. They also compared modern ratfish to their ancestral fossils, ConnectSci News explained.

The team noted that evidence of an early similar structure is also present in females, though it never actually develops as in males, where it grows from a small cluster of cells into a little white bump that elongates between the eyes. It connects to muscles that control the jaw and then breaks through the surface of the skin and sprouts teeth.

The new teeth are attached to a band of tissue called the dental lamina, which plays a role in tooth development in the jaw.

“When we saw the dental lamina for the first time, our eyes popped,” Cohen said. “It was so exciting to see this crucial structure outside the jaw.”

Scientists have long pondered the origin of teeth, vital structures to survival and evolution, but they have mostly focused on oral teeth, without really thinking that they could be found elsewhere, too. Discovering teeth on the tenaculum prompted researchers to wonder where else teeth might be growing and what this means for the understanding of dental history.

“Chimeras offer a rare glimpse into the past,” Cohen said. “I think the more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw.”