This fish has teeth on the forehead (not to eat ice cream)

When it comes to teeth, the vertebrates share a lot with each other. Whatever the shape, size or sharp, the teeth have similar genetic origins, close physical characteristics and, of course, a usual place: the jaw. Almost always.

A new study, conducted by scientists at the University of Washington and the University of Florida, in the United States, now come to question the assumption that the teeth only exist in the mouth.

According to the, published on Thursday in Proceedings of the National Academy of Sciencesthe “ghost sharks” (Hydrolagus colliei) have teeth lines at the top of the head, aligned in a cartilaginous excrescence called Tenaclewhich vaguely resembles a nose.

The mysterious species, native to the Northeast Pacific and the family of Chimerridnear the sharks, apparently uses its extra teeth to mate.

Researchers have long speculated on the source of teethstructures so vital to survival and evolution that we rarely stop to think about them.

The debate, however, always focused on the teeth present in the jaws, without considering the possibility of there are in other parts of the body.

With the discovery of teeth in the tenacle, scientists now wonder where they can arise more and how it can alter the history of teeth evolution.

“This absolutely spectacular feature challenges a longtime assumption in evolutionary biology: that teeth are strictly oral structures,” he says Karly Cohenresearcher at the University of Washington and first author of the study at the University.

“The tenacle is a trace of development, not an aberrationand the first clear example of a toothed structure outside the jaw. ”

The “ghost sharks”, as in 2023 the researcher of the University of Florida and the main author of the study, Gareth Fraserbelong to the category of chimeras, cartilage fish that diverged from the sharks in the evolutionary tree for millions of years.

Measure about 60 centimeters And they owe their name in English, Spotted Ratfish (“stained”) to the long, thin tail that corresponds to half of its length.

Only adult males have a tenacle on the forehead. At rest, it seems A small white peanut. When erect, the tenacle becomes curved and pointed, coated with teeth.

The males exhibit the tenacle to intimidate rivals and during mating, use it to hold the female by the pectoral fin and avoid moving away.

“Sharks have no armsBut they need to mate under water, ”Cohen explained.“ So many have developed grip structures that allow them to keep their partner contact during playback. ”

“Os Hydrolagus colliei They have really strange faces, ”commented Karly Cohen.“ When they are small, They look like a spread elephant inside a yolk bag. ”

The cells that form the oral region are more dispersed, which makes it plausible that, at some point, a group of cells responsible for dental formation migrated to the head and remained there.

To test these hypotheses, the researchers captured and analyzed hundreds of copies, using microtomographies and fabric samples to document the development of the tenacle.

While sharks are difficult to study, the H.colliei abound in Puget Sound and They frequent the waters in no deep Next to Friday Harbor Labs, on the island of San Juan.

Scientists also compared modern specimens with ancestral fossils. The analyzes revealed that Both males and females begin the formation of the tenacle from an early age.

In males, the process begins with a small cell cluster that gives rise to white protuberance between the eyes. It binds to jaw muscles, breaks the skin and, Finally, it generates teeth. In females, this structure Never get to mineralizebut remain initial traces.

The new teeth are rooted in a fabric range called dental bladepresent in the jaw but never before documented in other regions. “When we saw the dental blade for the first time, We gaped”Says Cohen.” It was exciting to find this crucial structure outside the jaw. “

In humans, the dental blade disappears after the growth of definitive dentition, but many vertebrates maintain the ability to teeth.

Sharks, for example, have “an authentic crashing hall”From new teeth, cohen explained. The dermal demists, including those present in the Spotted Ratfish classpers, have no dental blade.

The identification of this structure was a solid proof that the teeth of the Tenacle They are really teeth, not residual denticles. Genetic evidence reinforced the conclusion.

Tissue samples showed that the genes associated with teeth in vertebrates were expressed in the tenacle, but not in the denticles. In fossils, investigators also found evidence of teeth in the tenacle of appeared species.

“We have here a combination of experimental data and paleontological evidence that demonstrates how these fish reused pre-existing genetic instructions for teeth to create a new device essential to reproduction,” he said Michael CoatesDirector of the Department of Biology and Anatomy of the University of Chicago and co -author of the study.

“If these strange chimeras have teeth on the foreheadThis requires us to rethink the dynamism of dental development in general, ”says Gareth Fraser, professor of biology at the University of Florida and senior author of the study.

“The chimeras offer us a rare glimpse of the past. The more we look at spinous structures in vertebrates, the more teeth we will find outside the jaw, ”concludes Karly Cohen.