New research has identified an an uncommon feature in a common fish: Forehead teeth, used for mating.

When it comes to teeth, vertebrates have a lot in common. No matter the shape, size, or sharpness, teeth share genetic origins, physical characteristics, and, of course, a home in the jaw.

The new findings call into question one of these core assumptions.

Spotted ratfish, a shark-like species native to the northeastern Pacific Ocean, have rows of teeth on top of their heads, lining a cartilaginous appendage called the tenaculum that loosely resembles Squidward’s nose.

Researchers have long speculated about the origins of teeth—structures so vital to survival and evolution that most of us rarely stop to think about them. However, the debate centers on the evolution of oral teeth, without considering the possibility that teeth could be elsewhere, too.

With the discovery of teeth on the tenaculum, researchers wonder where else they might be growing, and how this could alter conceptions of dental history.

“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” says Karly Cohen, a postdoctoral researcher at the University of Washington’s Friday Harbor Labs.

“The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”

The findings appear in Proceedings of the National Academy of Sciences.

Spotted ratfish are one of the most abundant fish species in Puget Sound. They belong to a category of cartilaginous fish called chimaeras that split from sharks on the evolutionary tree millions of years ago. Measuring about 2 feet long, spotted ratfish are named for the long slender tails that account for half of their length. Only adult males have a tenaculum adorning their foreheads. At rest, it looks like a small white peanut between their eyes. When erect, the tenaculum is hooked and barbed with teeth.

Males flare their tenaculum to intimidate competitors. While mating, they grip females by the pectoral fin to avoid drifting apart.

“Sharks don’t have arms, but they need to mate underwater,” Cohen says. “So, a lot of them have developed grasping structures to connect themselves to a mate during reproduction.”

Spotted ratfish also have pelvic claspers that they use for this purpose.

Many common sharks, rays, and skates are covered in tooth-like structures called denticles. Aside from the denticles on their pelvic claspers, spotted ratfish are “pretty naked,” Cohen says, leading the researchers to wonder: Where did all their denticles go?

Before this study, they had two theories. One suggested that the “teeth” on their tenaculum were denticles, a vestige of the past. The other proposed that they were true teeth, like those present in the oral cavity.

“Ratfish have really weird faces,” Cohen says. “When they are small, they kind of look like an elephant squished into a little yolk sack.”

The cells that form the oral region are spread farther afield, making it plausible that at some point, a clump of tooth-forming cells might have migrated onto the head and stuck.

To test these theories, the researchers caught and analyzed hundreds of fish, using micro-CT scans and tissue samples to document tenaculum development. While sharks can be quite hard to study, spotted ratfish abound in Puget Sound. They frequent the shallows surrounding Friday Harbor Labs, the UW research facility located on San Juan Island. They also compared the modern ratfish to ancestral fossils.

The scans showed that both male and female ratfish begin making a tenaculum early on. In males, it grows from a small cluster of cells into a little white pimple that elongates between the eyes. It attaches to muscles controlling the jaw and finally, erupts through the surface of the skin and sprouts teeth. In females it never materializes—or mineralizes—but evidence of an early structure remains.

The new teeth are rooted in a band of tissue called the dental lamina that is present in the jaw but has never been documented elsewhere.

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

In humans, the dental lamina disintegrates after we grow our adult teeth, but many vertebrates retain the ability to replace their teeth. Sharks, for example, have “a constant conveyor belt” of new teeth, Cohen says. Dermal denticles, including the ones on the spotted ratfish’s pelvic claspers, do not have a dental lamina. Identifying this structure was compelling evidence that the teeth on the tenaculum really are teeth and not leftover denticles. Genetic evidence also backed this conclusion.

“Vertebrate teeth are extremely well united by a genetic toolbox,” Cohen says.

Tissue samples revealed that the genes associated with teeth across vertebrates were expressed in the tenaculum, but not the denticles. In the fossil record, they also observed evidence of teeth on the tenaculum of related species.

A yellowish fish covered in white spots called the spotted ratfish. It has fins on its back and sides and on its forehead a small white bump is visible. This is the tenaculum, which is covered in invisible teeth.

“We have a combination of experimental data with paleontological evidence to show how these fishes coopted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction,” says Michael Coates, a professor and the chair of organismal biology and anatomy at the University of Chicago and a coauthor of the paper.

The modern adult male spotted ratfish can grow seven or eight rows of hooked teeth on its tenaculum. These teeth retract and flex more than the average canine, enabling the fish to latch onto a mate while swimming. The size of the tenaculum also appears to be unrelated to the length of the fish. Its development aligns instead with the pelvic claspers, suggesting that the migrant tissue is now regulated by other networks.

“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,” says Gareth Fraser, a professor of biology at the University of Florida and the study’s senior author.

Sharks often serve as the model for studying teeth and development because they have so many oral teeth and are covered in denticles. But, Cohen adds, sharks possess just a sliver of the dental diversity captured by history.

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

Funding for this research came from the National Science Foundation, the Save Our Seas Foundation, and internal endowments at Friday Harbor Labs supporting innovative early-career research.

Source: University of Washington

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Super ancient fish skull holds oldest backboned animal brain fossil

Sawfish fossils hold clues to origin of teeth

A 319-million-year-old fossilized fish skull holds the oldest example of a well-preserved vertebrate brain.

Scientists pulled the skull from a coal mine in England more than a century ago. The brain and its cranial nerves are roughly an inch long and belong to an extinct bluegill-size fish. The discovery opens a window into the neural anatomy and early evolution of the major group of fishes alive today, the ray-finned fishes, according to the study in Nature.

The serendipitous find also provides insights into the preservation of soft parts in fossils of backboned animals. Most of the animal fossils in museum collections were formed from hard body parts such as bones, teeth, and shells.

The CT-scanned brain analyzed for the new study belongs to Coccocephalus wildi, an early ray-finned fish that swam in an estuary and likely dined on small crustaceans, aquatic insects, and cephalopods, a group that today includes squid, octopuses, and cuttlefish. Ray-finned fishes have backbones and fins supported by bony rods called rays.

When the fish died, the soft tissues of its brain and cranial nerves were replaced during the fossilization process with a dense mineral that preserved, in exquisite detail, their three-dimensional structure.

“An important conclusion is that these kinds of soft parts can be preserved, and they may be preserved in fossils that we’ve had for a long time—this is a fossil that’s been known for over 100 years,” says senior author Matt Friedman, a paleontologist and director of the Museum of Paleontology at the University of Michigan.

Is this really a brain?

“Not only does this superficially unimpressive and small fossil show us the oldest example of a fossilized vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking,” says lead author Rodrigo Figueroa, a doctoral student who did the work as part of his dissertation, under Friedman, in the earth and environmental sciences department.

“With the widespread availability of modern imaging techniques, I would not be surprised if we find that fossil brains and other soft parts are much more common than we previously thought. From now on, our research group and others will look at fossil fish heads with a new and different perspective.”

The skull fossil from England is the only known specimen of its species, so only nondestructive techniques could be used during the study.

The work on Coccocephalus is part of a broader effort by Friedman, Figueroa, and colleagues that uses computed tomography (CT) scanning to peer inside the skulls of early ray-finned fishes. The goal of the larger study is to obtain internal anatomical details that provide insights about evolutionary relationships.

In the case of C. wildi, Friedman wasn’t looking for a brain when he fired up his micro-CT scanner and examined the skull fossil.

“I scanned it, then I loaded the data into the software we use to visualize these scans and noticed that there was an unusual, distinct object inside the skull,” he says.

The unidentified blob was brighter on the CT image—and therefore likely denser—than the bones of the skull or the surrounding rock.

“It is common to see amorphous mineral growths in fossils, but this object had a clearly defined structure,” Friedman says.

The mystery object displayed several features found in vertebrate brains: It was bilaterally symmetrical, it contained hollow spaces similar in appearance to ventricles, and it had multiple filaments extending toward openings in the braincase, similar in appearance to cranial nerves, which travel through such canals in living species.

“It had all these features, and I said to myself, ‘Is this really a brain that I’m looking at?'” Friedman says. “So I zoomed in on that region of the skull to make a second, higher-resolution scan, and it was very clear that that’s exactly what it had to be. And it was only because this was such an unambiguous example that we decided to take it further.”

Fish evolution

Though preserved brain tissue has rarely been found in vertebrate fossils, scientists have had better success with invertebrates. For example, the intact brain of a 310-million-year-old horseshoe crab was reported in 2021, and scans of amber-encased insects have revealed brains and other organs. There is even evidence of brains and other parts of the nervous system recorded in flattened specimens more than 500 million years old.

The preserved brain of a 300-million-year-old shark relative was reported in 2009. But sharks, rays, and skates are cartilaginous fishes, which today hold relatively few species compared to the ray-finned fish lineage containing Coccocephalus.

Early ray-finned fishes like Coccocephalus can tell scientists about the initial evolutionary phases of today’s most diverse fish group, which includes everything from trout to tuna, seahorses to flounder.

There are roughly 30,000 ray-finned fish species, and they account for about half of all backboned animal species. The other half is split between land vertebrates—birds, mammals, reptiles, and amphibians—and less diverse fish groups like jawless fishes and cartilaginous fishes.

The Coccocephalus skull fossil is on loan to Friedman from England’s Manchester Museum. It was recovered from the roof of the Mountain Fourfoot coal mine in Lancashire and was first scientifically described in 1925. The fossil was found in a layer of soapstone adjacent to a coal seam in the mine.

Though only its skull was recovered, scientists believe that C. wildi would have been 6 to 8 inches long. Judging from its jaw shape and its teeth, it was probably a carnivore, Figueroa says.

When the fish died, scientists suspect it was quickly buried in sediments with little oxygen present. Such environments can slow the decomposition of soft body parts.

In addition, a chemical micro-environment inside the skull’s braincase may have helped to preserve the delicate brain tissues and to replace them with a dense mineral, possibly pyrite, Figueroa says.

Evidence supporting this idea comes from the cranial nerves, which send electrical signals between the brain and the sensory organs. In the Coccocephalus fossil, the cranial nerves are intact inside the braincase but disappear as they exit the skull.

“There seems to be, inside this tightly enclosed void in the skull, a little micro-environment that is conducive to the replacement of those soft parts with some kind of mineral phase, capturing the shape of tissues that would otherwise simply decay away,” Friedman says.

Skull scans

Detailed analysis of the fossil, along with comparisons to the brains of modern-fish specimens from the University of Michigan Museum of Zoology collection, revealed that the brain of Coccocephalus has a raisin-size central body with three main regions that roughly correspond to the forebrain, midbrain, and hindbrain in living fishes.

Cranial nerves project from both sides of the central body. Viewed as a single unit, the central body and the cranial nerves resemble a tiny crustacean, such as a lobster or a crab, with projecting arms, legs and claws.

Notably, the brain structure of Coccocephalus indicates a more complicated pattern of fish-brain evolution than is suggested by living species alone, according to the authors.

“These features give the fossil real value in understanding patterns of brain evolution, rather than simply being a curiosity of unexpected preservation,” Figueroa says.

For example, all living ray-finned fishes have an everted brain, meaning that the brains of embryonic fish develop by folding tissues from the inside of the embryo outward, like a sock turned inside out.

All other vertebrates have evaginated brains, meaning that neural tissue in developing brains folds inward.

“Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward,” Friedman says. “So, this fossil is capturing a time before that signature feature of ray-finned fish brains evolved. This provides us with some constraints on when this trait evolved—something that we did not have a good handle on before the new data on Coccocephalus.”

Comparisons to living fishes showed that the brain of Coccocephalus is most similar to the brains of sturgeons and paddlefish, which are often called “primitive” fishes because they diverged from all other living ray-finned fishes more than 300 million years ago.

Friedman and Figueroa are continuing to CT scan the skulls of ray-finned fish fossils, including several specimens that Figueroa brought to Ann Arbor on loan from institutions in his home country, Brazil. Figueroa says his doctoral dissertation was delayed by the COVID-19 pandemic but is expected to be completed in summer 2024.

Friedman and Figueroa says the discovery highlights the importance of preserving specimens in paleontology and zoology museums.

“Here we’ve found remarkable preservation in a fossil examined several times before by multiple people over the past century,” Friedman says. “But because we have these new tools for looking inside of fossils, it reveals another layer of information to us.

“That’s why holding onto the physical specimens is so important. Because who knows, in 100 years, what people might be able to do with the fossils in our collections now.”

The study includes data produced at University of Michigan’s Computed Tomography in Earth and Environmental Science facility, which is supported by the Department of Earth and Environmental Sciences and the College of Literature, Science, and the Arts.

Sam Giles of London’s Natural History Museum and the University of Birmingham is a senior author of the study. Additional coauthors are from the University of Chicago and the University of Michigan Museum of Paleontology.

Source: University of Michigan

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