500-million-year-old fish armor may be the reason your teeth hurt

Anyone who’s ever flinched during a dental cleaning knows how sensitive our teeth can be. That pain is actually useful – it helps us to detect pressure, temperature, and other sensations as we chew. But this sensitivity didn’t start in our mouths.

It turns out that the inner layer of teeth, known as dentine, originally evolved for an entirely different purpose, that of helping ancient fish sense the world around them.

Teeth started out as armor

Scientists from the University of Chicago have found that dentine first appeared in the armored skeletons of ancient fish. Their study looked at fossils from around 465 million years ago, during the Ordovician period.

Back then, this tissue wasn’t used for chewing. Instead, it helped fish detect movement and changes in the water – an essential survival skill in a predator-filled environment.

The researchers confirmed that early vertebrate fish had dentine-lined structures in their external armor. These were not teeth but sensory tools. They helped the fish react to their surroundings – similar to how modern animals use skin or antennae.

First vertebrate teeth

Some fossil teeth from even earlier – dating to the Cambrian period, between 485 and 540 million years ago – were also examined. These were thought to be the earliest examples of vertebrate teeth.

But when the scientists compared them to the shells and armor of invertebrates like crabs and shrimp, they saw striking similarities. It now appears that both invertebrates and vertebrates developed sensory armor independently.

“When you think about an early animal like this, swimming around with armor on it, it needs to sense the world. This was a pretty intense predatory environment and being able to sense the properties of the water around them would have been very important,” said Neil Shubin, senior author of the new study.

“So, here we see that invertebrates with armor, like horseshoe crabs, need to sense the world too, and it just so happens they hit on the same solution.”

Searching for the first vertebrate

Yara Haridy, a postdoctoral researcher at the University of Chicago, began the project hoping to find the oldest known vertebrate.

She asked museums across the country for Cambrian-era fossils to scan for vertebrate features. One feature she looked for was dentine inside small armor bumps known as odontodes.

Many of the samples were tiny – small enough to sit on a toothpick. To study them in detail, she brought them to Argonne National Laboratory.

There, the team spent an entire night scanning the fossils using a high-powered synchrotron at the Advanced Photon Source.

“It was a night at the particle accelerator; that was fun,” Haridy said.

One fossil, called Anatolepis, showed promise. The scans revealed tubules beneath its armor, filled with dentine-like material.

At first glance, it looked like a vertebrate. If confirmed, this would have pushed the record for the earliest vertebrate back by tens of millions of years.

“We were high fiving each other, like ‘oh my God, we finally did it,’” Haridy said. “That would have been the very first tooth-like structure in vertebrate tissues from the Cambrian. So, we were pretty excited when we saw the telltale signs of what looked like dentine.”

Anatolepis wasn’t a vertebrate after all

To be sure, the team started comparing Anatolepis to other ancient and modern animals. This included everything from sharks and skates to barnacles, snails, and even catfish that Haridy had raised in her own aquarium.

The comparisons led to a surprising discovery. The structures in Anatolepis looked almost identical to the sensilla found in the shells of modern arthropods – tiny sensory organs that help crabs and shrimp feel their environment. It turns out that Anatolepis wasn’t a vertebrate after all. It was most probably an arthropod.

“This shows us that ‘teeth’ can also be sensory even when they’re not in the mouth. So, there’s sensitive armor in these fish. There’s sensitive armor in these arthropods,” Haridy enthused.

“This explains the confusion with these early Cambrian animals. People thought that this was the earliest vertebrate, but it actually was an arthropod.”

Another fossil, this time a vertebrate called Eriptychius, did have dentine in similar structures, confirming that sensory armor was also present in true vertebrates.

Armor, defense, and teeth origins

The study also offers a fresh perspective on how teeth evolved, likely from a type of defensive armor used by the first vertebrates. Sharks, skates, and catfish today have tooth-like skin structures called denticles.

When Haridy examined her catfish, she found that these denticles were connected to nerves – just like real teeth.

“We think that the earliest vertebrates, these big, armored fish, had very similar structures, at least morphologically. They look the same in ancient and modern arthropods, because they’re all making this mineralized layer that caps their soft tissue and helps them sense the environment,” she explained.

There are two main theories about the origin of teeth. One says teeth evolved inside the mouth and later appeared on body armor.

The other suggests they started as external sensory structures and were adapted into teeth later. This study supports the second idea.

While they didn’t find the earliest vertebrate fish, the researchers still uncovered something important.

“For some of these fossils that were putative early vertebrates, we showed that they’re not. But that was a bit of misdirection,” Shubin said. “We didn’t find the earliest one, but in some ways, we found something way cooler.”

The full study was published in the journal Nature.

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