Ever wonder why teeth hurt? Blame it on a prehistoric armored fish.
While the outer layer of our teeth is coated in hard enamel, it’s the inner layer, called dentine, that feels pain. Dentine carries signals to the nerves when we bite into something hard, or feel the sting of ice cream or sweetness.
Scientists have long debated where teeth came from. One idea was that they evolved from small bumps on the tough outer shells of ancient fish. These bumps, known as odontodes, were once a mystery.
But now, a new study confirms that these structures in an early vertebrate fish from the Ordovician period, about 465 million years ago, contained dentine.
Fishy origins of sensation
Using 3D scans on fossils of the fish, researchers discovered that these bumps were sensitive and were likely used to sense their environment, like detecting cold water or pressure from nearby objects.
While studying the fossils, the team also found that odontodes in ancient fish looked a lot like sensilla — tiny sensory organs found in the shells of animals like crabs and shrimp. Sensilla are also seen in fossils of ancient invertebrates.
Surprisingly, these two features evolved in completely different animal groups: fish, which have backbones, and arthropods, which don’t. According to Dr. Yara Haridy, who led the study, this is a classic case of evolutionary convergence — when different species develop similar traits on their own.
“These jawless fish and Aglaspidid arthropods (extinct marine arthropods) have an extremely distant shared common ancestor that likely had no hard parts at all,” Haridy said. “We know that vertebrates and arthropods evolved hard parts independently and amazingly they evolved similar sensory mechanisms integrated into their hard skeleton independently.”
The findings also help explain a long-standing mix-up in the fossil record. For decades, a Cambrian-era fossil called Anatolepis was thought to be one of the earliest vertebrates, thanks to tooth-like bumps on its surface.
But when the researchers closely examined the fossil using high-resolution CT scans, they realized those bumps didn’t contain dentine after all. Instead, they looked just like the sensory structures — sensilla — seen in arthropods.
Mistaking crabs for kin
That meant Anatolepis wasn’t a vertebrate fish, but likely an ancient arthropod.
The confusion, it turns out, is understandable. Sensory armor evolved in both vertebrates and invertebrates, and it often looks strikingly similar under the microscope. That’s because both groups developed ways to sense their environment using nerve-connected structures embedded in hard outer coverings — whether it was fish skin or crab shell.
To compare these features more broadly, the team scanned fossils and modern specimens ranging from snails and barnacles to sharks and catfish. One discovery stood out: suckermouth catfish raised in Haridy’s own lab had small tooth-like scales on their skin — called denticles — that were directly connected to nerves.
These denticles, like the ancient odontodes and arthropod sensilla, weren’t just armor — they were sensory tools.
“We think that the earliest vertebrates, these big, armored fish, had very similar structures,” Haridy said.
“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.”
This research also adds weight to a key theory in evolutionary biology. Called the “outside-in” hypothesis, the theory suggests that teeth evolved from external sensory structures like these. In other words, long before animals had mouths full of teeth, they had sensitive armor that helped them survive.
While they didn’t pin down the earliest vertebrate fish, Neil Shubin, the senior author of the study, said this discovery was more than worth the effort.
“For some of these fossils that were putative early vertebrates, we showed that they’re not. But that was a bit of misdirection,” he said. “We didn’t find the earliest one, but in some ways, we found something way cooler.”
The study has been published in the journal Nature.