The oldest known evidence of early tooth-like structures and dentine, a key tissue in vertebrates, comes from fragmentary Cambrian fossils called Anatolepis heintzi. These fossils have tiny dentine tubules, leading scientists to consider them the first precursors of teeth, known as odontodes. However, debates persist on whether Anatolepis was a vertebrate, as imaging challenges and the absence of similar exoskeletal tissues make classification difficult.
New research from the University of Chicago reveals that dentine first appeared as a sensory tissue in the armored exoskeletons of ancient fish. Scientists previously suspected teeth evolved from these bumpy structures but weren’t sure of their function. The new study confirms that early vertebrate fish from the Ordovician period (about 465 million years ago) had dentine, likely helping them sense their surroundings in the water.
Additionally, features resembling teeth in Cambrian fossils (485–540 million years ago) were found to be similar to sensory organs in the armor of fossil invertebrates and modern arthropods like crabs and shrimp. This suggests that armored animals—both vertebrates and invertebrates—evolved separate sensory adaptations to navigate their environments.
Neil Shubin, senior author of the study, explained: “Early armored animals needed to sense their surroundings in a world full of predators. It turns out invertebrates like horseshoe crabs developed a similar sensory solution.”
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Paleontologist Yara Haridy searched for the earliest vertebrates in the fossil record, scanning Cambrian fossils (485–540 million years old) for signs of vertebrate traits. One key feature in later fish is dentine, a tissue found in small bumps on their external armor called odontodes.
Haridy collected hundreds of fossil specimens, some barely the size of a toothpick, and took them to Argonne National Laboratory for high-resolution CT scanning. During the process, one sample—a Cambrian fossil called Anatolepis—appeared to have dentine-lined tubules, suggesting it might be a vertebrate. If confirmed, this would push the vertebrate fossil record back by tens of millions of years.
Haridy and her team were thrilled when CT scans of Anatolepis revealed dentine, potentially making it the earliest known vertebrate with tooth-like structures. However, they needed to verify their findings.
They analyzed hundreds of fossils, comparing Anatolepis to a wide range of ancient and modern species, including arthropods like crabs and barnacles. Ultimately, they discovered that what looked like dentine-lined tubules were sensory structures similar to those found on crab shells, called sensilla. This meant Anatolepis wasn’t a vertebrate after all—it was an ancient arthropod.
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Meanwhile, another Ordovician vertebrate, Eriptychius, did contain true dentine, reaffirming that sensory adaptations in armored animals evolved separately in vertebrates and invertebrates. This finding helped clarify long-standing confusion in paleontology about early Cambrian fossils.
Haridy said, “This shows us that ‘teeth’ can also be sensory even when not in the mouth. So, there’s sensitive armor in these fish. There’s sensitive armor in these arthropods. This explains the confusion with these early Cambrian animals. People thought this was the earliest vertebrate, but it was an arthropod.”
Some modern fish, like sharks and catfish, have tooth-like denticles on their skin, making it rough to the touch. When Haridy examined her catfish, she found that these denticles were connected to nerves, similar to actual teeth. The resemblance between denticles, ancient armored fish structures (odontodes), and arthropod sensory organs (sensilla) was striking.
She explained that early vertebrates likely had similar sensory adaptations, forming mineralized layers that helped them detect their environment—just as modern arthropods do.
Scientists have debated how these structures evolved into teeth. One theory, the inside-out hypothesis, suggests teeth developed first and were later incorporated into exoskeletons.
However, Haridy’s findings support the outside-in hypothesis, which proposes that sensory structures emerged on exoskeletons first and were eventually repurposed for teeth using a shared genetic toolkit. This research helps clarify how vertebrates and invertebrates independently developed similar sensory mechanisms.
Although the team didn’t identify the earliest vertebrate fish, Shubin sees the discovery as a significant success.
“For some of these fossils that were thought to be early vertebrates, we proved they weren’t. But that was just a bit of misdirection,” Shubin explained. “We didn’t find the first vertebrate, but in a way, we uncovered something even more exciting.
Journal Reference:
- Haridy, Y., Norris, S.C.P., Fabbri, M. et al. The origin of vertebrate teeth and evolution of sensory exoskeletons. Nature (2025). DOI: 10.1038/s41586-025-08944-w
