The next time you chomp on a popsicle, then flinch from a lightning bolt of pain, you can now have something legitimate to blame: a 465-million-year old fish.
Researchers have found that dentine — a substance in the inner layer of teeth that sends sensory information to nerves inside the pulp — served a similar sensation-gathering purpose on the armed exoskeletons of ancient fish. The main difference is, in human teeth, it acts as an internal detector, while in ancient fish armor, it serves as an external sensation antenna.
Paleontologists have long suspected that teeth evolved from ancient fish exoskeletons, but couldn’t pinpoint a shared functional purpose. Researchers have now confirmed dentin as a common denominator. It exists in both the exoskeletons of early vertebrate fish from the Ordovician period and human teeth, according to a study published in the journal Nature.
Ancient Fish and Human Teeth
The study surmises that those ancient, armored fish used dentin-based detectors to help survey conditions in their watery environments. They also suggest that these devices resemble the sensory organs in shells of contemporary arthropods like crabs and shrimp. If true, such biological sensory devices may have evolved in many animals simultaneously, including both vertebrates and invertebrates.
“When you think about an early animal like this, swimming around with armor on it, it needs to sense the world,” Neil Shubin, biology professor at the University of Chicago and an author of the paper, said in a press release. “This was a pretty intense predatory environment and being able to sense the properties of the water around them would have been very important. 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.”
Read More: Pivotal Ancient Fish Fossils Mark a Key Turning Point in a Slow Evolution
Origins of Teeth
Researchers hadn’t set out to find the origins of teeth or dentin’s common denominator. Instead, Yara Haridy, a research fellow in Shubin’s lab, intended to identify the earliest vertebrate in the fossil record. She requested fossils from museums around the U.S., seeking specimens from the Cambrian period (485 million years to 540 million years ago). She planned to scan them in search of early vertebrate features.
One such sign — at least in later fish — is dentine inside its external bumps. She collected hundreds of specimens, then scanned them with a high energy light source. One image in particular caught her attention.
A scan of a Cambrian fossil from a genus of an extinct jawless fish called Anatolepis revealed some signature vertebrate features — particularly a series of small tube-like structures beneath its armor bumps. Such structures are hallmarks of vertebrate fish. The tubules beneath the bumps contained material that bore the chemical signatures of dentine.
Sensitive Armor in Fish and Arthropods
To confirm that the substance was, indeed, dentin, a group of researchers looked at images of other scanned specimens — ranging from ancient fossils to contemporary crabs. Eventually, they came across a known arthropod fossil from the Milwaukee Public Museum.
Once they compared Anatolepis to that fossil, they realized that what looked like dentine-lined tubules consistent with a vertebrate actually more closely resembled sensory organs on the shells of contemporary crabs. That meant Anatolepis needed re-classification from early vertebrate to ancient invertebrate arthropod.
“This shows us that ‘teeth’ can also be sensory even when they're not in the mouth,” Haridy said in a press release. “So, there's sensitive armor in these fish. There's sensitive armor in these arthropods. This explains the confusion with these early Cambrian animals.”
Read More: The Ancient Fish That Got Cold Feet and Returned to Water
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
