That zing in your molars after a cold drink? You can thank a prehistoric fish wrapped in bony armor that lived over 460 million years ago for that.
A new study published in Nature shows that dentine -- the sensitive tissue beneath your enamel -- first evolved as part of sensory armor in extinct jawless fish. These early vertebrates used mineralized tissue not just for protection but for detecting their environment.
"This shows us that 'teeth' can also be sensory even when they're not in the mouth," Yara Haridy, PhD, a paleontologist at the University of Chicago and lead author of the study, said in a press release. "So, there's sensitive armor in these fish. There's sensitive armor in these arthropods."
Originally, Haridy set out to find the earliest vertebrate in the fossil record. She examined hundreds of tiny specimens from the Cambrian and Ordovician periods, some no bigger than the tip of a toothpick. After hauling them to the Argonne National Laboratory, she used high-resolution CT scanning to look for signs of dentine.
"It was a night at the particle accelerator; that was fun," Haridy said.
One fossil in particular, Anatolepis heintzi, seemed promising. It had what looked like tubules filled with dentine -- telltale signs of vertebrate tissue. "We were high-fiving each other, like 'oh my god, we finally did it,'" said Haridy. "That would have been the very first tooth-like structure in vertebrate tissues from the Cambrian."
But the excitement didn't last. After comparing the fossil with others -- including modern arthropods like crabs and shrimp -- the team found that the tubules closely resembled sensory organs known as sensilla. This discovery confirmed that Anatolepis, once thought to be a vertebrate, was, in fact, an ancient invertebrate arthropod.
"When you think about an early animal like this, swimming around with armor on it, it needs to sense the world," said senior author Neil Shubin, PhD. "This was a pretty intense predatory environment, and being able to sense the properties of the water around them would have been very important."
The study supports the "outside-in" theory of tooth evolution -- that teeth didn't appear deep in the mouth first but began as external sensory armor. "Viewed through this evolutionary lens, the fact that teeth in the mouth are extremely sensitive is less of a mystery," the researchers wrote, "and more a reflection of their evolutionary origins within the sensory armor of early vertebrates."
It turns out your teeth have deep roots in armored fish. They've changed shape and location, but they still remember how to react.