Armored fish may have given rise to teeth in animals

The sensitive interior of human teeth might have originated from a seemingly unlikely place: sensory tissue in fish that were swimming in Earth’s oceans 465 million years ago.

While our teeth are covered in hard enamel, it’s dentine — the tooth’s inner layer responsible for carrying sensory information to the nerves — that reacts to the pressure of a hard bite, pain, or changes like extreme cold or sweetness.

When trying to determine the origins of teeth, one of the many possibilities considered by researchers over the years was that teeth may have evolved from bumps on the armored exoskeletons on ancient fish. But the true purpose of the structures, called odontodes, was unclear.

Now, a new study and 3D scans of fossils have yielded evidence that the external bumps contained dentine, which likely helped fish sense their surroundings. Scientists reported the findings Wednesday in the journal Nature.

“Covered in these sensitive tissues, maybe when it bumped against something it could sense that pressure, or maybe it could sense when the water got too cold and it needed to swim elsewhere,” said lead study author Dr. Yara Haridy, postdoctoral researcher in the department of organismal biology and anatomy at the University of Chicago, in an email.

During its analysis, the team also uncovered similarities between the odontodes and features called sensilla, which exist as sensory organs in the shells of modern animals such as crabs and shrimp, and can be found in fossilized invertebrate arthropods. The development of odontodes in fish, which are vertebrates, and sensilla in arthropods, which are invertebrates, is a prime example of evolutionary convergence — when similar features evolve independently in different animal groups, Haridy said.

“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.”

While arthropods have retained their sensilla, odontodes appear to be the direct precursors to teeth in animals.

As the researchers compared sensilla and odontodes, they also arrived at another finding: One species, once considered to be an ancient fish, was actually an arthropod.

The search for the oldest vertebrates

Haridy’s original aim was to solve the mystery of the oldest vertebrate animal that exists in the fossil record. She approached museums across the country and asked whether she could scan any fossil specimens they had from the Cambrian Period, 540 million years to 485 million years ago.

Then, she settled in for an all-nighter at the Argonne National Laboratory, where she used their Advanced Photon Source to capture high-resolution computer tomography, or CT, scans.

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

At first glance, a fossil of a creature called Anatolepis looked like a vertebrate fish – and indeed,  previous research from 1996 had identified it as one. Haridy and her colleagues noticed that there was a series of pores filled with a material that appeared to be dentine.

“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.”

To confirm their discovery, the team compared the scans with those of other ancient fossils, as well as modern crabs, snails, beetles, sharks, barnacles and even miniature suckermouth catfish that Haridy had raised herself.

Those comparisons showed that Anatolepis more closely resembled arthropod fossils, including one from the Milwaukee Public Museum. And what the team thought were tubules lined with dentine were actually more similar to sensilla.

But they did find dentine-containing odontodes in ancient fish like Eriptychius and Astraspis during the scans.

The confusion over Anatolepis’ true nature had stemmed from the fragmentary nature of the fossils. The most complete pieces are only about 3 millimeters (0.1 inches) in size, Haridy said, which proved to be a challenge for comparative research reliant on external imaging.

But the new scans she conducted enabled a 3D look at the fossils, revealing their internal anatomy.

“This shows us that ‘teeth’ can also be sensory even when they’re not in the mouth,” Haridy said. “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 that this was the earliest vertebrate, but it actually was an arthropod.”

The cutting edge modern imaging used in the study is resolving the debate over Anatolepis, said Dr. Richard Dearden, a postdoctoral research fellow at the Naturalis Biodiversity Center in Leiden, the Netherlands. Dearden was not involved in the new research.

“(The study authors) use cutting edge modern imaging approaches to try and settle this question, assembling an impressive swathe of comparative data to convincingly establish that Anatolepis is indeed not a vertebrate,” Dearden said in an email. 

Ashley Strickland