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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 scale-less fish and extinct marine arthropods called Aglaspidids share a very remote common ancestor that probably lacked any hardened body parts,” Haridy explained. “It’s known that both vertebrates and arthropods developed hard structures separately, and remarkably, they also evolved comparable sensing systems embedded within their rigid skeletons independently.
Although arthropods have kept their sensilla, odontodes seem to be the direct ancestors of teeth in creatures.
While comparing sensilla and odontodes, the researchers made another discovery: A species previously thought to be an ancient fish was actually an arthropod.
The search for the oldest vertebrates
Haridy initially aimed to uncover the identity of the earliest known vertebrate animal preserved in fossils. She reached out to various museums nationwide, requesting permission to examine and scan their fossil samples from the Cambrian Period, which spans from 540 million to 485 million years ago.
Next, she prepared for a night-long session at the Argonne National Laboratory, utilizing their Advanced Photon Source to obtain high-resolution computed tomography (CT) scans.
“It was a night at the particle accelerator; that was quite an experience,” Haridy mentioned.
At first glance, a fossil of a creature called Anatolepis looked like a vertebrate fish – and indeed,
previous research from 1996
Haridy and her team observed that it contained a sequence of pores filled with what seemed to be dentine.
We were giving each other high-fives, exclaiming ‘Oh my God, we actually pulled this off,'” Haridy stated. “This would’ve marked the earliest known tooth-like formation within vertebrate tissues from the Cambrian period. Therefore, we were quite thrilled upon spotting those distinctive indicators resembling 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 uncertainty surrounding Anatolepis’ actual identity originated from the incomplete state of the fossils. According to Haridy, the best-preserved sections measure just around 3 millimeters (0.1 inches), posing difficulties for comparative studies that depend on external imagery.
However, the fresh scans she performed allowed for a 3D view of the fossils, exposing their inner structure.
This demonstrates that ‘teeth’ can serve as sensors even outside of the mouth,” Haridy explained. “Therefore, these fish have sensitive armor, and so do these arthropods. This clarifies why there was such confusion about those ancient Cambrian creatures. Many believed them to be the earliest vertebrates, but they were actually arthropods.
The advanced contemporary imaging techniques utilized in this study are settling the argument surrounding Anatolepis, according to Dr. Richard Dearden, a postdoctoral research fellow at the Naturalis Biodiversity Center located in Leiden, the Netherlands. It should be noted that Dearden had no involvement with the recent investigation.
“(The researchers) employ advanced contemporary imaging techniques to address this issue, compiling an extensive range of comparative data that strongly confirms Anatolepis is not actually a vertebrate,” Dearden stated via email.
Armor against the elements
Armored jawless fishes such as Astraspis and Eriptychius shared their habitat with early arthropods like Anatolepis in the murky shallows of the Ordovician seas, which spanned from approximately 485.4 million to 443.8 million years ago.
Other contemporary creatures alongside these animals were massive cephalopods like giant squids, along with enormous sea scorpions. Traits such as odontodes and sensilla aided both fish and arthropods in differentiating between predators and prey.
When considering an ancient creature such as this one, which roamed the seas encased in armor, it had to interact with its surroundings,” explained Dr. Neil Shubin, lead researcher and the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago, in a press release. “This was a highly competitive ecosystem where sensing their aquatic environment accurately could make all the difference.” He added, “Invertebrates protected by shells similar to those seen in horseshoe crabs also needed these sensory capabilities. Interestingly enough, both evolved comparable solutions for addressing this challenge.
Several modern fishes have odontodes, while sharks, skates and some catfishes are covered in small toothlets called denticles, which cause their skin to feel like sandpaper, Haridy said.
Haridy examined the tissues of the catfish she cultivated and discovered that their denticles were linked to nerves similarly to how teeth are in various creatures. She noted that when contrasting teeth, odontodes, and sensilla, they exhibited remarkable similarities.
“We think that the earliest vertebrates, these big, armored fish, had very similar structures, at least morphologically. 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,” Haridy said.
It’s likely that the genes necessary to form odontodes also produced sensitive teeth in animals — including humans — later, according to the study authors.
The findings support the idea that sensory structures appeared first on exoskeletons, which then provided the genetic information that could then be used to create teeth as they became a necessary part of life, the study authors noted.
“Over time, fish evolved jaws, and it became advantageous to have pointy structures around and in the mouth,” Haridy said. “Little by little some fish with jaws had pointy odontodes at the edge of the mouth and then eventually some were directly in the mouth and then lost across the body. The relationship between odontodes and teeth is continuously being clarified by new fossils and modern genetics.”
The new research refines the timeline for the first appearance of hard tissues and the earliest ancestors of jawed fishes by removing Anatolepis from the fish tree of life, said Dr. Lauren Sallan, assistant professor and head of the macroevolution unit at the Okinawa Institute of Science and Technology in Japan. Sallan, who was not involved in the new study, said it also raises an intriguing new hypothesis that the scalelike precursors of teeth evolved to detect prey, friends or predators in the water.
This truly challenges our intuitive belief that hard tissues such as dentine and features like scales and teeth primarily developed for defensive purposes on the body or for feeding in the throat,” explained Sallan. “These could instead have been initially adapted and later altered for those functions, similar to how limbs evolved prior to being utilized for terrestrial locomotion. Additionally, it’s fascinating to observe the extent of similarity between early armoured arthropods and fish, which prompts inquiries into whether significant ecological overlap existed among these two types.
Haridy aims to keep searching for fossils that might reveal older vertebrates, as researchers anticipate finding specimens predating both Astraspis and Eriptychius. Despite not uncovering the desired discovery during their investigation, valuable insights were still gained, according to Shubin’s emailed statement.
“We were disappointed that (Anatolepis) wasn’t a vertebrate but we were amazed by the new ideas that arose,” Shubin said. “And that took us in an entirely new direction. That’s science.”
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