While the outer layer of our teeth is coated in hard enamel, it’s the inner layer, called dentine, that feels pain.
One idea was that they evolved from small bumps on the tough outer shells of ancient fish.
“We know that vertebrates and arthropods evolved hard parts independently and amazingly they evolved similar sensory mechanisms integrated into their hard skeleton independently.” The findings also help explain a long-standing mix-up in the fossil record.
“We think that the earliest vertebrates, these big, armored fish, had very similar structures,” Haridy said.
Called the “outside-in” hypothesis, the theory suggests that teeth evolved from external sensory structures like these.
Take a look at a prehistoric armored fish to understand why teeth hurt.
Our teeth’s inner layer, known as dentine, is the part that experiences pain, even though the outer layer is covered in hard enamel. When we feel the sting of ice cream or sweetness, or bite into something hard, dentine sends signals to the nerves.
For years, scientists have been debating the origin of teeth. They were thought to have originated from tiny lumps on the hard outer shells of extinct fish. Odontodes, the name for these lumps, were once unknown.
However, a recent study demonstrates that dentine was present in these structures in an early vertebrate fish from the Ordovician period, which occurred roughly 465 million years ago.
Sensations with fishy roots.
Researchers found these bumps were sensitive and probably used to sense their surroundings, such as detecting cold water or pressure from nearby objects, by using 3D scans on fish fossils.
The researchers discovered while examining the fossils that the odontodes of extinct fish resembled sensilla, which are microscopic sensory organs present in the shells of creatures like shrimp and crabs. Additionally, fossils of extinct invertebrates contain sensilla.
Remarkably, these two characteristics evolved in entirely different animal groups: arthropods, which lack backbones, and fish, which do. Dr. Yara Haridy, the study’s lead, claims that this is a classic example of evolutionary convergence, which occurs when distinct species independently acquire similar traits.
“There is a very distant common ancestor between these jawless fish and Aglaspidid arthropods (extinct marine arthropods) that probably had no hard parts at all,” Haridy stated. Vertebrates and arthropods are known to have independently evolved hard parts, and it’s amazing that they also independently evolved comparable sensory mechanisms incorporated into their hard skeletons. “.”.
Additionally, the discoveries clarify a long-standing confusion in the fossil record. For many years, a fossil from the Cambrian period called Anatolepis was believed to be among the first vertebrates because of bumps on its surface that resembled teeth.
But when the scientists used high-resolution CT scans to examine the fossil closely, they discovered that those bumps weren’t actually dentine. Rather, they resembled the arthropods’ sensilla, which are sensory structures.
mistaking crabs for relatives.
Since Anatolepis was an ancient arthropod, it was not a vertebrate fish.
It turns out that the confusion makes sense. Under a microscope, sensory armor, which evolved in both vertebrates and invertebrates, frequently has remarkably similar appearances. This is because both groups employed nerve-connected structures embedded in hard outer coverings, such as crab or fish shells, to develop ways of sensing their surroundings.
The team scanned fossils and contemporary specimens, from sharks and catfish to snails and barnacles, in order to compare these features more widely. One finding in particular caught our attention: suckermouth catfish raised in Haridy’s lab exhibited tiny scales on their skin that resembled teeth, known as denticles, and were directly linked to nerves.
Similar to ancient odontodes and arthropod sensilla, these denticles served as sensory instruments in addition to being armor.
“We believe these large, armored fish, the earliest vertebrates, had very similar structures,” Haridy stated.
They have the same appearance in both ancient and contemporary arthropods because they are all producing the mineralized layer that covers their soft tissues and aids in environmental sensing. “.”.
This study also supports a fundamental evolutionary biology theory. According to a theory known as the “outside-in” hypothesis, teeth originated from external sensory structures such as these. Animals had sensitive armor that allowed them to survive long before their mouths were full of teeth.
Neil Shubin, the study’s senior author, stated that this finding was well worth the effort, even though they were unable to identify the earliest vertebrate fish.
We demonstrated that some of these fossils, which were thought to be early vertebrates, are not. He said, “But that was a little misdirection.”. We didn’t discover the oldest one, but we did discover something much more amazing in some respects. “.”.
