The exact origin of the teeth and their primitive function remain unclear. It is believed that the evolutionary precursors of the teeth – structures called odontodes – would not have appeared in the mouth, but on the external breastplate of the first fish, some 500 million years ago.
Even today, sharks, lines or fish fish have microscopic teeth that make their skin as rough as sandpaper.
What were these odontodes for? Several hypotheses are advanced: protection against predation, aid for locomotion, mineral storage or even sensory function.
It was towards this last track that the study conducted by a team from the University of Chicago leans.
Initially, Yara Haridy, post-doctorating in biology of organizations and first author of the study, did not try to trace the origin of teeth. She was interested in another big question of paleontology: what is the oldest vertebrate fossil?
The researcher asked all American paleontology museums to send hundreds of specimens to her – including tiny fragments held on the tip of a toothpick – to analyze them with a very high precision scanner.
Then she set out to find out dentine traces, the inner layer of the teeth transmitting sensory information to the nerves located in the pulp.
A question of survival
“Anatolepis” seemed a good candidate. The exoskeleton of this Cambrian fossil has tubules – microscopic tubes – characteristic of dentin, which led paleontologists to consider it as the first fish.
But by comparing it to the images of other specimens she had scanned, Yara Haridy discovered that these structures were in fact much closer to the sensors of arthropods (crustaceans, insects …). Here is “anatolepis” demoted to the rank of invertebrate!
In modern arthropods, such as crabs, scorpions or spiders, sensors are sensory organs used to perceive temperature, vibrations or odors. And their strong resemblance to those of the fossils suggests that it was already the case 500 million years ago.
The researchers compared these structures with the odontodes lined with more recent fossil fish dentin, dating from the average ordovician (there are 470 to 458 million years). And have found “striking” morphological similarities going in the direction of organs with a sensory vocation.
“”We have carried out experiments on modern fish that have confirmed the presence of nerves in the external teeth of catfish, sharks and lines! This shows that the dental tissues of the odontodes located outside the mouth can be sensitive-and perhaps the very first odontodes were also“Explains Ms. Haridy to AFP.
Arthropods and first vertebrates have evolved “independently” but have developed “similar sensory solutions to respond to the same biological and ecological problem“, She continues.
These primitive animals, swimming with armor, evolved in “an intense predation environment”, details in a press release Neil Shubin, professor of biology of organizations and anatomy at the University of Chicago and the main study of the study. “”Be able to perceive water properties“Around them was” essential “.
Over the course of the evolution, the fish then “developed jaws, and it has become advantageous to have sharp structures around and inside the mouth“Says Ms. Haridy.”Little by little, some jaw fish have had sharp odontodes at the edge of the mouth, then some found themselves directly inside the mouth“.
“”A teeth is actually an ancient sensory characteristic that may have helped our aquatic ancestors to survive!“, She concludes.