A study of the earliest ancestors of vertebrates has shown that the rudiments of teeth appeared in them as early as 500 million years ago and were located all over the body. As scientists suggest, at first these teeth performed a sensitive function, notifying animals of the approach of danger. And even when the teeth moved to the jaws, they retained their sensitivity, which is now a common cause of toothache. Research published in the journal Nature.
The predecessors of teeth in vertebrates are considered to be odontodes, or dermal teeth, found on the skin of modern cartilaginous fish. They have a tooth-like structure: pulp surrounded by dentin and hard enamel. The oldest representative in which odontodes have been found is the animal Anatolepis heintzi from the Cambrian period.
However, scientists are still debating whether this animal is one of the first representatives of true vertebrates, since only fragments of its body have been found so far. Therefore, an American-Canadian team of scientists led by researchers from Harvard University decided to study this species to clarify its systematic position and clarify its role in the evolution of teeth in vertebrates.
The scientists studied specimens of Anatolepis heintzi, as well as extinct vertebrates and modern species of sharks, rays and catfish that have preserved odontodes. They did this using a synchrotron, which allowed them to see the internal structures of odontodes, such as dentinal tubules, which are also present in vertebrate teeth. As it turned out, these tubules are absent in A. heintzi, and instead their structures on the skin correspond in structure to the sensory organs of extinct arthropods of the order Aglaspidida.
The researchers then compared the odontodes of the vertebrate genus Eriptychius with the sensory organs of arthropods. They found structural similarities between them and suggested that they arose through convergent evolution, when unrelated groups develop similar features to perform the same function. In the case of odontodes and arthropod sensory organs, this function is to detect danger, such as the movement of nearby predators.
The fact that odontodes retain sensitivity and play a protective role in modern animals has been confirmed by the discovery of sensory neurons in these structures in rays, sharks and catfish. So scientists now suggest that during the evolution of bony fish and other vertebrates, odontodes moved to the front of the head, because this is where the animals most often encountered danger. Eventually, they found their place in the jaws and began to play a role in attacking prey, not just in defense.
Until now, scientists have been uncertain about the functions of odontodes in early vertebrates, attributing them to protection from predators that found it harder to swallow such prey, storing minerals, or facilitating movement in water. However, the discovery of their function as a sensory organ may explain why odontodes are not found in most modern bony fish and other vertebrates, as they have probably been replaced by more advanced sensory organs. Ultimately, studying the signaling of odontodes may help to understand the reasons for the hypersensitivity of similar teeth in humans.
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