https://www.nature.com/articles/d41586-025-01139-3
https://www.nature.com/articles/s41586-025-08944-w
The earliest vertebrates were jaw-less fishes, but had an heavily bony exoskeleton. This exoskeleton also contained tooth-like structures with dentine called odontodes which cartilaginous fishes (e.g. sharks) still have in their skin. Indeed, it’s long been argued that these odontodes are the precursors to teeth. Cartilaginous fishes lost the bony exoskeleton, but bony fishes (us included) retained some of the external bony armor as ‘dermal bones’, which includes most of the bones in your skull and clavicles. Yes, you have an exoskeleton. But in the first fishes, their exoskeleton was very extensive to protect themselves since they were small and jawless. Armoring up was the only option to protect themselves from the massive invertebrates like Eurypterid (sea scorpions) and giant relatives of nautilus with straight-shells. This later when vertebrates evolved Jaws, but why would these fishes have teeth in their armor?
The new paper published in Nature argues that the odontodes were sensory organs, analogous to the ones observed in the exoskeletons of crustaceans. Hence why the odontodes (unlike other types of dermal armor) are innervated by nerves.