Researchers studied modern fish with dermal teeth and determined they are innervated, meaning they deliver sensory signals through nerves like our own teeth would. The study supports the “outside-in” theory which suggests sensitive “teeth” developed on exoskeletons before they appeared in mouths. According to a new study published in , teeth may have originated as sensory organs that helped ancient fish navigate murky waters. To better understand whether these external teeth were innervated (connected to nerves), Haridy studied some modern fish with denticles. The team’s findings support what is known as the “outside-in” hypothesis, or the theory that sensitive structures developed on exoskeletons first and then sensitive teeth followed.