They are considered one of the most important synapomorphies of tetrapodomorphs, that allowed the passage from water to land.

[2] In tetrapods without secondary palates their function relates primarily to olfaction (sense of smell).

It is therefore not a structure but a space bounded as follows: The term is a latinization from the Greek χοάνη, "choanē" meaning funnel.

In the first tetrapodomorphs (~415 mya) the excurrent nostrils migrated to the edge of the mouth, occupying a position between the maxillary and premaxillary bones, directly below the lateral rostral (a bone that vanished in early tetrapods).

In animals with partial secondary palates (most birds and reptiles), the median choanal slit separates the two halves of the posterior half of the palate, connecting the nasal cavity with the buccal cavity (mouth) and the pharynx (throat).