Answer:
lungs
Step-by-step explanation:
Bony fishes and tadpoles have gills that they use to breathe. While amphibians and other groups such as reptiles, birds, and mammals use their lungs to breathe.
In the attached file you will find the cladogram showing the label "lung" between the bony fish and the amphibians. The position of this label means that the lungs are the structure that separates both groups and that every group that evolved after the amphibians possess lungs.
By evolution, we know that amphibians needed to develop these structures to survive on land. Tadpoles however still use gills to breathe and this is why they spend their first stages of life underwater.