Answer:
During the Devonian period which occurs 375 million years ago, gills are present in the fossil of tetrapods.
Step-by-step explanation:
Gills present in tetrapods which able the marine animals to breathe inside water but with the passage of time the sac which is present around the gills convert into lungs and these tetrapods are able to live on the land. Examples of tetrapods are frog, lion, birds and salamander etc.