Explanation:
Triassic-Mesozoic :- First mammal occur in late Triassic period of Mesozoic era about 200 million year ago.
Origin of mammals takes place in Triassic period. It means Dinosaurs and mammals originated in the same
period. Long before the arrival of true mammals, one group of extinct reptiles, the synapsids acquired several mammalian characteristics. They lived through out the Permian and Triassic periods. The more mammal like synapsids belong to the order Therapsida.