The three adaptive advantages of the mammalian anatomy are:
First; Mammals develop a well-developed offspring, and also provides proper nutrition to their young.
Second; Their Fur, their fur maintains their body temperature against changes of temperature in the surroundings, it can also be used as protection from the sun. Mammal Fur is easy to be reproduced than feathers since fur requires less body resource to reproduce.
Third; They don't lay eggs. Mammals keep their offspring inside their body, for further development and add proper nutrition, a mammal's offspring can develop further and get stronger to be prepared for the world, once they are born