The unique mammalian characteristics are
 

Hair, ear pinna and mammary glands are the unique features of mammals, not found in any other animals.
Hair helps regulate body temperature by providing insulation against cold weather. In cold conditions, hairs stand up, creating a layer of trapped air that acts as an insulator. Hair can also protect the skin from UV rays of the sun. It can also provide sensory feedback, for the detection of touch and vibrations.
The pinna collects sound waves and directs them into the external auditory meatus, which then funnels them
toward the tympanic membrane. Mammary glands of female mammals produce milk to nourish their young. Breast milk also contains antibodies that protect the newborn from infections.
(1) The skull of mammals is dicondylic, with two occipital condyles for articulation with the first vertebra
(atlas). Monocondylic skull, with a single occipital condyle, is found in reptiles and birds.
(2) Tympanic membrane is also present in amphibians, reptiles and birds.
(4) Development in mammals is direct.