Amphitherium is known from a lower jaw, which is often all we know of certain mammalian taxa. It is characterized by the structure of its teeth.
The molars of the earliest mammals had three cusps all in a row. In pantotheres, like Amphitherium, the cusps were arrange as a triangle.
Here the dental formula of 4 incisors, 1 canine, 5 premolars and 6 molars is in place.
Soft tissue evolution is assumed from the examples of living mammals. Amphitherium probably resembled a living oppossum.
It is likely that external ear flaps, what we call "ears" appeared at this point.
Vibrissae (whiskers) likely appeared at this point because egg-laying mammals don't have them and other mammals do.
The lower legs and tail were probably the only parts not covered in dense hair.
In pantotheres the underdeveloped embryos are born and climb without assistance to their mother's nipples. They don't let go for many months.
The cloaca divides into a anus and a birth and urine canal. In males the penis remains within the body until extended for mating. The scrotum containing the testicles hangs below the belly anterior to the penis.