Placentalia
Placentalia ("Placentals") is one of the three subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The placentals are primarily distinguished from other mammals in that the fetus is carried in the uterus of its mother where it is nourished via a placenta, until the live birth of a fully developed offspring occurs.
Anatomical features
Placental mammals are anatomically distinguished from other mammals by:
a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large offspring relative to the size of the mother.
the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. (Their function in non-placental mammals is to stiffen the body during locomotion, but in placentals they would inhibit the expansion of the abdomen during pregnancy.)
the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint.
