Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. This is the plesyomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy. Even non-placental eutherians probably reproduced this way. COUNT 3
QSN1 Which sub class are most viviparous mammals in? ANS1 Theria QSN2 Viviparous mammals today are in which two main infraclasses? ANS2 marsupial and placental QSN3 What is present in all non-placental mammals? ANS3 epipubic bones