Context: 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.

Question: Which sub class are most viviparous mammals in?
Answer: Theria

Question: Viviparous mammals today are in which two main infraclasses?
Answer: marsupial and placental

Question: What is present in all non-placental mammals?
Answer: epipubic bones

Question: What sub class are most placental eutherians in?
Answer: Unanswerable

Question: What class are placental eutherians in today?
Answer: Unanswerable

Question: Where on the mother are the epipubic bones located?
Answer: Unanswerable

Question: What prevents a short gestation period in full pregnancy?
Answer: Unanswerable

Question: In marsupials what is a plesyomorphic condition shorter than?
Answer: Unanswerable