Final answer:
Placental mammals are classified according to their adaptations, which are traits that enhance their survival in specific environments. The placenta plays a crucial role in their reproductive strategy by sustaining the fetus during development, contributing to their evolutionary success. Classifications of these mammals have evolved from structural to molecular-based phylogenetic systems.
Step-by-step explanation:
Placental mammals are classified according to the adaptations that help the animals live in their environment. Within the world of Therian mammals, which includes both placental mammals and marsupials, placental mammals are distinct in that they give birth to relatively large and mature fetuses. This is facilitated by the placenta, a vital organ that nourishes the fetus and acts as a barrier between the mother's immune system and the developing offspring. The placenta enables the fetus to have a prolonged period of growth and development in utero, giving placental mammals significant evolutionary advantages due to their offspring's maturity at birth.
The success of placental mammals is attributed to factors such as the ability to protect and nourish the fetus through the maternal placenta and giving birth to mature infants, which increases the chances of survival. Moreover, the traditional classification of placental mammals based on structural and functional similarities has been supplemented by molecular analyses and DNA studies, which suggest more accurate phylogenetic relationships. Such advances help scientists better understand mammalian evolution and adaptability.