Final answer:
Placental mammals have the distinctive feature of a placenta that sustains the fetus during pregnancy, resulting in the birth of large and mature infants. They have evolved efficient reproductive strategies that contributed to their success and dominance among mammals, excluding regions like Australia where marsupials prevail.
Step-by-step explanation:
Characteristics of Placental Mammals
Placental mammals are part of the therian mammals and possess specific traits that distinguish them from other mammals, such as marsupials and monotremes. The hallmark of placental mammals is the presence of a placenta, which sustains the fetus during gestation within the mother's uterus. This sophisticated organ allows for the exchange of oxygen, nutrients, and waste between the mother and the fetus. As a result, placental mammals, which include species such as humans, rabbits, and whales, give birth to relatively large and mature infants after a prolonged period of development inside the womb.
The success of placental mammals in comparison to marsupials and monotremes is largely due to their highly efficient reproductive strategy. They can nurture their young through an extended gestation period, leading to a better chance of survival after birth. Moreover, placentation helps in protecting the fetus from the mother's immune system, allowing for a harmonious prenatal development.
Placental mammals evolved from ancestors such as Eomaia and became the dominant group in most regions, except places like Australia where marsupials thrive. The diversity and adaptability of placental mammals have helped them outnumber other mammalian groups.