The Rise of Mammals A. Mammals originated in the Carboniferous when therapsid reptiles diverged from lineages that led ultimately to modern reptiles and birds. Brain capacity is increased, allowing more capacity for memory, learning, and conscious thought. Milk-secreting glands nourish the young. Mammals show behavioral flexibility, the ability to expand on the basics with novel forms of behavior. The placenta is the organ of exchange of nutrients and wastes between the maternal blood and the fetal blood.
Placental nourishment is more efficient than nourishment of the pouched animals.
3 Representatives of this group are found in virtually every aquatic and terrestrial environment. D. Pouched mammals, such as the opossum of North America, give birth to tiny, blind, hairless young that find their way to the mother's pouch where they are suckled and finish their development. E. Egg-laying mammals are represented today only by the platypus and spiny anteater in Australia. They are practically toothless. They lay eggs but suckle their young.