The Rise of Mammals Distinctly Mammalian Traits Brain capacity is increased, allowing more capacity for memory, learning, and conscious thought. Milk-secreting glands nourish the young. Dentition is extensive and specialized to meet dietary habits. During the Triassic, divergence from the small, hairless reptiles called synapsids gave rise to the therapsids. By Jurassic times, mouse-sized therians with jaws and hair had evolved.
Metabolic rates are lower.
They lay eggs but suckle their young. The marsupials found in Australia and in North America are distinctive in that the young are born tiny, blind, and hairless but find their way to the mother's pouch where they are suckled and finish their development. Placental mammals are found in virtually every aquatic and terrestrial environment. It is the organ of exchange of nutrients and wastes between the maternal blood and the fetal blood.