MаmmаlsMаmmals have been arоund since the time оf the dinоsaurs about 220 million years ago, originating as tiny, shrew-like, nocturnal, insect-eating creatures. They reached their maximum diversity in the Tertiary period after the mass extinction of dinosaurs.There are about 5,000 species of mammals, the lowest among vertebrates, with rodents, bats, shrews and moles comprising most species. Two fundamental mammalian traits are hair and mammary glands in females that produce milk to nourish young.Other key mammalian adaptations include specialized heterodont teeth adapted for specific diets, endothermy providing warm-bloodedness, and a muscularized diaphragm facilitating efficient respiration.The three major mammalian groups are:Prototheria (monotremes like platypus, echidnas) which lay eggs.Metatheria (marsupials like kangaroos, opossums) which give birth to underdeveloped young that complete development in a pouch.Eutheria (placental mammals like primates, rodents, carnivores) which nourish the embryo via a true placenta.