It is thought that birds are descended from dinosaurs, as evidenced, in part, by the scales on their feet. Birds' bones are light weight for flight. Generally, mating is accompanied by an elaborate courtship ritual. Mammals have a diaphragm to aid in respiration. They are endothermic.
Most mammals bear live young.
These mammals lays eggs like reptiles, but do have fur and milk. They have one posterior opening for their digestive, urinary, and genital tracts, hence the subclass name. The young are born as very immature embryos and must crawl to their mother's pouch to continue their development. Because of continental drift, there is a wide variety of marsupials in Australia, yet few elsewhere on Earth. In this taxon, young complete embryonic development within the mother's uterus and are nourished across a placenta .