It wasn't until about 65 million years ago, after the extinction of the dinosaurs, that larger mammals were able to evolve to take advantage of newly available habitats and food. Today mammals are found throughout the world in almost every type of habitat. There are only 3 species of living monotremes found only in Australia, Tazmania, and New Guineae, the duck-billed platypus, and 2 echnidas, Tachyglossus aculeatus , and Zaglossus bruijni . In marsupials the placenta that connects the fetus to the mother and provides nourishment is reabsorbed and severely limits how long embryos can survive in the womb. Females give birth to tiny, poorly developed young that then crawl into a pouch where they are fed milk and develop further.
Eutherians are the most diverse group of mammals.
They all have a complex placenta that grows to accomodate the growing embryos and allows much development during gestation. There are 4,629 described mammal species throughout the world. This number is relatively small compared with over 9,600 species of birds or 40,000 species of fish. Structurally and functionally, however, mammals are quite diverse. Today, mammals occupy every major habitat.