On present Earth there are three subclasses of mammals:
- Monotreme. They are primitive mammals whose mouth has a horned beak that reproduce by laying eggs.
Current representatives, platypus and echidnas are restricted to the Australian region (Australia and New Guinea);
- Marsupials. This group includes representatives of the Australian fauna, such as kangaroos and koalas, and representatives from the US and South America, such as our skunks and kookas. After a short phase of development in a purse-fold skin of the mother's abdomen, the marsupium;
The tiger cat, Dasyurus maculatus, one of Australia's largest carnivorous marsupials.
- Placental. It includes most mammals, separated into orders such as carnivores, rodents, ungulates, cetaceans, chiropterans, and primates, to which the human species belongs. In these animals, the uterus and placenta are well developed, allowing for development within the maternal organism.