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What are the typical features of mammals?

The typical features of mammals are: body (more or less) covered with hair; presence of the diaphragm muscle (that separates the thorax from the abdomen); mammary glands that produce milk (in females); enucleated blood red cells; middle ear with three ossicles.

What are the three main groups into which mammals are divided?

The three groups into which mammals are divided are: monotremes (or prototherian, e.g., ornithorhynchus), marsupials (or metatherian, for example, kangaroos) and placental (or eutherian, such as humans).

Do all mammals have placenta?

Mammals of the monotreme group (ornithorhyncus, echidnas) are oviparous, egg-laying, and they do not present placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not present placenta too; females of this group give birth to embryonic youngs that then continue development within the mother’s pouch. Placenta only forms in female placental mammals.