On what basis are mammals classified into different groups, and what these groups?

Mammals classified as monotremes, such as the Duck-billed Platypus and Echidna, lay eggs.

Babies born to marsupials are undernourished.

Instead of having a placenta, they carry their young inside a pouch that is joined to their stomach and provides them with nutrition. An example of this would be a kangaroo.

The placenta is an organ or lining that facilitates the transfer of nutrients from the mother's body to the developing baby through the umbilical cord. It also introduces the baby's excrement into the mother's bloodstream.

Mammals are classified into different groups based on several characteristics, including their reproductive strategies, anatomical features, and evolutionary relationships. The main groups of mammals are:

1. Monotremes: Egg-laying mammals, such as the platypus and echidna.
2. Marsupials: Mammals that give birth to relatively undeveloped young, which then continue to develop in a pouch, like kangaroos, koalas, and opossums.
3. Placentals: Mammals that give birth to fully developed young, nourished through a placenta during gestation. This group includes a vast diversity of mammals, including humans, dogs, cats, whales, and rodents. These groups are further divided into various orders, families, genera, and species based on more specific characteristics and evolutionary relationships.