Scientists have organized mammals into orders based on their characteristics and their structures. Scientists are still trying to figure out how some mammals are related.

All mammals share three characteristics not seen in other animals. They have three middle ear bones, they have hair, and they produce milk for their young. There are about 5,000 species of mammals. Scientists have organized mammals into about 26 orders based on characteristics and structure.

There are three subclasses of mammals:

Prototheria, Metatheria and Eutheria. Monotremata is the only order in the prototheria subclass. Monotremes are so different from other mammals that scientists think they may be descended from a separate mammal-like reptilian stock. The duck-billed platypus is in this order. Marsupials make up the metatheria subclass. With 19 orders, eutheria is the largest subclass of mammals.