The class of mammals belongs to the phylum chordates and the by far largest subphylum of this group, the vertebrates.
The mammals themselves are divided into three subclasses:
The protheria whose only living members today are the platypus and the echidnas,
At the moment about 5,500 species of recent mammals have been scientifically described and are distributed in 29 orders. Among them the rodents are the group with the most species.
The distinguishing characteristics of mammals are - apart from lactating their young with milk - the possession of hair and a constant body temperature.
In the course of their evolution, mammals have colonized all kinds of habitats. They are not restricted to land, several species have adapted to an aquatic life style of which the cetaceans (whales and dolphins) are the most specialized. Other taxa have conquered the air, among them the bats, who are apart from birds the only recent vertebrates capable of active flight.
Independently several other mammal groups have developed gliding membranes.
In regard to size there are amazing differences within the group: