The mammals are a distinct group of vertebrates in many respects:

• warm blood and a constant body temperature;

• high metabolic rate and unique homeostatic mechanisms that go with it;

• a characteristic mode of locomotion, or gait, in which the body is held upright with the legs underneath - as with the horse opposite for instance.

• mammalian brains are large;

• distinctive method of reproduction, including lactation;

• powerful jaws and a set of relatively durable teeth, differentiated into a number of tooth types.

Therefore, when the mammals evolved from the reptiles, there had to be changes on a large scale in many characters. It was a macroevolutionary change. How did this transition take place?