Like other mammals, monotremes are warm-blooded with a high metabolic rate, have hair on their bodies, produce milk to feed their young, have a single bone in their lower jaw, and have three middle ear bones.

 

Monotremes were very poorly understood for many years and to this day some of the 19th-century myths that grew up around them endure. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals.

 

It now seems plain that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the marsupial and placental groups.