Prototheria have developed from 

Reptilia gave rise to Prototheria. They are categorised as a subclass of mammals and are known as monotremes. These are egg laying. Prototheria have several skeletal characteristics with reptiles. Their legs are clearly on the sides of their bodies rather than directly beneath them. As a result, their look is that of reptiles. However, the show a lot of characters of mammals hence classified under it. Example is platypus. Also unlike Prototheria, birds have a pair of legs underneath them, and unlike Prototheria, amphibians do not have a tail-like structure. Eutheria on the other hand are subclass of mammals that give birth to the young ones like humans and dogs.