Mammal-like reptile

Mammal-like reptile is an old term for the therapsids: those synapsids which gave rise to the true mammals.
The term is both outmoded and a mistake, because mammals did not descend from reptiles. Both groups descended from early amniotes (egg-laying tetrapods), probably in the Lower or Middle Carboniferous. The precursors of reptiles are called sauropsids, and the precursors of mammals are called synapsids. The immediate ancestors of the mammals came from a group of therapsids called the cynodonts.
Mammals and reptiles.
There are a number of characteristics which cannot be seen on fossils, but which are of great importance.
Mammals are distinguished from reptiles by fundamental differences in the development of the blood system. These differences are such that it would be almost impossible for mammals to be derived from reptiles.
This supports the division of amniotes into sauropsida and synapsida.