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.[1]
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.[2][3]
↑Clack, Jennifer 2012. Gaining ground: the origin and early evolution of tetrapods. 2nd ed, Bloominton, Indiana: Indiana University Press, Chapter 8, p295. ISBN978-0-253-35675-8, (Mammal-like reptile at Google Books)
↑Benton, Michael J. 2005. Vertebrate paleontology, 3rd ed. Blackwell, Oxford: chapter 10, The mammals. ISBN0632056371
↑Prothero, Donald R. 2007. Evolution: what the fossils say and why it matters. New York: Columbia University Press, Chapter 13, 271–280, especially p274/5. ISBN978-0-231-13962-5