Pelycosaur

Pelycosaur (/ˈpɛlɪkəˌsɔːr/ PEL-ih-kə-sor) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used, and Pelycosauria was considered an order, but this is now thought to be incorrect and outdated.

Because it excludes the advanced synapsid group Therapsida, the term is paraphyletic and contrary to modern formal naming practice. Thus the name pelycosaurs, similar to the term mammal-like reptiles, fell out of favor among scientists by the 21st century, and is only used informally, if at all, in the modern scientific literature. The terms stem mammals, protomammals, and basal or primitive synapsids are instead used where needed.