Pelycosaur (/ˈpɛlɪkəˌsɔːr/ PEL-ih-kə-sor)[1] is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants.

Because it excludes the advanced synapsid group Therapsida, the term is paraphyletic and contrary to modern formal naming practice.

[3][4] Thus the name pelycosaurs, similar to the term mammal-like reptiles, had fallen out of favor among scientists by the 21st century, and is only used informally, if at all, in the modern scientific literature.

[citation needed] The pelycosaurs appear to have been a group of synapsids that have direct ancestral links with the mammals, having differentiated teeth and a developing hard palate.

The pelycosaurs appeared during the Late Carboniferous and reached their apex in the early part of the Permian, remaining the dominant land animals for some 40 million years.

[disputed – discuss] Fossil evidence from some varanopids shows that parts of the skin were covered in rows of osteoderms, presumably overlain by horny scutes.

[12] In 1940, the group was reviewed in detail, and every species known at the time described, with many illustrated, in an important monograph by Alfred Sherwood Romer and Llewellyn Price.