Synapsid Primitive and advanced synapsids
The mammal-like reptiles are traditionally divided into a primitive group and an advanced group, known respectively as pelycosaurs and therapsids. Pelycosaurs are a paraphyletic group of six primitive families of synapsids. They were all rather lizard-like with sprawling gait and possibly horny scutes. The therapsids contain the more advanced synapsids, having a more erect pose and possibly hair, at least in some forms. The latter are the ancestors of mammals.