The mammals of today are but one branch of the Synapsida, a great vertebrate group with a 300 million year history. Pre-mammalian synapsids -- including the famous "finback" Dimetrodon, shown at the top left -- dominated the land vertebrate fauna of the Permian and early Triassic before losing ground to the diversifying dinosaurs and other archosaurs. These pre-mammalian groups of synapsids have at times been called "mammal-like reptiles". This term is now discouraged because although many had characteristics in common with mammals, none of them were actually reptiles.
Visit Jack Conrad's pages on Synapsida for additional information.