Synapsid classification has undergone tremendous change in recent years; most of the traditional groupings have been discovered to be paraphyletic. To represent this, we have used multiple lines drawn to paraphyletic groups in the cladogram above. Except for the Mammalia, all synapsid groups are extinct.
Current hypotheses about early synapsid diversification suggest that "pelycosaurs" are the basal-most synapsids, and are certainly paraphyletic. This group includes familiar "sail-back" critters like Dimetrodon, but also includes a variety of lesser known early synapsids, such as the herbivorous Caseidae. All groups of pelycosaurs went extinct by the end of the Permian. The remaining groups in the above cladogram constitute the Therapsida, and most of them diversified in the Permian or Triassic -- and then disappeared as the dinosaurs came to dominate the terrestrial world.
In addition to our Hall of Mammals, we also have a page on Thrinaxodon, a non-mammalian cynodont.
Visit the synapsids at the Tree of Life project for additional information.