 The modern view of synapsid relationships was proposed by paleontologist Robert R. Reisz in 1986 , whose study included features mostly found in the skull rather than in the postcranial skeleton . Dimetrodon is still considered a sphenacodont under this phylogeny , but varanodontids are now considered more basal synapsids , falling outside clade Sphenacodontia . Within Sphenacodontia is the group Sphenacodontoidea , which in turn contains Sphenacodontidae and Therapsida . Sphenacodontidae is the group containing Dimetrodon and several other sail @-@ backed synapsids like Sphenacodon and Secodontosaurus , while Therapsida includes mammals and their mostly Permian and Triassic relatives . 
