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.