Within clade Synapsida, "Dimetrodon" is part of the clade Sphenacodontia, which was first proposed as an early synapsid group in 1940 by paleontologists Alfred Romer and Llewellyn Ivor Price, along with the groups Ophiacodontia and Edaphosauria. All three groups are known from the Late Carboniferous and Early Permian. Romer and Price distinguished them primarily by postcranial features such as the shapes of limbs and vertebrae. Ophiacodontia was considered the most primitive group because its members appeared the most reptilian, and Sphenacodontia was the most advanced because its members appeared the most like a group called Therapsida, which included the closest relatives to mammals. Romer and Price placed another group of early synapsids called varanopids within Sphenacodontia, considering them to be more primitive than other sphenacodonts like "Dimetrodon". They thought varanopids and "Dimetrodon"-like sphenacodonts were closely related because both groups were carnivorous, although varanopids are much smaller and more lizard-like, lacking sails.