 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 <unk> and <unk> . 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 . <unk> 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 . 
