Phylogenetic Classification
See also: Synapsid#Linnaean and cladistic classificationsDimetrodon is an early member of a group called synapsids, which include mammals and many of their extinct relatives. It is often mistaken as a dinosaur in popular culture, despite having become extinct around 40 million years (Ma) before the first dinosaurs appeared. As a synapsid, Dimetrodon is more closely related to mammals than it is to dinosaurs or any living reptile. In the late nineteenth and early twentieth centuries, most paleontologists called Dimetrodon a reptile in accordance with Linnean taxonomy. In the Linnean system, Reptilia was ranked as a class and Dimetrodon was ranked as a genus within that class. Although a close relationship between Dimetrodon and mammals was appreciated at the time, mammals belonged to their own separate class and thus Dimetrodon was a "mammal-like reptile". Paleontologists recognized that mammals evolved from this group in what they called a reptile-to-mammal transition. More recently, phylogenetic taxonomy has become a common system of classification in vertebrate paleontology whereby groups are defined by shared common ancestors. Reptiles and mammals have been placed into clades, which are groups that include a common evolutionary ancestor and all of its descendants. Under phylogenetic systematics, the descendants of the last common ancestor of Dimetrodon and all living reptiles must include all mammals because Dimetrodon is now known to be more closely related to mammals than it is to any living reptile. To avoid mammals being in the clade that includes these living reptiles, neither Dimetrodon nor any other "mammal-like reptile" can be considered part of that clade. Descendants of the last common ancestor of mammals and reptiles (which appeared around 310 Ma in the Late Carboniferous) are therefore split into two clades: Synapsida, which includes Dimetrodon and mammals, and Sauropsida, which includes living reptiles and all extinct reptiles more closely related to them than to mammals.
Within Synapsida, Dimetrodon is part of the clade Sphenacodontia. Sphenacodontia 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 syapsids 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. 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 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. Below is a cladogram modified from the analysis of Benson (in press) that follows this phylogeny:
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