Under phylogenetic systematics, the descendants of the last common ancestor of "Dimetrodon" and all living reptiles would include all mammals because "Dimetrodon" is more closely related to mammals than to any living reptile. Thus, if it is desired to avoid the clade that contains both mammals and the living reptiles, then "Dimetrodon" must not be included in that clade—nor any other "mammal-like reptile". 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.