 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 . 
