 Dimetrodon is assigned to a group traditionally called " mammal @-@ like reptiles " — more recently termed " stem @-@ mammals " or " non @-@ mammalian synapsids " . That is , many vertebrate paleontologists today group Dimetrodon together with mammals in an evolutionary group , or clade , called Synapsida , while they place dinosaurs with living reptiles and birds in a separate clade , Sauropsida . Single openings in the skull behind each eye , known as temporal fenestrae , and other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids . 
