The oldest known fossil synapsid is Protoclepsydrops from about 320 million years ago , while the oldest known sauropsid is probably Paleothyris , in the order Captorhinida , from the Middle Pennsylvanian epoch ( about 306-312 million years ago ) . The oldest known fossil synapsid is Protoclepsydrops from about 320 million years ago , while the oldest known sauropsid is probably Paleothyris , in the order Captorhinida , from the Middle Pennsylvanian epoch ( ca . 306-312 million years ago ) .