Original: Phylogenetically it is defined as a clade including the most recent common ancestor of Sinoconodon , morganuconodonts , docodonts , Monotremata , Marsupialia , Placentalia , extinct members of this clade , and all of its descendants .
Simplified: Instead of having many teeth that are frequently replaced , mammals have one set of baby teeth and later one set of adult teeth which fit together precisely .