A crown group is a living monophyletic group or clade, consisting of the last common ancestor of all living examples, plus all of its descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to extinct ones. Extinct members of crown groups are possible. Because the Dodo, although extinct, is still descended from the last common ancestor of all living birds, for example, it is nevertheless included in the bird Crown Group.