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.

However, some organisms fall outside the crown group at a particular level, and these require an extra systematic category to include them. A good example is Archaeopteryx which, although clearly bird-like, is not descended from the last common ancestor of all living birds, being instead more basal than any of them. Such organisms can be referred to the stem group of a clade (a term introduced by the palaeontologist Dick Jefferies). In this example Archaeopteryx is a stem-group bird. All organisms more closely related to crown-group birds than to any other living group are referable to the bird stem group. As living birds are by definition in the crown group, it follows that all members of the stem-group of a clade are extinct; and thus, stem groups only have fossil members.

Stem and crown groups are hierarchical; thus, an extinct crown-group bird may lie in the stem group of a subset of the birds, such as the passerines; and stem-group birds lie, together with crown group birds and crocodiles and several extinct groups, in the crown group of the Archosauria. Thus all extinct organisms lie in a stem group of some living crown group. A crown group and its stem group considered together are known as the total group. Although the stem group is paraphyletic, it can be objectively defined by subtracting the crown group from the total group.

Stem groups offer a route to classify fossils that otherwise do not obey systematics based on living organisms. Stem group organisms always lack one or more features that are present at the base of the crown group to which they are attached. Thus, Archaeopteryx still retains teeth, unlike crown-group birds. As a group evolves away from the last common ancestor of it and its nearest living relative, it accumulates the distinctive features of its crown group. Hence, the last common ancestor of birds and crocodiles – the first crown group archosaur -was neither bird nor crocodile, and possessed none of the distinctive features of either. Evolution up the bird stem group allowed the accumulation of distinctive bird features such as the beak and hollow bones, until all were finally present at the base of the crown group. Placing fossils in their right order in a stem group allows the order of these acquisitions to be established, and thus the ecological and functional setting of the evolution of the major features of the group in question. Stem groups thus offer a route to integrate unique palaeontological data into questions of the evolution of living organisms. Furthermore, they show that fossils that were considered to lie in their own separate group because they did not show all the diagnostic features of a living clade, can nevertheless be related to it by lying in its stem group. Such fossils have been of particular importance in considering the origins of the tetrapods, mammals, and animals, among others.

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