Some organisms fall close to but outside a particular crown group. A good example is Archaeopteryx which, although clearly bird-like, is not descended from the last common ancestor of all living birds. Such organisms can be classified within the stem group of a clade. In this example Archaeopteryx is a stem-group bird, but it falls within the vertebrate crown-group. 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. A crown group and its stem group considered together are known as the total group. Although the stem group is paraphyletic, it can be
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| - Some organisms fall close to but outside a particular crown group. A good example is Archaeopteryx which, although clearly bird-like, is not descended from the last common ancestor of all living birds. Such organisms can be classified within the stem group of a clade. In this example Archaeopteryx is a stem-group bird, but it falls within the vertebrate crown-group. 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. A crown group and its stem group considered together are known as the total group. Although the stem group is paraphyletic, it can be
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| abstract
| - Some organisms fall close to but outside a particular crown group. A good example is Archaeopteryx which, although clearly bird-like, is not descended from the last common ancestor of all living birds. Such organisms can be classified within the stem group of a clade. In this example Archaeopteryx is a stem-group bird, but it falls within the vertebrate crown-group. 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. 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. The crown-and-stem group concept, first mooted in 1979, was not commonly used until its reintroduction in the 2000s.
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![[RDF Data]](/fct/images/sw-rdf-blue.png)