Contents of The Crown Group
It is not necessary for a species to have living descendents in order for it to be included in the crown group. Extinct side branches on the family tree that are descended from the last common ancestor of living members will still be part of a crown group. For example, if we consider the crown-birds (i.e. all extant birds and the rest of the family tree back to their last common ancestor), extinct side branches like the dodo or great auk are still descended from the last common ancestor of all living birds, so fall within the bird crown group. One very simplified cladogram for birds is shown below:
In this diagram, the clade labelled "Neornithes" is the crown group of birds: it includes the ancestor of all living birds and its descendants, living or not. Although considered to be birds (i.e. members of the clade Aves), Archaeopteryx and other extinct groups are not included in the crown group, as they fall outside the Neornithes clade, being descended from an earlier ancestor.
An alternative definition does not require all members of a crown group to be extant, only to have resulted from a "major cladogenesis event". The first definition forms the basis of this article.
Often, the crown group is given the designation "crown-", to separate it from the group as commonly defined. Both birds and mammals are traditionally defined by their traits, and contain fossil members that lived before the last common ancestors of the living groups or, like the mammal Haldanodon, were not descended from that ancestor although they lived later. Crown-Aves and Crown-Mammalia therefore differ slightly in content from the common definition of Aves and Mammalia. This has caused some confusion in the literature.
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