Crown group

A clade comprising the last common ancestor of a group’s living members and all its descendants, including extinct descendants.

Clade: A group consisting of an ancestor and all its descendants. A crown group is a particular kind of clade, anchored by living members.
Stem group: Extinct taxa more closely related to a crown group than to its living sister group, but outside the crown group. Stem-group taxa sit outside the crown despite their close relationship to its living members.
Crown tetrapod: The clade comprising the last common ancestor of living amphibians, reptiles, birds, and mammals and all its descendants. Its definition separates living-lineage ancestry from early fossil tetrapods outside the crown.
Willi Hennig: A German entomologist whose work established modern phylogenetic systematics. His emphasis on evolutionary branching provided the framework for defining clades by ancestry.
Last common ancestor: The most recent ancestor shared by two or more organisms or lineages. This ancestor of the living members anchors the crown group.
Total group: A crown group together with all extinct taxa more closely related to it than to its living sister group. It includes the crown group and its stem, extending the boundary to earlier extinct relatives.
Crown mammal: The clade comprising the last common ancestor of living mammals and all its descendants. Fossils can be tested against the ancestry shared by living mammals.
Phylogenetic systematics: A method of classifying organisms according to shared ancestry and branching relationships. Crown-group reasoning depends on the ancestry-based classification this approach formalized.
Extant taxon: A taxon whose members are living today. Extant members determine which ancestor defines a crown group.
Sister group: The closest relative of a clade, sharing with it an immediate common ancestor. The crown group's sister lineage helps define which extinct taxa belong to its stem.

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